Search

We designed and validated a test system that simulates a growth environment for Zea mays L. maize seedlings under conditions of low moisture gradient in darkness. This system allowed us to simultaneously measure mesocotyl elongation and the primary root hydrotropic response in seedlings before the emergence phase in a collection of maize hybrids. We found great variation in these two traits with statistically significant reduction of their elongations under the low moisture gradient condition that indicate the richness of maize genetic diversity. Hence, the objective of designing a new test system that evaluates the association between these underground traits with the potential use to measure other traits in maize seedlings related to early vigor was achieved.

Efficient foraging by plant roots relies on the ability to sense multiple physical and chemical cues in soil and to reorient growth accordingly (tropism). Root tropisms range from sensing gravity (gravitropism), light (phototropism), water (hydrotropism), touch (thigmotropism), and more. Electrotropism, also known as galvanotropism, is the phenomenon of aligning growth with external electric fields and currents. Although root electrotropism has been observed in a few species since the end of the 19th century, its molecular and physical mechanisms remain elusive, limiting its comparison with the more well-defined sensing pathways in plants. Here, we provide a quantitative and molecular characterization of root electrotropism in the model system Arabidopsis (Arabidopsis thaliana), showing that it does not depend on an asymmetric distribution of the plant hormone auxin, but instead requires the biosynthesis of a second hormone, cytokinin. We also show that the dose–response kinetics of the early steps of root electrotropism follows a power law analogous to the one observed in some physiological reactions in animals. Future studies involving more extensive molecular and quantitative characterization of root electrotropism would represent a step toward a better understanding of signal integration in plants and would also serve as an independent outgroup for comparative analysis of electroreception in animals and fungi.

Tropisms are growth-based plant directional movements, allowing plants to respond to their living environments. Plant roots have developed various tropic responses, including gravitropism, hydrotropism, chemotropism, and halotropism, in response to the gravity, moisture gradient, nutrient gradient, and salinity gradient, respectively. Revealed mechanisms of several tropic responses suggested that plant hormone gradient and cell division activity play key roles in determining these responses. Approaches to measure cell division and hormone gradients, however, have rarely been applied in root tropic analyses. Here, we describe a number of methods to quantify cell division and hormone gradients during root tropic analysis. These approaches are mainly based on our previous researches on root hydrotropism.

We hereby show that root systems adapt to a spatially discontinuous pattern of water availability even when the gradients of water potential across them are vanishingly small. A paper microfluidic approach allowed us to expose the entire root system of Brassica rapa plants to a square array of water sources, separated by dry areas. Gradients in the concentration of water vapor across the root system were as small as 10-4 mM·m-1 (~4 orders of magnitude smaller than in conventional hydrotropism assays).\ud In spite of such minuscule gradients (which greatly limit the possible influence of the well-understood gradient-driven hydrotropic response), our results show that (i) individual roots as well as the root system as a whole adapt to the pattern of water availability to maximize access to water, and that (ii) this adaptation increases as water sources become more rare. \ud These results suggest that either plant roots are more sensitive to water gradients than humanmade water sensors by 3 to 5 orders of magnitude, or they might have developed, like other organisms, mechanisms for water foraging that allow them to find water in the absence of an external gradient in water potential.

Despite the vital roles of jasmonoyl-isoleucine (JA-Ile) in governing plant growth and environmental acclimation, it remains unclear what intracellular processes lead to its induction. Here, we provide compelling genetic evidence that mechanical and osmotic regulation of turgor pressure represents a key factor in eliciting JA-Ile biosynthesis. After identifying cell wall mutant alleles in KORRIGAN1 (KOR1) with elevated JA-Ile in seedling roots, we found that ectopic JA-Ile resulted from cell non-autonomous signals deriving from enlarged cortex cells compressing inner tissues and stimulating JA-Ile production. Restoring cortex cell size by cell-type-specific KOR1 complementation, by isolating a genetic kor1 suppressor, and by lowering turgor pressure with hyperosmotic treatments, abolished JA-Ile signalling. Strikingly, heightened JA-Ile levels guided kor1 roots towards greater water availability, uncovering a previously unrecognized JA-Ile function in root hydrotropism. Collectively, these findings enhance our understanding of JA-Ile biosynthesis initiation, and reveal a novel role of JA-Ile in orchestrating environmental resilience.

Arabidopsis roots sense the moisture gradient in soils and grow toward an area with higher water potential – a process called hydrotropism. Our previous study has shown that the apoplastic proton extrusion in root tips is influenced by brassinosteroids (BRs) receptor BR-INSENSITIVE1 (BRI1) and is crucial for hydrotropic response in Arabidopsis. Here we show that BRI1 interacts directly not only with Arabidopsis plasma membrane H+-ATPase 2 (AHA2) but also with Arabidopsis plasma membrane H+-ATPase 7 (AHA7). Therefore, BRI1 may affect hydrotropic response via regulating the activities of AHA2 and AHA7.

In most soils, the spatial distribution of nutrients and water in the rooting zone of plants is heterogeneous and changes over time. To access localized resources more efficiently, plants induce foraging responses by modulating individual morphological root traits, such as the length of the primary root or the number and length of lateral roots. These adaptive responses require the integration of exogenous and endogenous nutrient- or water-related signals into the root developmental program. Recent studies corroborated a central role of auxin in shaping root architectural traits in response to fluctuating nutrient and water availabilities. In this review, we highlight current knowledge on nutrient- and water-related developmental processes that impact root foraging and involve auxin as a central player. A deeper understanding and exploitation of these auxin-related processes and mechanisms promises advances in crop breeding for higher resource efficiency.

Plant roots respond to soil moisture gradients and remodel their growth orientation toward water through hydrotropism, a process vital for acclimation to a changing soil environment. Mechanisms underlying the root hydrotropic response, however, remain poorly understood. Here, we examined hydrotropism in 31 Arabidopsis (Arabidopsis thaliana) ecotypes collected from different parts of the world and grown along moisture gradients in a specially designed soil-simulation system. Comparative transcriptome profiling and physiological analyses were carried out on three selected ecotypes, Wassilewskija (Ws), Columbia (Col-0) (strongly hydrotropic), Col-0 (moderately hydrotropic), and C24 (weakly hydrotropic), and in mutant lines with altered root hydrotropic responses. We show that H+ efflux, Ca2+ influx, redox homeostasis, epigenetic regulation, and phytohormone signaling may contribute to root hydrotropism. Among phytohormones, the role of brassinosteroids (BRs) was examined further. In the presence of an inhibitor of BR biosynthesis, the strong hydrotropic response observed in Ws was reduced. The root H+ efflux and primary root elongation also were inhibited when compared with C24, an ecotype that showed a weak hydrotropic response. The BR-insensitive mutant bri1-5 displayed higher rates of root growth inhibition and root curvature on moisture gradients in vertical or oblique orientation when compared with wild-type Ws. We also demonstrate that BRI1 (a BR receptor) interacts with AHA2 (a plasma membrane H+-ATPase) and that their expression patterns are highly coordinated. This synergistic action may contribute to the strong hydrotropism observed in Ws. Our results suggest that BR-associated H+ efflux is critical in the hydrotropic response of Arabidopsis roots.

