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The interaction of plant roots with bacteria is influenced by chemical signaling, where auxins play a critical role. Auxins exert positive or negative influences on the plant traits responsible of root architecture configuration such as root elongation and branching and root hair formation, but how bacteria that modify the plant auxin response promote or repress growth, as well as root structure, remains unknown. Here, we isolated and identified via molecular and electronic microscopy analysis a Micrococcus luteus LS570 strain as a plant growth promoter that halts primary root elongation in Arabidopsis seedlings and strongly triggers root branching and absorptive potential. The root biomass was exacerbated following root contact with bacterial streaks, and this correlated with inducible expression of auxin-related gene markers DR5:GUS and DR5:GFP. Cellular and structural analyses of root growth zones indicated that the bacterium inhibits both cell division and elongation within primary root tips, disrupting apical dominance, and as a consequence differentiation programs at the pericycle and epidermis, respectively, triggers the formation of longer and denser lateral roots and root hairs. Using Arabidopsis mutants defective on auxin signaling elements, our study uncovers a critical role of the auxin response factors ARF7 and ARF19, and canonical auxin receptors in mediating both the primary root and lateral root response to M. luteus LS570. Our report provides very basic information into how actinobacteria interact with plants and direct evidence that the bacterial genus Micrococcus influences the cellular and physiological plant programs ultimately responsible of biomass partitioning.

Boron (B) has been considered either as a nutrient or as a toxic non-essential element for plants and considerable debate arose recently regarding its functions and mechanisms of action. To gain further detail on the roles of boron in growth, development, and cell viability, Arabidopsis wild-type and mediator18 mutants, these later showing genetically fixed cell death in the pro-vasculature of roots, were germinated and grown side by side in agar-solidified plates with a standard nutrient solution supplemented with increasing concentrations (0.25–8 mM) of boric acid (BA). In the WT and med18-1 mutants, BA exerted a dose-dependent inhibition of leaf formation and primary root growth, but did not significantly promote root branching in the WT or med18-1 mutants, these later manifesting an enhanced lateral root formation capacity under a wide range of BA concentrations. Although the overall effect of BA in roots was growth repressing, no signs of cell death in meristems of primary roots of WT seedlings could be appreciated, instead, it appears to diminish the cell death manifested in pro-vasculature of med18 mutants, which correlates with reduced expression of the ERF115 transcription factor, which is induced in inner tissues upon damage, and recovery of auxin-inducible gene expression within the root tip. Irrespective of the overall growth-repressing effects in the shoot and root systems, it seems clear that important protective functions are orchestrated by BA, which supports cell viability in a mutant with spontaneous tissue damage.

Plants host a diverse microbiome and differentially react to the fungal species living as endophytes or around their roots through emission of volatiles. Here, using divided Petri plates for Arabidopsis-T. atroviride co-cultivation, we show that fungal volatiles increase endogenous sugar levels in shoots, roots and root exudates, which improve Arabidopsis root growth and branching and strengthen the symbiosis. Tissue-specific expression of three sucrose phosphate synthase-encoding genes (AtSPS1F, AtSPS2F and AtSPS3F), and AtSUC2 and SWEET transporters revealed that the gene expression signatures differ from those of the fungal pathogens Fusarium oxysporum and Alternaria alternata and that AtSUC2 is largely repressed either by increasing carbon availability or by perception of the fungal volatile 6-pentyl-2H-pyran-2-one. Our data point to Trichoderma volatiles as chemical signatures for sugar biosynthesis and exudation and unveil specific modulation of a critical, long-distance sucrose transporter in the plant.

