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Summary Cover crop integration into grain crop rotations is a promising strategy for mitigating nematode‐induced diseases in agriculture. However, the precise mechanisms underlying this phenomenon remain elusive. Here, we first assessed the impact of five commonly used cover crops on the suppression of rice root‐knot nematodes (RKNs). We then chose ryegrass as a model to explore the mechanistic basis of the suppression effect. Contrary to expectations, while ryegrass rotation significantly enhances soil fertility, this increased fertility has minimal impact on RKN suppression. Furthermore, neither integrated ryegrass residues nor root exudates exhibit direct toxicity towards RKNs. We demonstrated that ryegrass rotation primarily suppresses RKNs by enriching beneficial soil microbiota. By complementing with isolated bacteria strains, we further demonstrated that ryegrass‐enriched bacteria not only directly reduce RKN infectivity and preference, but also activate plant immunity via the OsLRR‐RLK‐MAPK‐WRKY‐JA cascade, thereby diminishing RKN infection. Our study highlights the crucial role of soil microbiota in plant–nematode interactions, challenging conventional views on the direct effects of cover crops in nematode suppression. It offers a mechanistic understanding of the regulation potential and action modes of cover crops in mitigating nematode diseases, providing valuable insights for sustainable agriculture. [ABSTRACT FROM AUTHOR]

This article explores the role of rice orobanchol synthase in the hydroxylation of a noncanonical strigolactone called methyl 4-oxo-carlactonoate. Strigolactones are plant hormones that regulate growth and development, and they play a role in root parasitic weeds and beneficial symbiosis with fungi. The study focuses on the biosynthesis of strigolactones in rice and provides insights into the enzymatic processes involved. The researchers used LC-MS analysis to identify and quantify strigolactones in rice samples and found that the enzyme MAX1-1400 can convert both canonical and noncanonical strigolactones. They also discovered a new metabolite produced by MAX1-1400. The study concludes that OsMAX1-1400 is involved in noncanonical strigolactone biosynthesis, but further research is needed to understand the function and significance of these compounds in rice. The study was supported by funding from King Abdullah University of Science and Technology and the Bill & Melinda Gates Foundation. [Extracted from the article]

Summary: Nitrogen (N) and phosphorus (P) are the most important macronutrients required for plant growth and development. To cope with the limited and uneven distribution of N and P in complicated soil environments, plants have evolved intricate molecular strategies to improve nutrient acquisition that involve adaptive root development, production of root exudates, and the assistance of microbes. Recently, great advances have been made in understanding the regulation of N and P uptake and utilization and how plants balance the direct uptake of nutrients from the soil with the nutrient acquisition from beneficial microbes such as arbuscular mycorrhiza. Here, we summarize the major advances in these areas and highlight plant responses to changes in nutrient availability in the external environment through local and systemic signals. [ABSTRACT FROM AUTHOR]

Summary: Warming and elevated CO2 (eCO2) are expected to facilitate vascular plant encroachment in peatlands. The rhizosphere, where microbial activity is fueled by root turnover and exudates, plays a crucial role in biogeochemical cycling, and will likely at least partially dictate the response of the belowground carbon cycle to climate changes.We leveraged the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment, to explore the effects of a whole‐ecosystem warming gradient (+0°C to 9°C) and eCO2 on vascular plant fine roots and their associated microbes. We combined trait‐based approaches with the profiling of fungal and prokaryote communities in plant roots and rhizospheres, through amplicon sequencing.Warming promoted self‐reliance for resource uptake in trees and shrubs, while saprophytic fungi and putative chemoorganoheterotrophic bacteria utilizing plant‐derived carbon substrates were favored in the root zone. Conversely, eCO2 promoted associations between trees and ectomycorrhizal fungi. Trees mostly associated with short‐distance exploration‐type fungi that preferentially use labile soil N. Additionally, eCO2 decreased the relative abundance of saprotrophs in tree roots.Our results indicate that plant fine‐root trait variation is a crucial mechanism through which vascular plants in peatlands respond to climate change via their influence on microbial communities that regulate biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Summary: Endoplasmic reticulum (ER) bodies are ER‐derived structures that contain a large amount of PYK10 myrosinase, which hydrolyzes tryptophan (Trp)‐derived indole glucosinolates (IGs). Given the well‐described role of IGs in root–microbe interactions, we hypothesized that ER bodies in roots are important for interaction with soil‐borne microbes at the root–soil interface.We used mutants impaired in ER bodies (nai1), ER body‐resident myrosinases (pyk10bglu21), IG biosynthesis (myb34/51/122), and Trp specialized metabolism (cyp79b2b3) to profile their root microbiota community in natural soil, evaluate the impact of axenically collected root exudates on soil or synthetic microbial communities, and test their response to fungal endophytes in a mono‐association setup.Tested mutants exhibited altered bacterial and fungal communities in rhizoplane and endosphere, respectively. Natural soils and bacterial synthetic communities treated with mutant root exudates exhibited distinctive microbial profiles from those treated with wild‐type (WT) exudates. Most tested endophytes severely restricted the growth of cyp79b2b3, a part of which also impaired the growth of pyk10bglu21.Our results suggest that root ER bodies and their resident myrosinases modulate the profile of root‐secreted metabolites and thereby influence root–microbiota interactions. [ABSTRACT FROM AUTHOR]