Because water is essential to life, organisms have evolved a wide range of strategies to cope with water limitations, including actively searching for their preferred moisture levels to avoid dehydration. Plants use moisture gradients to direct their roots through the soil once a water source is detected, but how they first detect the source is unknown. We used the model plant Pisum sativum to investigate the mechanism by which roots sense and locate water. We found that roots were able to locate a water source by sensing the vibrations generated by water moving inside pipes, even in the absence of substrate moisture. When both moisture and acoustic cues were available, roots preferentially used moisture in the soil over acoustic vibrations, suggesting that acoustic gradients enable roots to broadly detect a water source at a distance, while moisture gradients help them to reach their target more accurately. Our results also showed that the presence of noise affected the abilities of roots to perceive and respond correctly to the surrounding soundscape. These findings highlight the urgent need to better understand the ecological role of sound and the consequences of acoustic pollution for plant as well as animal populations.

Roots display directional growth toward moisture in response to a water potential gradient. Root hydrotropism is thought to facilitate plant adaptation to continuously changing water availability. Hydrotropism has not been as extensively studied as gravitropism. However, comparisons of hydrotropic and gravitropic responses identified mechanisms that are unique to hydrotropism. Regulatory mechanisms underlying the hydrotropic response appear to differ among different species. We recently performed molecular and genetic analyses of root hydrotropism in Arabidopsis thaliana. In this review, we summarize the current knowledge of specific mechanisms mediating root hydrotropism in several plant species.

Summary The role of amyloplasts in the interactions between hydrotropism and gravitropism has been previously described. However, the effect of light-dark on the interactions between the two tropisms remains unclear. Here, by developing a method that makes it possible to mimic natural conditions more closely than the conventional lab conditions, we show that hydrotropism is higher in wild-type Arabidopsis seedlings whose shoots are illuminated but whose roots are grown in the dark compared with seedlings that are fully exposed to light. Root gravitropism is substantially decreased because of the reduction of amyloplast content in the root tip with decreased gene expression in PGM1 (a key starch biosynthesis gene), which may contribute to enhanced root hydrotropism under darkness. Furthermore, the starch-deficient mutant pgm1-1 exhibits greater hydrotropism compared with wild-type. Our results suggest that amyloplast response and starch reduction occur under light-dark modulation, followed by decreased gravitropism and enhanced hydrotropism in Arabidopsis root.

Hydrotropism is the directed growth of roots toward the water found in the soil. However, mechanisms governing interactions between hydrotropism and gravitropism remain largely unclear. In this study, we found that an air system and an agar–sorbitol system induced only oblique water-potential gradients; an agar–glycerol system induced only vertical water-potential gradients; and a sand system established both oblique and vertical water-potential gradients. We employed obliquely oriented and vertically oriented experimental systems to study hydrotropism in Arabidopsis and tomato plants. Comparative analyses using different hydrotropic systems showed that gravity hindered the ability of roots to search for obliquely oriented water, whilst facilitating roots’ search for vertically oriented water. We found that the gravitropism-deficient mutant aux1 showed enhanced hydrotropism in the oblique orientation but impaired root elongation towards water in the vertical orientation. The miz1 mutant exhibited deficient hydrotropism in the oblique orientation but normal root elongation towards water in the vertical orientation. Importantly, in contrast to miz1, the miz1/aux1 double mutant exhibited hydrotropic bending in the oblique orientation and attenuated root elongation towards water in the vertical orientation. Our results suggest that gravitropism is required for MIZ1-regulated root hydrotropism in both the oblique orientation and the vertical orientation, providing further insight into the role of gravity in root hydrotropism.

Drought alone causes more annual loss in crop yield than all pathogens combined. To adapt to moisture gradients in soil, plants alter their physiology, modify root growth and architecture, and close stomata on their aboveground segments. These tissue-specific responses modify the flux of cellular signals, resulting in early flowering or stunted growth and, often, reduced yield. Physiological and molecular analyses of the model plant Arabidopsis thaliana have identified phytohormone signaling as key for regulating the response to drought or water insufficiency. Here we discuss how engineering hormone signaling in specific cells and cellular domains can facilitate improved plant responses to drought. We explore current knowledge and future questions central to the quest to produce high-yield, drought-resistant crops., A.I.C.-D. has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement 683163). A.I.C.-D. is a recipient of a BIO2016-78150-P grant funded by the Spanish Ministry of Economy and Competitiveness and Agencia Estatal de Investigación (MINECO/AEI) and Fondo Europeo de Desarrollo Regional (FEDER). A.G. has received funding by a postdoctoral fellowship from the “Severo Ochoa Programme for Centers of Excellence in R&D” 2016–2019 from the Ministerio de Ciencia e Innovación (SEV-2015-0533). A.G. and A.R.-M. have received funding from ERC-2015-CoG–683163 granted to the A.I.C.-D. laboratory. A.R.-M. is a predoctoral fellow from Fundación Tatiana Pérez de Guzmán el Bueno. This work was supported by the CERCA Programme/Generalitat de Catalunya.

Salinity is one of the most devastating abiotic stresses accounting for major crop losses yearly. Seedlings display a directed root growth response away from increasing salt levels in soil or agar plates, which has been named halotropism. GWAS on root growth of 333 Arabidopsis accessions identified WRKY25, CHX13 and a gene of unknown function DOB1, as new components contributing to early main root halotropic responses. We showed that halotropism required MIZ1 and MIZ2, genes previously shown to be involved in hydrotropism, and these responses are also dependent on components of both ethylene and ABA signalling pathways. In addition, we physiologically characterised KAB1 a putative K+ regulator, which does not play a direct role in halotropism. KAB1 contributes to root architecture, germination, stomata closure and shoot growth under both control and stress conditions in plants. Majority of the components were transcriptionally upregulated during salt stress and these proteins also function in maintaining Na+/K+ homeostasis during increased salinity. Deciphering the underlying mechanism of root halotropism and acclimation to abiotic stresses is a stepping stone in further elucidation of salt stress responses in planta. Phytohormones and Na+/K+ transporters are key players and are required for plant acclimation and adaptation to various stresses.