N-acyl-L-homoserine lactones (AHLs) are involved in cell-to-cell communication in Gram-negative bacteria through a process termed quorum-sensing (QS). In this report, we evaluated the response of Arabidopsis thaliana primary roots to abscisic acid (ABA) in wild-type (WT) and decanamide resistant root 1 (drr1) mutant, previously reported to be resistant to N-decanoyl-L-homoserine lactone (C10-HL). When compared to WT seedlings, drr1 mutants were hypersensitive to ABA and had primary roots shorter, which correlated with lower cell division in meristems, a higher concentration of endogenous ABA, and a greater expression of ABI5 gene; and this shortened primary root phenotype was reversed in drr1abi5 double mutants. An analysis of expression of ABSCISIC ACID INSENSITIVE 4 (ABI4) showed an ABA-inducible pattern in primary root tips, which was further increased in drr1 mutant seedlings. Comparison of seed germination in WT, drr1, abi5, and drr1abi5 lines showed higher germination percentages in the following order under control condition: abi5 > drr1abi5 > WT > drr1, while abi5 and drr1abi5 germinated faster and drr1 germinated slower with respect to WT under ABA condition. Taken together, our results suggest that DRR1 is a negative regulator of ABA signaling probably acting upstream of the transcription factors ABI4 and ABI5, which influence ABA responsiveness in primary roots and seed germination.

Members of the fungal genus Trichoderma stimulate growth and reinforce plant immunity. Nevertheless, how fungal signaling elements mediate the establishment of a successful Trichoderma-plant interaction is largely unknown. In this work, we analyzed growth, root architecture and defense in an Arabidopsis-Trichoderma co-cultivation system, including the wild-type (WT) strain of the fungus and mutants affected in NADPH oxidase. Global gene expression profiles were assessed in both the plant and the fungus during the establishment of the interaction. Trichoderma atroviride WT improved root branching and growth of seedling as previously reported. This effect diminished in co-cultivation with the ∆nox1, ∆nox2 and ∆noxR null mutants. The data gathered of the Arabidopsis interaction with the ∆noxR strain showed that the seedlings had a heightened immune response linked to jasmonic acid in roots and shoots. In the fungus, we observed repression of genes involved in complex carbohydrate degradation in the presence of the plant before contact. However, in the absence of NoxR, such repression was lost, apparently due to a poor ability to adequately utilize simple carbon sources such as sucrose, a typical plant exudate. Our results unveiled the critical role played by the Trichoderma NoxR in the establishment of a fine-tuned communication between the plant and the fungus even before physical contact. In this dialog, the fungus appears to respond to the plant by adjusting its metabolism, while in the plant, fungal perception determines a delicate growth-defense balance.

Roots provide physical and nutritional support to plant organs that are above ground and play critical roles for adaptation via intricate movements and growth patterns. Through screening the effects of bacterial isolates from roots of halophyte Mesquite (Prosopis sp.) on Arabidopsis thaliana, we identified Achromobacter sp. 5B1 as a probiotic bacterium that influences plant functional traits. Detailed genetic and architectural analyses in Arabidopsis grown in vitro and in soil, cell division measurements, auxin transport and response gene expression and brefeldin A treatments demonstrated that root colonization with Achromobacter sp. 5B1 changes the growth and branching patterns of roots, which were related to auxin perception and redistribution. Expression analysis of auxin transport and signaling revealed a redistribution of auxin within the primary root tip of wild-type seedlings by Achromobacter sp. 5B1 that is disrupted by brefeldin A and correlates with repression of auxin transporters PIN1 and PIN7 in root provasculature, and PIN2 in the epidermis and cortex of the root tip, whereas expression of PIN3 was enhanced in the columella. In seedlings harboring AUX1, EIR1, AXR1, ARF7ARF19, TIR1AFB2AFB3 single, double or triple loss-of-function mutations, or in a dominant (gain-of-function) mutant of SLR1, the bacterium caused primary roots to form supercoils that are devoid of lateral roots. The changes in growth and root architecture elicited by the bacterium helped Arabidopsis seedlings to resist salt stress better. Thus, Achromobacter sp. 5B1 fine tunes both root movements and the auxin response, which may be important for plant growth and environmental adaptation.