Summary: Rhizomicrobiome plays important roles in plant growth and health, contributing to the sustainable development of agriculture. Plants recruit and assemble the rhizomicrobiome to satisfy their functional requirements, which is widely recognized as the 'cry for help' theory, but the intrinsic mechanisms are still limited.In this study, we revealed a novel mechanism by which plants reprogram the functional expression of inhabited rhizobacteria, in addition to the de novo recruitment of soil microbes, to satisfy different functional requirements as plants grow. This might be an efficient and low‐cost strategy and a substantial extension to the rhizomicrobiome recruitment theory.We found that the plant regulated the sequential expression of genes related to biocontrol and plant growth promotion in two well‐studied rhizobacteria Bacillus velezensis SQR9 and Pseudomonas protegens CHA0 through root exudate succession across the plant developmental stages. Sixteen key chemicals in root exudates were identified to significantly regulate the rhizobacterial functional gene expression by high‐throughput qPCR.This study not only deepens our understanding of the interaction between the plant–rhizosphere microbiome, but also provides a novel strategy to regulate and balance the different functional expression of the rhizomicrobiome to improve plant health and growth. [ABSTRACT FROM AUTHOR]

Summary: Root traits including root exudates are key factors affecting plant interactions with soil and thus play an important role in determining ecosystem processes. The drivers of their variation, however, remain poorly understood. We determined the relative importance of phylogeny and species ecology in determining root traits and analyzed the extent to which root exudate composition can be predicted by other root traits.We measured different root morphological and biochemical traits (including exudate profiles) of 65 plant species grown in a controlled system. We tested phylogenetic conservatism in traits and disentangled the individual and overlapping effects of phylogeny and species ecology on traits. We also predicted root exudate composition using other root traits.Phylogenetic signal differed greatly among root traits, with the strongest signal in phenol content in plant tissues. Interspecific variation in root traits was partly explained by species ecology, but phylogeny was more important in most cases. Species exudate composition could be partly predicted by specific root length, root dry matter content, root biomass, and root diameter, but a large part of variation remained unexplained.In conclusion, root exudation cannot be easily predicted based on other root traits and more comparative data on root exudation are needed to understand their diversity. [ABSTRACT FROM AUTHOR]

Summary: Jasmonic acid (JA) is involved in the modulation of defence and growth activities in plants. The best‐characterized growth‐defence trade‐offs stem from antagonistic crosstalk among hormones.In this study, we first confirmed that JA negatively regulates root‐knot nematode (RKN) susceptibility via the root exudates (REs) of tomato plants. Omics and toxicological analyses implied that kaempferol, a type of flavonol, from REs has a negative effect on RKN infection.We demonstrated that SlMYB57 negatively regulated kaempferol contents in tomato roots, whereas SlMYB108/112 had the opposite effect. We revealed that JA fine‐tuned the homeostasis of kaempferol via SlMYB‐mediated transcriptional regulation and the interaction between SlJAZs and SlMYBs, thus ensuring a balance between lateral root (LR) development and RKN susceptibility.Overall, this work provides novel insights into JA‐modulated LR development and RKN susceptibility mechanisms and elucidates a trade‐off model mediated by JA in plants encountering stress. [ABSTRACT FROM AUTHOR]