Roots display directional growth toward moisture in response to a water potential gradient. Root hydrotropism is thought to facilitate plant adaptation to continuously changing water availability. Hydrotropism has not been as extensively studied as gravitropism. However, comparisons of hydrotropic and gravitropic responses identified mechanisms that are unique to hydrotropism. Regulatory mechanisms underlying the hydrotropic response appear to differ among different species. We recently performed molecular and genetic analyses of root hydrotropism in Arabidopsis thaliana. In this review, we summarize the current knowledge of specific mechanisms mediating root hydrotropism in several plant species.

Recent studies mainly in Arabidopsis have renewed interest and discussion in some of the key issues in hydrotropism of roots, such as the site of water sensing and the involvement of auxin. We examined hydrotropism in maize (Zea mays) primary roots. We determined the site of water sensing along the root using a nonintrusive method. Kinematic analysis was conducted to investigate spatial root elongation during hydrotropic response. Indole-3-acetic acid (IAA) and other hormones were quantified using LC-MS/MS. The transcriptome was analyzed using RNA sequencing. Main results: The very tip of the root is the most sensitive to the hydrostimulant. Hydrotropic bending involves coordinated adjustment of spatial cell elongation and cell flux. IAA redistribution occurred in maize roots, preceding hydrotropic bending. The redistribution is caused by a reduction of IAA content on the side facing a hydrostimulant, resulting in a higher IAA content on the dry side. Transcriptomic analysis of the elongation zone prior to bending identified IAA response and lignin synthesis/wall cross-linking as some of the key processes occurring during the early stages of hydrotropic response. We conclude that maize roots differ from Arabidopsis in the location of hydrostimulant sensing and the involvement of IAA redistribution.

One of the greatest challenges of terrestrial vegetation is to acquire water through soil-grown roots. Owing to the scarcity of high-quality water in the soil and the environment’s spatial heterogeneity and temporal variability, ranging from extreme flooding to drought, roots have evolutionarily acquired tremendous plasticity regarding their geometric arrangement of individual roots and their three-dimensional organization within the soil. Water deficiency has also become an increasing threat to agriculture and dryland ecosystems due to climate change. As a result, roots have become important targets for genetic selection and modification in an effort to improve crop resilience under water-limiting conditions. This review addresses root plasticity from different angles: Their structures and geometry in response to the environment, potential genetic control of root traits suitable for water-limiting conditions, and contemporary and future studies of the principles underlying root plasticity post-Darwin’s ‘root-brain’ hypothesis. Our increasing knowledge of different disciplines of plant sciences and agriculture should contribute to a sustainable management of natural and agricultural ecosystems for the future of mankind.

Plants take up water and nutrients through roots, and uptake efficiency depends on root behavior. Roots recognize the moisture gradient in the soil and grow toward the direction of high moisture. This phenomenon is called hydrotropism, and it contributes to efficient water uptake. As nutrients in soil are also unevenly distributed, it is beneficial for plants to grow their roots in the direction of increasing nutrient concentrations, but such a phenomenon has not been demonstrated. Here, we describe the directional growth of roots in response to a nutrient gradient. Using our assay system, the gradient of a nitrogen nutrient, NH4+, was sufficient to stimulate positive tropic responses of rice lateral roots. This phenomenon is a tropism of plant roots to nutrients; hence, we propose the name ‘nutritropism’. As well as other tropisms, differential cell elongation was observed before the elongation zone during nutritropism, but the pattern promoting cell elongation preferentially on the non-stimulated side was opposite to those in root hydrotropism and gravitropism. Our evaluation of the NH4+ gradient suggested that the root tips responded to a sub-micromolar difference in NH4+ concentration on both sides of the root. Hydrotropism, gravitropism and phototropism were described in plants as the ‘power of movement’ by Charles and Francis Darwin in 1880, and these three tropisms have attracted the attention of plant scientists for more than 130 years. Our discovery of nutritropism represents the fourth ‘power of movement’ in plants and provides a novel root behavioral property used by plants to acquire nutrients efficiently.

Dwindling fresh water resources and climate change poses serious threats to rice production. Roots play crucial role in sensing water gradient and directing growth of the plant towards water through a mechanism called hydrotropism. Since very little information is available on root hydrotropism in major food crops, this study was carried out to clone and characterize an ortholog of Arabidopsis MIZU-KUSSEI1 (MIZ1) from rice. Contrasting rice genotypes for drought and salt tolerance were selected based on phenotyping for root traits. Nagina 22 and CR-262-4 were identified as most tolerant and Pusa Sugandh 5 and Pusa Basmati 1121 were identified as most susceptible varieties for both drought and salt stresses. Allele mining of MIZ1 in these varieties identified a 12 bp Indel but did not show specific allelic association with stress tolerance. Analysis of allelic variation of OsMIZ1 in 3024 rice genotypes of 3K genome lines using Rice SNP-Seek database revealed 49 InDels. Alleles with the 12 bp deletions were significantly prevalent in indica group as compared to that of japonica group. Real-time RT-PCR analysis revealed that OsMIZ1 expression levels were upregulated significantly in tolerant cv. Nagina 22 and CR-262-4 under osmotic stress, while under salt stress, it was significantly upregulated only in CR-262-4 but maintained in Nagina 22 under salt stress. However, in the roots of susceptible genotypes, OsMIZ1 expression decreased under both the stresses. These results highlight the possible involvement of OsMIZ1 in drought and salt stress tolerance in rice. Furthermore, expression studies using publically available resources showed that enhanced expression of OsMIZ1 is regulated in response to disease infections, mineral deficiency, and heavy metal stresses and is also expressed in reproductive tissues in addition to roots. These findings indicate potential involvement of MIZ1 in developmental and stress response processes in rice.

The phenomenon of plant root tips sensing moisture gradient in soil and growing towards higher water potential is designated as root hydrotropism, which is critical for plants to survive when water is a limited factor. Molecular mechanisms regulating such a fundamental process, however, are largely unknown. Here we report our identification that cytokinins are key signaling molecules directing root growth orientation in a hydrostimulation (moisture gradient) condition. Lower water potential side of the root tip shows more cytokinin response relative to the higher water potential side. Consequently, two cytokinin downstream type-A response regulators, ARR16 and ARR17, were found to be up-regulated at the lower water potential side, causing increased cell division in the meristem zone, which allows the root to bend towards higher water potential side. Genetic analyses indicated that various cytokinin biosynthesis and signaling mutants, including the arr16 arr17 double mutant, are significantly less responsive to hydrostimulation. Consistently, treatments with chemical inhibitors interfering with either cytokinin biosynthesis or cell division completely abolished root hydrotropic response. Asymmetrically induced expression of ARR16 or ARR17 effectively led to root bending in both wild-type and miz1, a previously known hydrotropism-defective mutant. These data demonstrate that asymmetric cytokinin distribution is a primary determinant governing root hydrotropism.