Plants adapt to soil injury and biotic stress via cell regeneration. In Arabidopsis, root tip damage by genotoxic agents, antibiotics, UV light and cutting induces a program that recovers the missing tissues through activation of stem cells and involves ethylene response factor 115 (ERF115), which triggers cell replenishment. Here, we show that mutation of the gene encoding an MED18 subunit of the transcriptional MEDIATOR complex and chromate [Cr(VI)], an environmental pollutant, synergistically trigger a developmental program that enables the splitting of the meristem in vivo to produce twin roots. Expression of the quiescent centre gene marker WOX5, auxin-inducible DR5:GFP reporter and the ERF115 factor traced the changes in cell identity during the conversion of single primary root meristems into twin roots and were induced in an MED18 and chromate-dependent manner during the root twinning events, which also required auxin redistribution and signalling mediated by IAA14/SOLITARY ROOT (SLR1). Splitting of the root meristem allowed dichotomous root branching in Arabidopsis, a poorly understood process in which stem cells may act to enable whole organ regeneration.

The interaction of plants with bacteria and the long-term success of their adaptation to challenging environments depend upon critical traits that include nutrient solubilization, remodeling of root architecture, and modulation of host hormonal status. To examine whether bacterial promotion of phosphate solubilization, root branching and the host auxin response may account for plant growth, we isolated and characterized ten bacterial strains based on their high capability to solubilize calcium phosphate. All strains could be grouped into six Pseudomonas species, namely P. brassicae, P. baetica, P. laurylsulfatiphila, P. chlororaphis, P. lurida, and P. extremorientalis via 16S rRNA molecular analyses. A Solibacillus isronensis strain was also identified, which remained neutral when interacting with Arabidopsis roots, and thus could be used as inoculation control. The interaction of Arabidopsis seedlings with bacterial streaks from pure cultures in vitro indicated that their phytostimulation properties largely differ, since P. brassicae and P. laurylsulfatiphila strongly increased shoot and root biomass, whereas the other species did not. Most bacterial isolates, except P. chlororaphis promoted lateral root formation, and P. lurida and P. chlororaphis strongly enhanced expression of the auxin-inducible gene construct DR5:GUS in roots, but the most bioactive probiotic bacterium P. brassicae could not enhance the auxin response. Inoculation with P. brassicae and P. lurida improved shoot and root growth in medium supplemented with calcium phosphate as the sole Pi source. Collectively, our data indicate the differential responses of Arabidopsis seedlings to inoculation with several Pseudomonas species and highlight the potential of P. brassicae to manage phosphate nutrition and plant growth in a more eco-friendly manner.

Plant growth-promoting fungi are integral components of the root microbiome that help the host resist biotic and abiotic stress while improving nutrient acquisition. Trichoderma atroviride is a common inhabitant of the rhizosphere, which establishes a perdurable symbiosis with plants through the emission of volatiles, diffusible compounds, and robust colonization. Currently, little is known on how the environment influences the Trichoderma-plant interaction. In this report, we assessed plant growth and root architectural reconfiguration of Arabidopsis seedlings grown in physical contact with T. atroviride under contrasting nitrate and ammonium availability. The shoot and root biomass accumulation and lateral root formation triggered by the fungus required high nitrogen supplements and involved nitrate reduction via AtNIA1 and NIA2. Ammonium supplementation did not restore biomass production boosted by T. atroviride in nia1nia2 double mutant, but instead fungal inoculation increased nitric oxide accumulation in Arabidopsis primary root tips depending upon nitrate supplements. N deprived seedlings were largely resistant to the effects of nitric oxide donor SNP triggering lateral root formation. T. atroviride enhanced expression of CHL1:GUS in root tips, particularly under high N supplements and required an intact CHL1 nitrate transporter to promote lateral root formation in Arabidopsis seedlings. These data imply that the developmental programs strengthened by Trichoderma and the underlying growth promotion in plants are dependent upon adequate nitrate nutrition and may involve nitric oxide as a second messenger.