Summary: Several plant species have been reported to inhibit nitrification via their root exudates, the so‐called biological nitrification inhibition (BNI). Given the potential of BNI‐producing plants to sustainably mitigate N losses in agrosystems, identification of BNI activity in existing germplasms is of paramount importance.A hydroponic system was combined with an optimized Nitrosomonas europaea‐based bioassay to determine the BNI activity of root exudates. The pipeline allows collecting and processing hundreds of root exudates simultaneously. An additional assay was established to assess the potential bactericide effect of the root exudates.The pipeline was used to unravel the impact of developmental stage, temperature and osmotic stress on the BNI trait in selected wheat genotypes. Biological nitrification inhibition activity appeared consistently higher in wheat at the pretillering stage as compared to the tillering stage. While low‐temperatures did not alter BNI activities in root exudates, osmotic stress appeared to change the BNI activity in a genotype‐dependent manner. Further analysis of Nitrosomonas culture after pre‐exposure to root exudates suggested that BNI activity has no or limited bactericide effects.The present pipeline will be instrumental to further investigating the dynamics of BNI activity and to uncover the diversity of the BNI trait in plant species. [ABSTRACT FROM AUTHOR]

If no dormancy was present, and only spore mortality occurred, then the observed C SB 50(year I n i ) sb will necessarily increase over time because a larger concentration of spores added at year 0 is needed to achieve the same C SB 50 sb of viable spores. Changing balance between dormancy and mortality determines the trajectory of ectomycorrhizal fungal spore longevity over a 15-yr burial experiment Therefore, even in the case of an erroneous cue (e.g. host seedling germination and root exudates initiates spore germination that is then terminated by seedling death), only a small fraction of the spore bank will be exhausted. Keywords: dormancy; ectomycorrhiza; longevity; long-term; Rhizopogon spores EN dormancy ectomycorrhiza longevity long-term Rhizopogon spores 11 15 5 03/06/23 20230401 NES 230401 Introduction Temporal variability, ranging in scale from minutes to decades, imposes both challenges (Roff, [27]) and opportunities (Hutchinson, [13]; Roy & Chattopadhyay, [28]) for the survival of most organisms (Levin, [19]). [Extracted from the article]

Summary: Plant growth is greatly influenced by the rhizosphere microbiome, which has been traditionally investigated from a bottom‐up perspective assessing how resources such as root exudates stimulate microbial growth and drive microbiome assembly. However, the importance of predation as top‐down force on the soil microbiome remains largely underestimated.Here, we planted wheat both in natural and in sterilized soils inoculated with the key microbiome predators – bacterivorous nematodes – to assess how plant performance responds to top‐down predation of the soil microbiome and specific plant growth‐promoting bacteria, namely phosphate‐solubilizing bacteria.We found that nematodes enriched certain groups (e.g. Actinobacteria, Chloroflexi, and Firmicutes) and strengthened microbial connectance (e.g. Actinobacteria and Proteobacteria). These changes in microbiome structure were associated with phosphate‐solubilizing bacteria that facilitated phosphorus (P) cycling, leading to greater P uptake and biomass of wheat in both soils. However, the enhancement varied between nematode species, which may be attributed to the divergence of feeding behavior, as nematodes with weaker grazing intensity supported greater abundance of phosphate‐solubilizing bacteria and better plant performance compared with nematodes with greater grazing intensity.These results confirmed the ecological importance of soil nematodes for ecosystem functions via microbial co‐occurrence networks and suggested that the predation strength of nematodes determines the soil bacteria contribution to P biogeochemical cycling and plant growth. [ABSTRACT FROM AUTHOR]

Summary: Root hairs and soil water content are crucial in controlling the release and diffusion of root exudates and shaping profiles of biochemical properties in the rhizosphere. But whether root hairs can offset the negative impacts of drought on microbial activity remains unknown.Soil zymography, 14C imaging and neutron radiography were combined to identify how root hairs and soil moisture affect rhizosphere biochemical properties. To achieve this, we cultivated two maize genotypes (wild‐type and root‐hair‐defective rth3 mutant) under ambient and drought conditions.Root hairs and optimal soil moisture increased hotspot area, rhizosphere extent and kinetic parameters (Vmax and Km) of β‐glucosidase activities. Drought enlarged the rhizosphere extent of root exudates and water content. Colocalization analysis showed that enzymatic hotspots were more colocalized with root exudate hotspots under optimal moisture, whereas they showed higher dependency on water hotspots when soil water and carbon were scarce.We conclude that root hairs are essential in adapting rhizosphere properties under drought to maintain plant nutrition when a continuous mass flow of water transporting nutrients to the root is interrupted. In the rhizosphere, soil water was more important than root exudates for hydrolytic enzyme activities under water and carbon colimitation. See also the Commentary on this article by Zhang et al., 237: 707–709. [ABSTRACT FROM AUTHOR]