Maize illustrates one of the most complex cases of embryogenesis in higher plants that results in the development of early embryo with distinctive organs such as the mesocotyl, seminal and primary roots, coleoptile, and plumule. After seed germination, the elongation of root and mesocotyl follows opposite directions in response to specific tropisms (positive and negative gravitropism and hydrotropism). Tropisms represent the differential growth of an organ directed toward several stimuli. Although the life cycle of roots and mesocotyl takes place in darkness, their growth and functions are controlled by different mechanisms. Roots ramify through the soil following the direction of the gravity vector, spreading their tips into new territories looking for water; when water availability is low, the root hydrotropic response is triggered toward the zone with higher moisture. Nonetheless, there is a high range of hydrotropic curvatures (angles) in maize. The processes that control root hydrotropism and mesocotyl elongation remain unclear; however, they are influenced by genetic and environmental cues to guide their growth for optimizing early seedling vigor. Roots and mesocotyls are crucial for the establishment, growth, and development of the plant since both help to forage water in the soil. Mesocotyl elongation is associated with an ancient agriculture practice known as deep planting. This tradition takes advantage of residual soil humidity and continues to be used in semiarid regions of Mexico and USA. Due to the genetic diversity of maize, some lines have developed long mesocotyls capable of deep planting while others are unable to do it. Hence, the genetic and phenetic interaction of maize lines with a robust hydrotropic response and higher mesocotyl elongation in response to water scarcity in time of global heating might be used for developing more resilient maize plants.

Hydrotropism is the directional root growth response determined by water stimulus. In a water potential gradient system (WPGS) the roots of the Arabidopsis wild type have a diminished root growth compared to normal medium (NM). In contrast, the altered hydrotropic response1 (ahr1) mutant roots maintain their robust growth in the same WPGS. The aims of this work were to ascertain how ahr1 roots could sustain growth in the WPGS, with a special focus on the integration of cellular processes involved in the signaling that determines root growth during abiotic stress and their relation to hydrotropism. Cellular analysis of the root apical meristem of ahr1 mutant contrary to the wild type showed an absence of changes in the meristem length, the elongation zone length, the length of fully elongated cells, and the cell cycle duration. The robust and steady root growth of ahr1 seedlings in the WPGS is explained by the mutant capacity to maintain cell production and cell elongation at the same level as in the NM. Analysis of auxin response at a transcriptional level showed that roots of the ahr1 mutant had a lower auxin response when grown in the WPGS, compared to wild type, indicating that auxin signaling participates in attenuation of root growth under water stress conditions. Also, wild type plants exhibited a high increase in proline content while ahr1 mutants showed minimum changes in the Normal Medium→Water Stress Medium (NM→WSM), a lower water potential gradient system than the WPGS. Accordingly, in this condition, gene expression of Δ1-6 Pyrroline-5-Carboxylate Synthetase1 (P5CS1) involved in proline synthesis strongly increased in wild type but not in ahr1 seedlings. The ahr1 phenotype shows unique features since the mutant root cells continue to proliferate and grow in the presence of a progressively negative water potential gradient at a level comparable to wild type growing in the NM. As such, it represents an exceptional resource for understanding hydrotropism.

The endangered status and narrow distribution in Syringa pinnatifolia , a sub-alpine shrub species, were primarily caused by its poor drought adaptation related to its lower degree of physiological adjustment despite its greater degree of morphological plasticity. Although deep rooting is usually considered a drought-tolerant trait, we found that Syringa pinnatifolia, a deep rooting and hydrotropic shrub, has a limited distribution in arid areas. To elucidate the mechanisms for its narrow distribution, we conducted two experiments to examine the physiological and morphological responses to water availability and heterogeneity in S. pinnatifolia and a widespread congeneric species, S. oblata. We measured gas exchange, water use efficiency, and plasticity index in plants of these two species grown at different levels of soil water regimes and in containers with patched water distribution. Our results showed that high photosynthetic capacity in the narrowly distributed S. pinnatifolia was an important factor enabling its survival in the harsh sub-alpine environment. High photosynthetic capacity in S. pinnatifolia, however, was obtained at the expense of high transpiratory water loss, resulting in lower integrative water use efficiency. Biomass allocation to roots in S. pinnatifolia increased by 73 % when soil water increased from 75 to 95 % field capacity, suggesting that S. pinnatifolia could be less competitive for above-ground resources under favorable water regimes. The horizontal root hydrotropism and vertical root hydrotropism of S. pinnatifolia in soil with patched water patterns were likely related to compensation for leaf water loss at low soil water level, indicating a limited capacity for homeostasis within the plant for water conservation and lower level of inherent drought-tolerance. In summary, greater degree of morphological plasticity but lower degree of physiological adjustment may be the main causes for the hydrotropism and narrow distribution of S. pinnatifolia in the sub-alpine habitats.

Water is a key factor in plant life. Therefore, reaching and holding water is a crucial part in plant survival. Plants sense water through a set of sensors which includes sensors for water activity (potential), for specific components of water potential, or for specific solutes contributing to water potential and for hydraulic signals. While these sensors are common to different plants and other organisms, their functions and modes of action are yet far from being understood. It is also unknown how these sensing mechanisms are linked to cellular and whole-plant responses to changes in water status in the soil or in the atmosphere. Advanced technologies that would provide means for single-cell physiological manipulations together with high-throughput noninvasive real-time monitoring systems of shoots and roots and advanced biochemistry and structural studies at atomic resolution of sensor proteins and protein complexes are imperative for understanding water sensing by plants.