Research over the last three decades showed that chromium, particularly the oxyanion chromate Cr(VI) behaves as a toxic environmental pollutant that strongly damages plants due to oxidative stress, disruption of nutrient uptake, photosynthesis and metabolism, and ultimately, represses growth and development. However, mild Cr(VI) concentrations promote growth, induce adventitious root formation, reinforce the root cap, and produce twin roots from single root meristems under conditions that compromise cell viability, indicating its important role as a driver for root organogenesis. In recent years, considerable advance has been made towards deciphering the molecular mechanisms for root sensing of chromate, including the identification of regulatory proteins such as SOLITARY ROOT and MEDIATOR 18 that orchestrate the multilevel dynamics of the oxyanion. Cr(VI) decreases the expression of several glutamate receptors, whereas amino acids such as glutamate, cysteine and proline confer protection to plants from hexavalent chromium stress. The crosstalk between plant hormones, including auxin, ethylene, and jasmonic acid enables tissues to balance growth and defense under Cr(VI)-induced oxidative damage, which may be useful to better adapt crops to biotic and abiotic challenges. The highly contrasting responses of plants manifested at the transcriptional and translational levels depend on the concentration of chromate in the media, and fit well with the concept of hormesis, an adaptive mechanism that primes plants for resistance to environmental challenges, toxins or pollutants. Here, we review the contrasting facets of Cr(VI) in plants including the cellular, hormonal and molecular aspects that mechanistically separate its toxic effects from biostimulant outputs.

The RNA polymerase II drives the biogenesis of coding and non-coding RNAs for gene expression. Here, we describe new roles for its second-largest subunit, NRPB2, on root organogenesis and regeneration. Down-regulation of NRPB2 activates a determinate developmental program, which correlated with a reduction in mitotic activity, cell elongation, and size of the root apical meristem. Noteworthy, nrpb2-3 mutants manifest cell death in pro-vascular cells within primary root tips of plants grown in darkness or exposed to light, which triggers the expression of the regeneration gene marker ERF115 in neighbor cells close to damage. Auxin and stem cell niche (SCN) gene expression as well as structural analysis revealed that NRPB2 maintains SCN activity through distribution of PIN transporters in root tissues. Wild-type seedlings regenerated the root tip after excision of the QC and SCN, but nrpb2-3 mutants did not rebuild the missing tissues, and this process could be genotypified using pERF115:GFP, DR5:GFP, and pWOX5:GFP reporter constructs. The levels of reactive oxygen species increased in the mutants four days after germination and strongly decreased at later times, whereas nitric oxide accumulated as the root tip differentiates. These results show the importance of the transcriptional machinery for root organogenesis, cell viability, and regenerative capacity for reconstruction of tissues and organs upon injury.

The plant growth promoting rhizobacterium Azospirillum brasilense Sp245 enhances biomass production in cereals and horticultural species and is an interesting model to study the physiology of the phytostimulation program. Although auxin production by Azospirillum appears to be critical for root architectural readjustments, the role of cytokinins in the growth promoting effects of Azospirillum remains unclear. Here, Arabidopsis thaliana seedlings were co-cultivated in vitro with A. brasilense Sp245 to assess whether direct contact of roots with bacterial colonies or exposure to the bacterial volatiles using divided Petri plates would affect biomass production and root organogenesis. Both interaction types increased root and shoot fresh weight but had contrasting effects on primary root length, lateral root formation and root hair development. Cell proliferation in root meristems analyzed with the CYCB1;1::GUS reporter decreased over time with direct contact, but was augmented by plant exposure to volatiles. Noteworthy, the expression of the cytokinin-inducible reporters TCS::GFP and ARR5::GUS increased in root tips in response to bacterial contact, without being affected by the volatiles. In A. thaliana having single (cre1-12, ahk2-2, ahk3-3), double (cre1-12/ahk2-2, cre1-12/ahk3-3, ahk2-2/ahk3-3) or triple (cre1-12/ahk2-2/ahk3-3) mutations in canonical cytokinin receptors, only the triple mutant had a marked effect on plant growth in response to A. brasilense. These results show that different mechanisms are elicited by A. brasilense, which influence the cytokinin-signaling pathway.