Summary: Invasive plants can change soil properties resulting in improved growth. Although invaders are known to alter soil chemistry, it remains unclear if chemicals secreted by roots facilitate invasive plant–soil mutualisms.With up to 19 confamilial pairs of invasive and native plants, and most of which were congeners, we explored the root exudate‐induced changes in plant–arbuscular mycorrhizal (AM) fungal mutualisms.We found that, relative to natives, invaders had greater AM colonization, greater biomass and their root exudates contained higher concentrations of two common chemical signals – quercetin and strigolactones – which are known to stimulate AM fungal growth and root colonization. An exudate exchange experiment showed that root exudates from invaders increased AM colonization more than exudates from natives. However, application of activated carbon led to greater reduction in AM colonization and plant biomass for invaders than natives, suggesting stronger effects of chemical signals in root exudates from invaders.We show that nonnative plants promote interactions with soil mutualists via enhancing root exudate chemicals, which could have important implications for invasion success. See also the Commentary on this article by Lekberg & Callaway, 236: 797–799. [ABSTRACT FROM AUTHOR]

Summary: Strigolactones (SLs) are rhizosphere signalling molecules and phytohormones. The biosynthetic pathway of SLs in tomato has been partially elucidated, but the structural diversity in tomato SLs predicts that additional biosynthetic steps are required. Here, root RNA‐seq data and co‐expression analysis were used for SL biosynthetic gene discovery.This strategy resulted in a candidate gene list containing several cytochrome P450s. Heterologous expression in Nicotiana benthamiana and yeast showed that one of these, CYP712G1, can catalyse the double oxidation of orobanchol, resulting in the formation of three didehydro‐orobanchol (DDH) isomers.Virus‐induced gene silencing and heterologous expression in yeast showed that one of these DDH isomers is converted to solanacol, one of the most abundant SLs in tomato root exudate. Protein modelling and substrate docking analysis suggest that hydroxy‐orbanchol is the likely intermediate in the conversion from orobanchol to the DDH isomers.Phylogenetic analysis demonstrated the occurrence of CYP712G1 homologues in the Eudicots only, which fits with the reports on DDH isomers in that clade. Protein modelling and orobanchol docking of the putative tobacco CYP712G1 homologue suggest that it can convert orobanchol to similar DDH isomers as tomato. [ABSTRACT FROM AUTHOR]

Summary: The simultaneous symbiosis of leguminous plants with two root mutualists, endophytic fungi and rhizobia is common in nature, yet how two mutualists interact and co‐exist before infecting plants and the concomitant effects on nodulation are less understood.Using a combination of metabolic analysis, fungal deletion mutants and comparative transcriptomics, we demonstrated that Bradyrhizobium and a facultatively biotrophic fungus, Phomopsis liquidambaris, interacted to stimulate fungal flavonoid production, and thereby primed Bradyrhizobial nodulation signaling, enhancing Bradyrhizobial responses to root exudates and leading to early nodulation of peanut (Arachis hypogaea), and such effects were compromised when disturbing fungal flavonoid biosynthesis.Stress sensitivity assays and reactive oxygen species (ROS) determination revealed that flavonoid production acted as a strategy to alleviate hyphal oxidative stress during P. liquidambaris–Bradyrhizobial interactions. By investigating the interactions between P. liquidambaris and a collection of 38 rhizobacteria, from distinct bacterial genera, we additionally showed that the flavonoid‐ROS module contributed to the maintenance of fungal and bacterial co‐existence, and fungal niche colonization under soil conditions.Our results demonstrate for the first time that rhizobial nodulation signaling can be primed by fungi before symbiosis with host plants and highlight the importance of flavonoid in tripartite interactions between legumes, beneficial fungi and rhizobia. [ABSTRACT FROM AUTHOR]