[EN] The phytohormone abscisic acid (ABA) plays a key role regulating root growth, root system architecture, and root adaptive responses, such as hydrotropism. The molecular and cellular mechanisms that regulate the action of core ABA signaling components in roots are not fully understood. ABA is perceived through receptors from the PYR/PYL/RCAR family and PP2C coreceptors. PYL8/RCAR3 plays a nonredundant role in regulating primary and lateral root growth. Here we demonstrate that ABA specifically stabilizes PYL8 compared with other ABA receptors and induces accumulation of PYL8 in root nuclei. This requires ABA perception by PYL8 and leads to diminished ubiquitination of PYL8 in roots. The ABA agonist quinabactin, which promotes root ABA signaling through dimeric receptors, fails to stabilize the monomeric receptor PYL8. Moreover, a PYL8 mutant unable to bind ABA and inhibit PP2C is not stabilized by the ligand, whereas a PYL85KR mutant is more stable than PYL8 at endogenous ABA concentrations. The PYL8 transcript was detected in the epidermis and stele of the root meristem; however, the PYL8 protein was also detected in adjacent tissues. Expression of PYL8 driven by tissue-specific promoters revealed movement to adjacent tissues. Hence both inter- and intracellular trafficking of PYL8 appears to occur in the root apical meristem. Our findings reveal a non-cell-autonomous mechanism for hormone receptors and help explain the nonredundant role of PYL8-mediated root ABA signaling., Work in the P.L.R. and F.M. laboratories was supported by the Ministerio de Ciencia e Innovacion, Fondo Europeo de Desarrollo Regional and Consejo Superior de Investigaciones Cientificas Grants BIO2014-52537-R and BIO2017-82503-R (to P.L.R.) and BIO2015-64307-R (to F.M.). J.L.-J. was supported by a Juan de la Cierva contract from Ministerio de Economia y Competitividad (MINECO) and by the Marie Sklodowska-Curie Action H2020-MSCA-IF-2015-707477. B.B.-P. was funded by Programa VALi+d GVA APOSTD/2017/039. J.J. was supported by a FPI contract from MINECO and M.A.F. by a Formacion de Profesorado Universitario contract from MINECO. D.D. and M.J.B. were supported by Biotechnology and Biological Sciences Research Council Grant BB/M002136/1 and Leverhulme Trust Grant RPG-2016-409.

Previous experimental results show that planting spacing has significant effects on root distribution and soil suction (negative pore water pressure) due to inter-plant competition. However, there is a lack of theoretical study on this aspect. This study proposes a new physically based mathematical model to capture planting spacing effects on root growth and soil suction considering three key factors, namely hydrotropism, soil mechanical impedance and inter-plant competition. The model is mainly composed of four parts: (i) extension of root zone front; (ii) increase in root density; (iii) root water uptake and (iv) water flow in soil matrix. Root growth and root water uptake are fully coupled. In order to validate the model, laboratory and field tests were conducted on one tree (Schefflera heptaphylla) and one shrub species (Schefflera arboricola), respectively, with different planting spacings. Even though the investigated tree and shrub species had different values of leaf area index and root length density, consistent conclusions on planting spacing effects can be drawn. When planting spacing became smaller, the size of root system decreased while root density increased, hence causing higher soil suction. The model can capture the root distributions as well as induced soil suction during both evapotranspiration and rainfall events quite well for both tree and shrub species.

Significance Plant roots grow toward water, a phenomenon termed hydrotropism. The nature of the root signal that governs hydrotropism is elusive and remains to be elucidated. Here, we show that, in response to water potential differences across the root tip, a slow, asymmetric long-distance Ca2+ signal is transmitted via the phloem to the elongation zone, where it is asymmetrically distributed across the root to promote root curvature. Furthermore, we demonstrate that the hydrotropism-associated protein MIZ1 plays a role in generating the Ca2+ signal by its direct binding to and inhibition of ECA1, an endoplasmic reticulum Ca2+ pump, which resembles the animal sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs). This study elucidates the mechanism underlying systemic Ca2+ signaling in plant roots, which is essential for water tracking., Ever since Darwin postulated that the tip of the root is sensitive to moisture differences and that it “transmits an influence to the upper adjoining part, which bends towards the source of moisture” [Darwin C, Darwin F (1880) The Power of Movement in Plants, pp 572–574], the signal underlying this tropic response has remained elusive. Using the FRET-based Cameleon Ca2+ sensor in planta, we show that a water potential gradient applied across the root tip generates a slow, long-distance asymmetric cytosolic Ca2+ signal in the phloem, which peaks at the elongation zone, where it is dispersed laterally and asymmetrically to peripheral cells, where cell elongation occurs. In addition, the MIZ1 protein, whose biochemical function is unknown but is required for root curvature toward water, is indispensable for generating the slow, long-distance Ca2+ signal. Furthermore, biochemical and genetic manipulations that elevate cytosolic Ca2+ levels, including mutants of the endoplasmic reticulum (ER) Ca2+-ATPase isoform ECA1, enhance root curvature toward water. Finally, coimmunoprecipitation of plant proteins and functional complementation assays in yeast cells revealed that MIZ1 directly binds to ECA1 and inhibits its activity. We suggest that the inhibition of ECA1 by MIZ1 changes the balance between cytosolic Ca2+ influx and efflux and generates the cytosolic Ca2+ signal required for water tracking.

The mechanism of root hydrotropism has been a mystery for many years, due to the complexity of the interactions between the external environment and plants themselves. To gain an engineering perspective, the time-dependent hydrotropism of a single root has been modeled, initially using a two-dimensional model. Based on the water and nutrient distribution in rhizosphere as computed with the conservation equations, together with a basic reaction-kinetics-type growth model and an intuitive root bending model, it has been found that the root already possesses the property of hydrotropism. For the first time, hydrotropism could be tracked by a process engineering model, which is a new idea based on chemical engineering concept, suggesting an alternative mechanism of hydrotropism. The effects of different initial root widths, lengths, and other growth/transport coefficients on root hydrotropism have then been explored. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1331–1346, 2016

Approximately a century ago, in 1916, the term ‘hydrotropy’ was coined by the scientist Carl A. Neuberg to address anionic organic salts which considerably augmented the aqueous solubility of poorly soluble solutes. Currently hydrotropic solutions possess high industrial demand due to their unique features such as easy availability, good recovery, absence of fire hazards, higher separation factors without any solutes emulsification problem and eco-friendly nature. The present review takes the readers through a concise overview, geometrical features of hydrotropic agents, hypothetical mechanisms and their different advances toward drug delivery. Moreover, this review would provide an insight of the future perspectives concerned with the drug delivery and hydrotropism.

A three-dimensional dynamic model which is combined with root-system development and water extraction is proposed to simulate the interaction between root growth and soil-water flow. Hydrotropism is considered as one controlling factor of root growth in the model. In the paper, the dynamic root model is simulated based on L-systems. The space colonization algorithm is employed to compute hydrotropism caused by soil moist gradient. The soil-water flow, caused by water extraction rather than by evaporation and irrigation, is solved by finite-element method. A root simulation study is presented for water uptake by a maize root system. Effects of hydrotropism on root are demonstrated. The results show that the proposed method can couple root development to surroundings and analyze effects of hydrotropism to water uptake and root length fraction. Based on grid technology, the proposed method can provide a flexible root model which can be easily coupled with soil processes. A novel algorithm (space colonization) is used to calculate root hydrotropism.A model is proposed to couple interaction between root growth and soil-water flow.We compare effects of hydrotropism to water extraction and root length fraction.The model can be used to analyze water extraction process and root architecture.Based on grid, proposed method is flexible to couple with other soil processes.