KEY MESSAGE The role of the root cap in the plant response to phosphate deprivation has been scarcely investigated. Here we describe early structural, physiological and molecular changes prior to the determinate growth program of the primary roots under low Pi and unveil a critical function of the transcription factor SOMBRERO in low Pi sensing. Mineral nutrient distribution in the soil is uneven and roots efficiently adapt to improve uptake and assimilation of sparingly available resources. Phosphate (Pi) accumulates in the upper layers and thus short and branched root systems proliferate to better exploit organic and inorganic Pi patches. Here we report an early adaptive response of the Arabidopsis primary root that precedes the entrance of the meristem into the determinate developmental program that is a hallmark of the low Pi sensing mechanism. In wild-type seedlings transferred to low Pi medium, the quiescent center domain in primary root tips increases as an early response, as revealed by WOX5:GFP expression and this correlates with a thicker root tip with extra root cap cell layers. The halted primary root growth in WT seedlings could be reversed upon transfer to medium supplemented with 250 µM Pi. Mutant and gene expression analysis indicates that auxin signaling negatively affects the cellular re-specification at the root tip and enabled identification of the transcription factor SOMBRERO as a critical element that orchestrates both the formation of extra root cap layers and primary root growth under Pi scarcity. Moreover, we provide evidence that low Pi-induced root thickening or the loss-of-function of SOMBRERO is associated with expression of phosphate transporters at the root tip. Our data uncover a developmental window where the root tip senses deprivation of a critical macronutrient to improve adaptation and surveillance.

The transit from indeterminate to determinate root developmental program compromises growth and causes the differentiation of the meristem, but a direct link between this process with auxin signaling and/or viability of initial cells remains untested. Here, through the isolation and characterization of the halted primary root1 (hpr1) mutant of Arabidopsis, which develops primary and lateral roots with genetically stable determinate growth after germination, we show that the differentiation of the root meristem correlates with enhanced auxin responsiveness and is preceded by the death of provasculature initial cells in both primary and lateral roots. Supplementation of indole-3-acetic acid causes both a dose-dependent repression of primary root growth and an induction of DR5:uidA expression in wild-type seedlings, and these effects were exacerbated in hpr1 mutants. The damage of provasculature initial cells in the root of hpr1 mutants occurred at earlier times than the full differentiation of the meristem, and correlates with a reduced expression domain of CycB1:uidA and PRZ:uidA. Thus, HPR1 plays critical functions for stem cell maintenance, auxin homeostasis, cell division in the meristem, and indeterminate root growth.

A MAPK module, of which MPK6 kinase is an important component, is involved in the coordination of the responses to Pi and Fe in the primary root meristem of Arabidopsis thaliana. Phosphate (Pi) deficiency induces determinate primary root growth in Arabidopsis through cessation of cell division in the meristem, which is linked to an increased iron (Fe) accumulation. Here, we show that Mitogen-Activated Protein Kinase6 (MPK6) has a role in Arabidopsis primary root growth under low Pi stress. MPK6 activity is induced in roots in response to low Pi, and such induction is enhanced by Fe supplementation, suggesting an MPK6 role in coordinating Pi/Fe balance in mediating root growth. The differentiation of the root meristem induced by low Pi levels correlates with altered expression of auxin-inducible genes and auxin transporter levels via MPK6. Our results indicate a critical role of the MPK6 kinase in coordinating meristem cell activity to Pi and Fe availability for proper primary root growth.

Plant growth-promoting rhizobacteria modulate root development through different mechanisms. This work was conducted to evaluate the effects of root colonization by Pseudomonas putida and Pseudomonas fluorescens in biomass production, lateral root formation, and activation of auxin signaling in Arabidopsis thaliana. Selected strains of P. putida and P. fluorescens were tested for modification of DR5::uidA, BA3::uidA and HS::AXR3NT-GUS auxin-related gene expression, and to promote root hair and lateral root formation in WT and tir1-1, tir1-1afb2-1afb3-1, arf7-1, arf19-1, arf7-1arf19-1, and rhd6 mutants. Production of cyclodipeptides with possible roles in auxin signaling was also determined in P. putida and P. fluorescens culture supernatants by gas chromatography–mass spectrometry. P. putida and P. fluorescens stimulated lateral root and root hair formation and increased plant biomass, which correlated with an induction of the auxin response. Genetic analyses suggested that growth promotion involves auxin signaling as tir1-1, tir1-1afb2-1afb3-1, arf7-1, arf19-1, and arf7-1arf19-1 mutants showed decreased lateral root response to inoculation and because P. putida and P. fluorescens restored root hair development in the rhd6 mutant. It was also found that these bacteria produce the cyclodipeptides cyclo(L-Pro-L-Val), cyclo(L-Pro-L-Phe), and cyclo(L-Pro-L-Tyr), which modulates auxin-responsive gene expression in roots. Our results suggest a role of cyclodipeptides for bacterial phytostimulation.