Summary: Carbon (C) exuded via roots is proposed to increase under drought and facilitate important ecosystem functions. However, it is unknown how exudate quantities relate to the total C budget of a drought‐stressed tree, that is, how much of net‐C assimilation is allocated to exudation at the tree level.We calculated the proportion of daily C assimilation allocated to root exudation during early summer by collecting root exudates from mature Fagus sylvatica and Picea abies exposed to experimental drought, and combining above‐ and belowground C fluxes with leaf, stem and fine‐root surface area.Exudation from individual roots increased exponentially with decreasing soil moisture, with the highest increase at the wilting point. Despite c. 50% reduced C assimilation under drought, exudation from fine‐root systems was maintained and trees exuded 1.0% (F. sylvatica) to 2.5% (P. abies) of net C into the rhizosphere, increasing the proportion of C allocation to exudates two‐ to three‐fold. Water‐limited P. abies released two‐thirds of its exudate C into the surface soil, whereas in droughted F. sylvatica it was only one‐third.Across the entire root system, droughted trees maintained exudation similar to controls, suggesting drought‐imposed belowground C investment, which could be beneficial for ecosystem resilience. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Zhang et al. (2023), 237: 780–792. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Yu et al. (2022), 236: 1140–1153. [ABSTRACT FROM AUTHOR]

Summary: Root exudates are a pathway for plant–microbial communication and play a key role in ecosystem response to environmental change. Here, we collate recent evidence that shows that plants of different growth strategies differ in their root exudation, that root exudates can select for beneficial soil microbial communities, and that drought affects the quantity and quality of root exudation. We use this evidence to argue for a central involvement of root exudates in plant and microbial response to drought and propose a framework for understanding how root exudates influence ecosystem form and function during and after drought. Specifically, we propose that fast‐growing plants modify their root exudates to recruit beneficial microbes that facilitate their regrowth after drought, with cascading impacts on their abundance and ecosystem functioning. We identify outstanding questions and methodological challenges that need to be addressed to advance and solidify our comprehension of the importance of root exudates in ecosystem response to drought. [ABSTRACT FROM AUTHOR]

Summary: Plants produce a wide array of secretions both above and below ground. Known as mucilages or exudates, they are secreted by seeds, roots, leaves and stems and fulfil a variety of functions including adhesion, protection, nutrient acquisition and infection. Mucilages are generally polysaccharide‐rich and often occur in the form of viscoelastic gels and in many cases have adhesive properties. In some cases, progress is being made in understanding the structure–function relationships of mucilages such as for the secretions that allow growing ivy to attach to substrates and the biosynthesis and secretion of the mucilage compounds of the Arabidopsis seed coat. Work is just beginning towards understanding root mucilage and the proposed adhesive polymers involved in the formation of rhizosheaths at root surfaces and for the secretions involved in host plant infection by parasitic plants. In this article, we summarise knowledge on plant exudates and mucilages within the concept of their functions in microenvironmental design, focusing in particular on their bioadhesive functions and the molecules responsible for them. We draw attention to areas of future knowledge need, including the microstructure of mucilages and their compositional and regulatory dynamics. [ABSTRACT FROM AUTHOR]

Summary: Root–soil interactions fundamentally affect the terrestrial carbon (C) cycle and thereby ecosystem feedbacks to climate change. This study addressed the question of whether the secondary metabolism of different temperate forest tree species can affect soil microbial respiration. We hypothesized that phenolics can both increase and decrease respiration depending on their function as food source, mobilizer of other soil resources, signaling compound, or toxin. We analyzed the phenolic compounds from root exudates and root tissue extracts of six tree species grown in a glasshouse using high‐performance liquid chromatography. We then tested the effect of individual phenolic compounds, representing the major identified phenylpropanoid compound classes, on microbial respiration through a 5‐d soil incubation. Phenolic root profiles were highly species‐specific. Of the eight classes identified, flavonoids were the most abundant, with flavanols being the predominating sub‐class. Phenolic effects on microbial respiration ranged from a 26% decrease to a 46% increase, with reduced respiration occurring in the presence of compounds possessing a catechol ring. Tree species variation in root phenolic composition influences the magnitude and direction of root effects on microbial respiration. Our data support the hypothesis that functional group rather than biosynthetic class determines the root phenolic effect on soil C cycling. [ABSTRACT FROM AUTHOR]