The widely accepted Cholodny-Went theory posits that asymmetrical auxin redistribution is required for tropic growth of plant organs. We demonstrate, using the sensitive auxin-sensor protein DII-VENUS that, in the presence and absence of auxin polar transport inhibitors, root tropism toward moisture (hydrotropism) does not involve auxin redistribution. Moreover, highly specific auxin signaling antagonists accelerate hydrotropism. Thus, hydrotropism is an exception to the Cholodny-Went theory.

While fast plant movements are spectacular but rare, almost all plants exhibit relatively slow, growth-mediated tropic movements that are key to their survival in the natural world. In this brief review, we discuss recent insights into the molecular mechanisms underlying phototropism, gravitropism, hydrotropism, and autostraightening. Careful molecular genetic and physiological studies have helped confirm the importance of lateral auxin gradients in gravitropic and phototropic responses. However, auxin signaling does not explain all tropisms: recent work has shown that abscisic acid signaling mediates root hydrotropism and has implicated mechanosensing in autostraightening, the organ straightening process recently modeled as a proprioceptive response. The interactions between distinct tropic signaling pathways and other internal and external sensory processes are also now being untangled.

Root hydrotropism is an essential growth response to water potential gradients in plants. To understand the mechanism, fundamental elements such as MIZU-KUSSEI 1 (MIZ1) have been investigated extensively. We investigated the physiological role of a plasma membrane-associated cation-binding protein (PCaP1) and examined the effect of PCaP1 loss-of-function mutations on root hydrotropism. pcap1 knockout mutants showed a defect in root bending as a hydrotropic response, although gravitropism was normal in pcap1 mutants. When pcap1 seedlings were treated with abscisic acid, a negative regulator of gravitropism, the seedlings showed normal gravitropism. The hydrotropism defect in pcap1 mutants was clearly rescued by introducing the genomic sequence of PCaP1 with an endodermis-specific promoter. Analysis of PCaP1-greenfluorescent protein-expressing roots by confocal laser scanning microscopy revealed that PCaP1 was stably associated with the plasma membrane in most cells, but in the cytoplasm of endodermal cells at the bending region. Furthermore, we prepared a transgenic line overexpressing MIZ1 on the pcap1 background and found that the pcap1 hydrotropism defect was rescued. Our results indicate that PCaP1 in the endodermal cells of the root elongation zone is involved in the hydrotropic response. We suggest that PCaP1 contributes to hydrotropism through a MIZ1-independent pathway or as one of the upstream components that transduce water potential signals to MIZ1.

Fresh water is an increasingly scarce resource for agriculture. Plant roots mediate water uptake from the soil and have developed a number of adaptive traits such as hydrotropism to aid water foraging. Hydrotropism modifies root growth to respond to a water potential gradient in soil and grow towards areas with a higher moisture content. Abscisic acid (ABA) and a small number of genes, including those encoding ABA signal transducers, MIZ2/GNOM, and the hydrotropism-specific MIZ1, are known to be necessary for the response in Arabidopsis thaliana, whereas the role of auxin in hydrotropism appears to vary depending on the plant species. This review will describe recent progress characterizing the hormonal regulation of hydrotropism. Recent advances in identifying the sites of hydrotropic perception and response, together with its interaction with gravitropism, will also be discussed. Finally, I will describe putative mechanisms for perception of the water potential gradient and a potential role for hydrotropism in acclimatizing plants to drought conditions.

The direction of auxin transport changes in gravistimulated roots, causing auxin accumulation in the lower side of horizontally reoriented roots. This study found that auxin was similarly involved in hydrotropism and gravitropism in rice and pea roots, but hydrotropism in Lotus japonicus roots was independent of both auxin transport and response. Application of either auxin transport inhibitors or an auxin response inhibitor decreased both hydrotropism and gravitropism in rice roots, and reduced hydrotropism in pea roots. However, Lotus roots treated with these inhibitors showed reduced gravitropism but an unaltered or an enhanced hydrotropic response. Inhibiting auxin biosynthesis substantially reduced both tropisms in rice and Lotus roots. Removing the final 0.2 mm (including the root cap) from the root tip inhibited gravitropism but not hydrotropism in rice seedling roots. These results suggested that modes of auxin involvement in hydrotropism differed between plant species. In rice roots, although auxin transport and responses were required for both gravitropism and hydrotropism, the root cap was involved in the auxin regulation of gravitropism but not hydrotropism. Hydrotropism in Lotus roots, however, may be regulated by a novel mechanism that is independent of both auxin transport and the TIR1/AFBs auxin response pathway.

Roots of higher plants change their growth direction in response to moisture, avoiding drought and gaining maximum advantage for development. This response is termed hydrotropism. There have been few studies of root hydrotropism in grasses, particularly in maize. Our goal was to test whether an enhanced hydrotropic response of maize roots correlates with a better adaptation to drought and partial/lateral irrigation in field studies. We developed a laboratory bioassay for testing hydrotropic response in primary roots of 47 maize elite DTMA (Drought Tolerant Maize for Africa) hybrids. After phenotyping these hybrids in the laboratory, selected lines were tested in the field. Three robust and three weak hybrids were evaluated employing three irrigation procedures: normal irrigation, partial lateral irrigation and drought. Hybrids with a robust hydrotropic response showed growth and developmental patterns, under drought and partial lateral irrigation, that differed from weak hydrotropic responders. A correlation between root crown biomass and grain yield in hybrids with robust hydrotropic response was detected. Hybrids with robust hydrotropic response showed earlier female flowering whereas several root system traits, such as projected root area, median width, maximum width, skeleton width, skeleton nodes, average tip diameter, rooting depth skeleton, thinner aboveground crown roots, as well as stem diameter, were considerably higher than in weak hydrotropic responders in the three irrigation procedures utilized. These results demonstrate the benefit of intensive phenotyping of hydrotropism in primary roots since maize plants that display a robust hydrotropic response grew better under drought and partial lateral irrigation, indicating that a selection for robust hydrotropism might be a promising breeding strategy to improve drought avoidance in maize.