The influence of Phomopsis viticola, Diaporthe phaseolorum and Pseudofusicoccum stromaticum isolated from avocado stem-end rot on growth of Arabidopsis seedlings was investigated. We found ...

The plant hormone ethylene induces auxin biosynthesis and transport and modulates root growth and branching. However, its function on root stem cells and the identity of interacting factors for the control of meristem activity remains unclear. Genetic analysis for primary root growth in wild-type (WT) Arabidopsis thaliana seedlings and ethylene-related mutants showed that the loss-of-function of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) inhibits cell division and elongation. This phenotype is associated with an increase in the expression of the auxin transporter PIN2 and a drastic decrease in the expression of key factors for stem cell niche maintenance such as PLETHORA1, SHORT ROOT and SCARECROW. While the root stem cell niche is affected in ctr1 mutants, its maintenance is severely compromised in the ctr1-1eir1-1(pin2) double mutant, in which an evident loss of proliferative capacity of the meristematic cells leads to a fully differentiated root meristem shortly after germination. Root traits affected in ctr1-1 mutants could be restored in ctr1-1ein2-1 double mutants. These results reveal that ethylene perception via CTR1 and EIN2 in the root modulates the proliferative capacity of root stem cells via affecting the expression of genes involved in the two major pathways, AUX-PIN-PLT and SCR-SHR, which are key factors for proper root stem cell niche maintenance.

Plants being sessile organisms are exposed to various biotic and abiotic factors, thus causing stress. The Pseudomonas aeruginosa bacterium is an opportunistic pathogen for animals, insects, and plants. Direct exposure of Arabidopsis thaliana to the P. aeruginosa PAO1 strain induces plant death by producing a wide variety of virulence factors, which are regulated mainly by quorum sensing systems. Besides virulence factors, P. aeruginosa PAO1 also produces cyclodipeptides (CDPs), which possess auxin-like activity and promote plant growth through activation of the target of the rapamycin (AtTOR) pathway. On the other hand, plant defense mechanisms are regulated through the production of phytohormones, such as salicylic acid (SA) and jasmonic acid (JA), which are induced in response to pathogen-associated molecular patterns (PAMPs), activating defense genes associated with SA and JA such as PATHOGENESIS-RELATED-1 (PR-1) and LIPOXYGENASE2 (LOX2), respectively. PR proteins are suggested to play critical roles in coordinating the Systemic Acquired Resistance (SAR). In contrast, LOX proteins (LOX2, LOX3, and LOX4) have been associated with the production of JA by producing its precursors, oxylipins. The activation of defense mechanisms involves signaling cascades such as Mitogen-Activated Protein Kinases (MAPKs) or the TOR pathway as a switch for re-directing energy towards defense or growth. In this work, we challenged A. thaliana (wild type, mpk6 or mpk3 mutants, and overexpressing TOR) seedlings with P. aeruginosa PAO1 strains to identify the role of bacterial CDPs in the plant immune response. Results showed that the pre-exposure of these Arabidopsis seedlings to CDPs significantly reduced plant infection of the pathogenic P. aeruginosa PAO1 strains, indicating that plants that over-express AtTOR or lack MPK3/MPK6 protein-kinases are more susceptible to the pathogenic effects. In addition, CDPs induced the GUS activity only in the LOX2::GUS plants, indicative of JA-signaling activation. Our findings indicate that the CDPs are molecules that trigger SA-independent and JA-dependent defense responses in A. thaliana; hence, bacterial CDPs may be considered elicitors of the Arabidopsis immune response to pathogens.