Summary: Sorgoleone, a major component of the hydrophobic root exudates of Sorghum spp., is probably responsible for many of the allelopathic properties attributed to members of this genus. Much of the biosynthetic pathway for this compound has been elucidated, with the exception of the enzyme responsible for the catalysis of the addition of two hydroxyl groups to the resorcinol ring. A library prepared from isolated Sorghum bicolor root hair cells was first mined for P450‐like sequences, which were then analyzed by quantitative reverse transcription‐polymerase chain reaction (RT‐qPCR) to identify those preferentially expressed in root hairs. Full‐length open reading frames for each candidate were generated, and then analyzed biochemically using both a yeast expression system and transient expression in Nicotiana benthamiana leaves. RNA interference (RNAi)‐mediated repression in transgenic S. bicolor was used to confirm the roles of these candidates in the biosynthesis of sorgoleone in planta. A P450 enzyme, designated CYP71AM1, was found to be capable of catalyzing the formation of dihydrosorgoleone using 5‐pentadecatrienyl resorcinol‐3‐methyl ether as substrate, as determined by gas chromatography‐mass spectroscopy (GC‐MS). RNAi‐mediated repression of CYP71AM1 in S. bicolor resulted in decreased sorgoleone contents in multiple independent transformant events. Our results strongly suggest that CYP71AM1 participates in the biosynthetic pathway of the allelochemical sorgoleone. [ABSTRACT FROM AUTHOR]

Keywords: arbuscular mycorrhiza (AM); community ecology; plant invasion; resource exchange; restoration EN arbuscular mycorrhiza (AM) community ecology plant invasion resource exchange restoration 813 814 2 10/13/22 20221101 NES 221101 What inspired your interest in plant science? Ylva is also an Adjunct Professor at the University of Montana.Ylva joined New Phytologist as an advisor in 2021.For more information on Ylva, visit www.mpgranch.com or email her at ylekberg@mpgranch.comORCID: Ylva Lekberg https://orcid.org/0000-0003-1033-8032 Why did you decide to pursue a career in research? Arbuscular mycorrhiza (AM), community ecology, plant invasion, restoration, resource exchange. [Extracted from the article]

Some plant species are able to distinguish between neighbours of different genetic identity and attempt to pre-empt resources through root proliferation in the presence of unrelated competitors, but avoid competition with kin. However, studies on neighbour recognition have met with some scepticism because the mechanisms by which plants identify their neighbours have remained unclear., In order to test whether root exudates could mediate neighbour recognition in plants, we performed a glasshouse experiment in which plants of Deschampsia caespitosa were subjected to root exudates collected from potential neighbours of different genetic identities, including siblings and individuals belonging to the same or a different population or species., Our results show that root exudates can carry specific information about the genetic relatedness, population origin and species identity of neighbours, and trigger different responses at the whole root system level and at the level of individual roots in direct contact with locally applied exudates. Increased root density was mainly achieved through changes in morphology rather than biomass allocation, suggesting that plants are able to limit the energetic cost of selfish behaviour., This study reveals a new level of complexity in the ability of plants to interpret and react to their surroundings. [ABSTRACT FROM AUTHOR]

Summary [ABSTRACT FROM AUTHOR]

Root carbon (C) inputs may regulate decomposition rates in soil, and in this study we ask: how do labile C inputs regulate decomposition of plant residues, and soil microbial communities? In a 14 d laboratory incubation, we added C compounds often found in root exudates in seven different concentrations (0, 0.7, 1.4, 3.6, 7.2, 14.4 and 21.7 mg C g-1 soil) to soils amended with and without 13C-labeled plant residue. We measured CO2 respiration and shifts in relative fungal and bacterial rRNA gene copy numbers using quantitative polymerase chain reaction (qPCR). Increased labile C input enhanced total C respiration, but only addition of C at low concentrations (0.7 mg C g-1) stimulated plant residue decomposition (+2%). Intermediate concentrations (1.4, 3.6 mg C g-1) had no impact on plant residue decomposition, while greater concentrations of C (> 7.2 mg C g-1) reduced decomposition ()50%). Concurrently, high exudate concentrations (> 3.6 mg C g-1) increased fungal and bacterial gene copy numbers, whereas low exudate concentrations (< 3.6 mg C g-1) increased metabolic activity rather than gene copy numbers. These results underscore that labile soil C inputs can regulate decomposition of more recalcitrant soil C by controlling the activity and relative abundance of fungi and bacteria. [ABSTRACT FROM AUTHOR]