Background and Aims Root hydrotropism is a response to water-potential gradients that makes roots bend towards areas of higher water potential. The gene MIZU-KUSSEI1 (MIZ1) that is essential for hydrotropism in Arabidopsis roots has previously been identified. However, the role of root hydrotropism in plant growth and survival under natural conditions has not yet been proven. This study assessed how hydrotropic response contributes to drought avoidance in nature. Methods An experimental system was established for the study of Arabidopsis hydrotropism in soil. Characteristics of hydrotropism were analysed by comparing the responses of the miz1 mutant, transgenic plants overexpressing MIZ1 (MIZ1 OE) and wild-type plants. Key Results Wild-type plants developed root systems in regions with higher water potential, whereas the roots of miz1 mutant plants did not show a similar response. This pattern of root distribution induced by hydrotropism was more pronounced in MIZ1 OE plants than in wild-type plants. In addition, shoot biomass and the number of plants that survived under drought conditions were much greater in MIZ1 OE plants. Conclusions These results show that hydrotropism plays an important role in root system development in soil and contributes to drought avoidance, which results in a greater yield and plant survival under water-limited conditions. The results also show that MIZ1 overexpression can be used for improving plant productivity in arid areas.

Plants continuously sense a wide variety of perturbations and produce various responses known as tropisms in plants. It is essential for all plants to have survival sensory mechanisms and actuators responsible for a specific plant response process. Plants are ideal adaptive structures with smart sensing capabilities based on different types of tropisms, such as chemiotropism, geotropism, heliotropism, hydrotropism, magnetotropism, phototropism, thermotropism, electrotropism, thigmotropism, and host tropism. Plants can sense mechanical, electrical and electromagnetic stimuli, gravity, temperature, direction of light, insect attack, chemicals and pollutants, pathogens, water balance, etc. Here we show how plants sense different environmental stresses and stimuli and how phytoactuators response to them. Plants generate various types of intracellular and intercellular electrical signals in response to these environmental changes. This field has both theoretical and practical significance because these phytosensors and phytoactuators employ new principles of stimuli reception and signal transduction and play a very important role in the life of plants.

The default growth pattern of primary roots of land plants is directed by gravity. However, roots possess the ability to sense and respond directionally to other chemical and physical stimuli, separately and in combination. Therefore, these root tropic responses must be antagonistic to gravitropism. The role of reactive oxygen species (ROS) in gravitropism of maize and Arabidopsis (Arabidopsis thaliana) roots has been previously described. However, which cellular signals underlie the integration of the different environmental stimuli, which lead to an appropriate root tropic response, is currently unknown. In gravity-responding roots, we observed, by applying the ROS-sensitive fluorescent dye dihydrorhodamine-123 and confocal microscopy, a transient asymmetric ROS distribution, higher at the concave side of the root. The asymmetry, detected at the distal elongation zone, was built in the first 2 h of the gravitropic response and dissipated after another 2 h. In contrast, hydrotropically responding roots show no transient asymmetric distribution of ROS Decreasing ROS levels by applying the antioxidant ascorbate, or the ROS-generation inhibitor diphenylene iodonium attenuated gravitropism while enhancing hydrotropism. Arabidopsis mutants deficient in Ascorbate Peroxidase 1 showed attenuated hydrotropic root bending. Mutants of the root-expressed NADPH oxidase RBOH C, but not rbohD, showed enhanced hydrotropism and less ROS in their roots apices (tested in tissue extracts with Amplex Red). Finally, hydrostimulation prior to gravistimulation attenuated the gravistimulated asymmetric ROS and auxin signals that are required for gravity-directed curvature. We suggest that ROS, presumably H2O2, function in tuning root tropic responses by promoting gravitropism and negatively regulating hydrotropism.

Because of their sessile nature, plants evolved several mechanisms to tolerate or avoid conditions where water is scarce. The molecular mechanisms contributing to drought tolerance have been studied extensively, whereas the molecular mechanism underlying drought avoidance is less understood despite its importance. Several lines of evidence showed that the roots sense the moisture gradient and grow toward the wet area: so-called hydrotropism. We previously identified MIZU-KUSSEI (MIZ) 1 and MIZ2/GNOM as genes responsible for this process. To gain new insight into the molecular mechanism of root hydrotropism, we generated overexpressors of MIZ1 (MIZ1OEs) and analyzed their hydrotropic response. MIZ1OEs had a remarkable enhancement of root hydrotropism. Furthermore, a greater number of MIZ1OE root cells remained viable under hydrostimulated conditions than those of the wild type, which might contribute to retaining root growth under hydrostimulated conditions. Although overexpression of MIZ1 also caused a slight decrease in the root gravitropic response, it was not attributable to the enhanced hydrotropic response. In addition, miz2 mutation or the auxin response inhibitor nullified the enhanced hydrotropic response in MIZ1OEs. Furthermore, the expression of MIZ1 did not alter the expression of typical genes involved in drought tolerance. These results suggest that MIZ1 positively regulates hydrotropism at an early stage and its overexpression results in an enhancement of signal transduction unique to root hydrotropism to increase the degree of hydrotropic root bending.

Plant roots undergo tropic growth in response to environmental cues, and each tropic response is affected by several environmental stimuli. Even its importance, molecular regulation of hydrotropism has not been largely uncovered. Tropic responses including hydrotropism were impacted by other environmental signal. We found that hydrotropism was reduced in dark-grown seedling. Moreover, we found that the expression of MIZ1, an essential gene for hydrotropism, was regulated by light signal. From our genetic analysis, phytochrome A (phyA)-, phyB- and HY5-mediated blue-light signalling play curial roles in light-mediated induction of MIZ1 and hydrotropism. In addition, we found that abscisic acid (ABA) also induced MIZ1 expression. ABA treatment could recover weak hydrotropism and MIZ1 expression level of hy5, and ABA synthesis inhibitor, abamineSG, further reduced hydrotropic curvature of hy5. In contrast, ABA treatment did not affect ahydrotropic phenotype of miz1. These results suggest that ABA signalling regulates MIZ1 expression independently from light signalling. Our results demonstrate that environmental signals, such as light and stresses mediated by ABA signalling, are integrated into MIZ1 expression and thus regulate hydrotropism. These machineries will allow plants to acquire sufficient amounts of water.