N,N-dimethyl-hexadecylamine (DMHDA) is a volatile organic compound (VOC) produced by some plant growth-promoting rhizobacteria (PGPR), which inhibits the growth of pathogenic fungi and induces iron uptake by roots. In this report, through the application of a wide range of concentrations, we found that DMHDA affects Arabidopsis primary root growth and lateral root formation in a dose-dependent manner where 1 and 2 µM promoted root growth and higher (4–32 µM) concentrations repressed growth. Cytokinin-inducible TCS::GFP and ARR5::uidA gene constructs showed an increased expression in columella cells and root meristem, respectively, at 2 µM DMHDA, but their expression domains strongly diminished at growth repressing treatments. To test if either primary root growth promotion or repression could involve members of the cytokinin receptor family, the growth of WT and double mutant combinations cre1-12 ahk2-2, cre1-12 ahk3-3, and ahk2-2 ahk3-3 was tested in control conditions or supplemented with 2 µM or 16 µM DMHDA. Noteworthy, the root growth promotion disappeared in cre1-12 ahk2-2 and ahk2-2 ahk3-3 combinations, whereas all double mutants had higher repression than the WT at high doses. We further show that DMHDA fails to mimic the effects of ethylene in Arabidopsis seedlings grown in darkness that include an exaggerated apical hook, stem and root shortening, and root hair elongation. Our data help unravel how Arabidopsis senses a growth-modulating bacterial volatile through changes in cytokinin responsiveness.

Phosphate (Pi ) is a critical macronutrient for the biochemical and molecular functions of cells. Under phosphate limitation, plants manifest adaptative strategies to increase phosphate scavenging. However, how low phosphate sensing links to the transcriptional machinery remains unknown. The role of the MEDIATOR (MED) transcriptional co-activator, through its MED16 subunit in Arabidopsis root system architecture remodeling in response to phosphate limitation was assessed. Its critical function acting over the SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1)-ALUMINUM-ACTIVATED MALATE TRANSPORT1 (ALMT1) signaling module was tested through a combination of genetic, biochemical, and genome-wide transcriptomic approaches. Root system configuration in response to phosphate scarcity involved MED16 functioning, which modulates the expression of a large set of low-phosphate-induced genes that respond to local and systemic signals in the Arabidopsis root tip, including those directly activated by STOP1. Biomolecular fluorescence complementation analysis suggests that MED16 is required for the transcriptional activation of STOP1 targets, including the membrane permease ALMT1, to increase malate exudation in response to low phosphate. Our results unveil the function of a critical transcriptional component, MED16, in the root adaptive responses to a scarce plant macronutrient, which helps understanding how plant cells orchestrate root morphogenesis to gene expression with the STOP1-ALMT1 module.

As sessile organisms, plants respond to changing environments modulating their genetic expression, metabolism and postembryonic developmental program (PDP) to adapt. Among environmental stressor, lead (Pb) is one of the most hazardous pollutants which limits crop productivity. Here, we describe in detail the effects of a wide range of concentrations of Pb on growth and development and a possible convergence with phosphate (Pi) starvation response. We found that the response to Pb presents a biphasic curve dose response in biomass accumulation: below 400 µM show a stimulatory effect meanwhile at Pb doses up to 600 µM effects are inhibitory. We found that +Pb (800 µM) modifies root system architecture (RSA) and induces acidification media, according to in silico ion interaction, in the growing medium Pb and Pi coprecipitate and plants grow in both Pi deficiency and Pb stress at the same time, however in spite of seedlings are under Pi starvation AtPT2 expression are Pb downregulated indicating that in addition to Pi starvation stress, Pb regulates physiological responses in root system. Using the mutants stop1, lpr1/2 and lpi3, which are affected in Pi starvation response, we found that changes in RSA by +Pb is genetically regulated and there are shared pathways with Pi starvation response mediated by PDR2-LPR1/2 and LPI3 pathways since lpr1/2 and lpi3 mutants are insensitive to +Pb and Pi starvation. Taking together, these results indicate that similar changes in RSA induced by independent environmental stimuli +Pb and Pi starvation are due to similar mediated response by PDR2-LPR1/2 pathway.

Plant diseases caused by pathogenic fungi result in considerable losses in agriculture. The use of fungicides is an important alternative to combat these pathogens, but may affect both the environment and human health. Plants produce many bioactive compounds to defend themselves from biotic challenges and an increasing number of secondary metabolites have been identified, which may be used to control fungal infections. Here, the bioactivity of a synthetic capsaicinoid, N-vanillyl-octanamide, also termed ABX-I, in the growth of five phytopathogenic fungi was assessed in vitro. The compound inhibited growth of Colletotrichum gloeosporioides, Botrytis cinerea, Colletotrichum acutatum, Fusarium sp., and Rhizoctonia solani AG2, while the magnitude of this effect differed from capsaicin. To investigate if ABX-I could effectively protect crops against phytopathogens, fungal challenges were performed in tomato leaves and fruits, as well as avocado fruits co-infiltrated with Botrytis cinerea or Colletotrichum gloeosporioides, respectively. In both tomato leaves and fruits and avocado fruits, ABX-I decreased the fungal damage not only in vegetative but also in edible tissues, and diminished decay symptoms compared with untreated fruits, which were highly sensitive to the pathogens. Furthermore, ABX-I spray application to tomato or avocado plants did not compromise growth and development, whereas it repressed spore germination and growth of C. gloeosporioides, which suggests its potential as an affordable and promising resource to control fungal diseases in the agronomic sector.

Plant growth promoting rhizobacteria influence host functional and adaptive traits via complex mechanisms that are just started to be clarified. Azospirillum brasilense acts as a probiotic bacterium, but detailed information about its molecular mechanisms of phytostimulation is scarce. Three interaction systems were established to analyze the impact of A. brasilense Sp245 on the phenotype of Arabidopsis seedlings, and underlying molecular responses were assessed under the following growth conditions: (1) direct contact of roots with the bacterium, (2) chemical communication via diffusible compounds produced by the bacterium, (3) signaling via volatiles. A. brasilense Sp245 improved shoot and root biomass and lateral root production in the three interaction systems assayed. Cell division, quiescent center, and differentiation protein reporters pCYCB1;1::GUS, WOX5::GFP, and pAtEXP7::GUS had a variable expression in roots depending of the nature of interaction. pCYCB1;1::GUS and WOX5::GFP increased with volatile compounds, whereas pAtEXP7::GUS expression was enhanced towards the root tip in plants with direct contact with the bacterium. The auxin reporter DR5::GUS was highly expressed with diffusible and volatile compounds, and accordingly, auxin signaling mutants pin3, slr1, arf7arf19, and tir1afb2afb3 showed differential phytostimulant responses when compared with the wild type. By contrast, ethylene signaling was not determinant to mediate root changes in response to the different interactions, as observed using the ethylene-related mutants etr1, ein2, and ein3. Our data highlight the diverse effects by which A. brasilense Sp245 improves plant growth and root architectural traits and define a critical role of auxin but not ethylene in mediating root response to bacterization.

Cyclodipeptides (CDPs) are the smallest peptidic molecules that can be produced by diverse organisms such as bacteria, fungi, and animals. They have multiple biological effects. In this paper, we examined the CDPs produced by the bacteria Pseudomonas aeruginosa PAO1, which are known as opportunistic pathogens of humans and plants on TARGET OF RAPAMYCIN (TOR) signaling pathways, and regulation of root system architecture. This bacterium produces the bioactive CDPs: cyclo(L-Pro-L-Leu), cyclo(L-Pro-L-Phe), cyclo(L-Pro-L-Tyr), and cyclo(L-Pro-L-Val). In a previous report, these molecules were found to modulate basic cellular programs not only via auxin mechanisms but also by promoting the phosphorylation of the S6 ribosomal protein kinase (S6K), a downstream substrate of the TOR kinase. In the present work, we found that the inoculation of Arabidopsis plants with P. aeruginosa PAO1, the non-pathogenic P. aeruginosa ΔlasI/Δrhll strain (JM2), or by direct exposure of plants to CDPs influenced growth and promoted root branching depending upon the treatment imposed, while genetic evidence using Arabidopsis lines with enhanced or decreased TOR levels indicated a critical role of this pathway in the bacterial phytostimulation.