Root hydrotropism is the directional growth of roots toward a source of moisture in response to a water potential gradient. Root hydrotropism is therefore one of the most important factors for efficient water acquisition under water-limited conditions. However, the mechanisms underlying hydrotropism are not well understood. A previous study of the hydrotropism of primary roots in Arabidopsis thaliana identified the MIZU-KUSSEI (MIZ) 1 and MIZ2 genes as essential for this process. However, despite the important role of lateral roots in water acquisition under water deficient conditions, the mechanism of hydrotropism in lateral roots has not been investigated to date. The present study established an experimental system for the induction of lateral root hydrotropism in Arabidopsis and analyzed its characteristics in comparison with those of primary roots. Lateral roots were found to exhibit hydrotropism irrespective of their length. Expression of a pMIZ1::GUS fusion gene at the tip of lateral roots and assessment of lateral root hydrotropism in the ahydrotropic mutant miz1, which did not display hydrotropism, demonstrated that the lateral roots of Arabidopsis have the capacity to respond to water potential gradients, and that this activity is regulated by MIZ1.

Because plants cannot change their environmental circumstances by changing their location, they must instead adapt to a wide variety of environmental conditions, especially soil conditions. One of the most effective ways for a plant to adapt to a given soil condition is by modifying its root system architecture. We aim to identify the genetic factors controlling root growth angle, a trait that affects root system architecture. The present study consisted of a genetic analysis of the seminal root growth angle in wheat; the parental varieties of the doubled haploid lines (DHLs) used in this study exhibited significantly different root growth directions. Using the ‘basket’ method, the ratio of deep roots (DRR; the proportion of total roots with GA > 45 degrees) was observed for evaluating deep rooting. We were able to identify novel quantitative trait loci (QTLs) controlling the gravitropic and hydrotropic responses of wheat roots. Moreover, we detected one QTL for seminal root number per seedling (RN) on chromosome 5A and two QTLs for seminal root elongation rate (ER) on chromosomes 5D and 7D. Gravitropic and hydrotropic responses of wheat roots, which play a significant role in establishing root system architecture, are controlled by independent genetic factors.

Plant organ development is important for adaptation to a changing environment. Genetic and physiological studies have revealed that plant hormones play key roles in lateral root formation. In this study, we show that MIZU-KUSSEI1 (MIZ1), which was identified originally as a regulator of hydrotropism, functions as a novel regulator of hormonally mediated lateral root development. Overexpression of MIZ1 (MIZ1OE) in roots resulted in a reduced number of lateral roots being formed; however, this defect could be recovered with the application of auxin. Indole-3-acetic acid quantification analyses showed that free indole-3-acetic acid levels decreased in MIZ1OE roots, which indicates that alteration of auxin level is critical for the inhibition of lateral root formation in MIZ1OE plants. In addition, MIZ1 negatively regulates cytokinin sensitivity on root development. Application of cytokinin strongly induced the localization of MIZ1-green fluorescent protein to lateral root primordia, which suggests that the inhibition of lateral root development by MIZ1 occurs downstream of cytokinin signaling. Surprisingly, miz2, a weak allele of gnom, suppressed developmental defects in MIZ1OE plants. Taken together, these results suggest that MIZ1 plays a role in lateral root development by maintaining auxin levels and that its function requires GNOM activity. These data provide a molecular framework for auxin-dependent organ development in Arabidopsis (Arabidopsis thaliana).

This paper describes the principles and theoretical investigations, supported by experimental measurements, aimed at designing and developing a novel mechatronic system for soil exploration, inspired by the apical part of the plant roots, named apex. Each single plant root has to move through the substrate, orienting itself along the gravity vector and locating water and nutrients. In the same way, the mechatronic apex can steer in all directions and it embeds a gravity sensor, a soil moisture gradient detector, as well as the electronics for sensory data acquisition and steering control. A bio-inspired algorithm reproducing the gravitropism and hydrotropism behaviors, typical of plants, was developed and tested on a purposive prototype of the mechatronic apex system, actuated by hydraulic pumps. Moreover, the design and testing of a novel bio-inspired osmotic actuator module, composed of three cells separated by couples of osmotic and ion-selective membranes, is also presented. Preliminary prototypes developed in acrylic material for testing the gravitropism and hydrotropism behaviors are shown.

Because of their sessile nature, plants require appropriate strategies to adapt to the surrounding environment. Tropism is a directional growth in response to environmental stimulus that allows plants to adapt to changes in sunlight, nutrients and water. Plant roots display hydrotropism in response to a moisture gradient, and this phenomenon is believed to play an important role in the ability of the plant to obtain water. However, the molecular mechanism underlying hydrotropism has yet to be fully elucidated. To investigate the transcriptional changes associated with root hydrotropism, we performed a whole-genome microarray analysis of Arabidopsis to monitor the transcription levels of 22,810 genes during the early phase of the hydrotropic response. The transcript levels of 793 genes were significantly changed 1 or 2 hours (h) after hydrotropic stimulation. A large number of genes responsive to abscisic acid (ABA) or water-stress were among the hydrostimulation-responsive genes. In contrast, there appeared to be little overlap in transcript abundance between hydrostimulation-responsive and gravistimulation-responsive genes. Our results suggest that ABA and water-stress responses are important signal transduction mechanisms involved in the root hydrotropic response, and that the signaling pathways involved in hydrotropism differ from those of gravitropism.

Two essential functions are associated with the root tip: first of all, it ensures a sustained growth of the root system thanks to its role in protecting the stem cell zone responsible for cell division and differentiation. In addition, it is capable of detecting environmental changes at the root cap level, and this property provides a crucial advantage considering that this tissue is located at the forefront of soil exploration. Using results obtained mainly with the plant model Arabidopsis, we summarize the description of the structure of root cap and the known molecular mechanisms regulating its functioning. We briefly review the various responses of the root cap related to the interaction between the plant and its environment, such as phototropism, gravitropism, hydrotropism, mineral composition of the soil and protection against pathogens.

Plants have evolved a variety of mechanisms for responding to environmental cues, which allows them to survive in the presence of limited resources or environmental stresses. One of the most significant growth adaptations plants have attained is tropism, a growth response that involves bending of plant organs toward or away from a stimulus. Roots exhibit hydrotropism in response to moisture gradients, which is thought to be critical in acquiring water and establishing their stand in the soil. However, the mechanism underlying hydrotropism remains unsolved. Here, we report that the no hydrotropic response (nhr1) mutant of Arabidopsis, which is impaired in hydrotropism, is tolerant to drought. The no hydrotropic response phenotype of nhr1 was repressed by AbamineSG, an inhibitor of abscisic acid (ABA) biosynthesis, indicating that ABA negatively regulates hydrotropism. Furthermore, the content of ABA was higher in nhr1 compared to those of wild type (wt). However, the higher ABA levels in nhr1 plants were not due to higher transcript levels of 9-cis-epoxycarotenoid dioxygenase (NCED3), since these were diminished compared to those of wt. Our results indicated that the root hydrotropic response of the nhr1 mutant is modulated by ABA and that the higher ABA levels of the mutant might confer it drought resistance.