•White lupin ( Lupinus albus) forms specialized cluster roots characterized by exudation of organic anions under phosphorus (P) deficiency. Here, the role of nitric oxide (NO) in P deficiency-induced cluster-root formation and citrate exudation was evaluated. •White lupin plants were treated with the NO donor sodium nitroprusside (SNP) and scavenger or inhibitor of NO synthase under conditions of P deficiency (0 μM) or P sufficiency (50 μM). •Phosphorus deficiency enhanced NO production in primary and lateral root tips, with a greater increase in cluster roots than in noncluster roots. NO concentrations decreased with cluster root development from the pre-emergent stage, through the juvenile stage, to the mature stage. The P deficiency-induced increase in NO production was inhibited by antagonists of NO synthase and xanthine oxidoreductase, suggesting the involvement of these enzymes in NO production. SNP markedly increased the number of cluster roots. Citrate exudation from different root segments in P-deficient roots was positively correlated with endogenous root NO concentrations. •These findings demonstrate differential patterns of NO production in white lupin, depending on root zone, developmental stage and P nutritional status. NO appears to play a regulatory role in the formation of cluster roots and citrate exudation in white lupin under conditions of P deficiency. [ABSTRACT FROM AUTHOR]

•Three Bornean pitcher plant species, Nepenthes lowii, N. rajah and N. macrophylla, produce modified pitchers that ‘capture’ tree shrew faeces for nutritional benefit. Tree shrews ( Tupaia montana) feed on exudates produced by glands on the inner surfaces of the pitcher lids and defecate into the pitchers. •Here, we tested the hypothesis that pitcher geometry in these species is related to tree shrew body size by comparing the pitcher characteristics with those of five other ‘typical’ (arthropod-trapping) Nepenthes species. •We found that only pitchers with large orifices and lids that are concave, elongated and oriented approximately at right angles to the orifice capture faeces. The distance from the tree shrews’ food source (that is, the lid nectar glands) to the front of the pitcher orifice precisely matches the head plus body length of T. montana in the faeces-trapping species, and is a function of orifice size and the angle of lid reflexion. •Substantial changes to nutrient acquisition strategies in carnivorous plants may occur through simple modifications to trap geometry. This extraordinary plant–animal interaction adds to a growing body of evidence that Nepenthes represents a candidate model for adaptive radiation with regard to nitrogen sequestration strategies. [ABSTRACT FROM AUTHOR]

• Nutrient exchange is the key symbiotic feature of arbuscular mycorrhiza (AM). As evidence is accumulating that plants sense presymbiotic factors from AM fungi and prepare for colonization, we investigated whether modifications in plant sugar metabolism might be part of the precolonization program. • Inoculation of Lotus japonicus roots in a double Millipore sandwich with the AM fungus Gigaspora margarita prevented contact between the symbionts but allowed exchange of signal molecules. Starch content was used as a marker for root carbohydrate status. • Mycorrhizal colonization of L. japonicus roots led to a decrease in starch concentration. In roots inoculated in the double sandwich, the polysaccharide accumulated after 1 wk and persisted for at least 4 wk. The response was absent in the castor myc− mutant, sym4-2, while transcript levels of both CASTOR and POLLUX were slightly enhanced in the wild-type L. japonicus roots, suggesting a requirement of the corresponding proteins for the starch-accumulation response. Exudates obtained from fungal spores germinated in the absence of the plant also induced starch accumulation in wild-type L. japonicus roots. • We conclude that factors released from germinating AM fungal spores induce changes in the root carbon status, possibly by enhancing sugar import, which leads to starch accumulation when colonization is prevented. [ABSTRACT FROM AUTHOR]

• Root exudates play a key role during the presymbiotic growth phase and have been shown to stimulate hyphal branching and the catabolic metabolism of arbuscular mycorrhizal (AM) fungal spores. • Here, the effect of root exudates on presymbiotic growth, uptake of exogenous carbon and transcript levels for genes putatively involved in the carbon metabolism of germinating spores were determined. • Crude root exudates led to a slight acceleration of spore germination, increased germ tube branching and stimulated uptake and catabolic metabolism of acetate, and to a greater extent of glucose, but had no effect on gene expression. By contrast, partially purified root exudates increased the transcript levels of acyl-CoA dehydrogenase (ß-oxidation of fatty acids to acetyl-CoA), malate synthase (glyoxylate cycle) and glutamine-fructose-6-phosphate aminotransferase (chitin biosynthesis), but did not differ from crude root exudates in their effect on substrate uptake and respiration. The expression of glycogen synthase (glycogen biosynthesis), glucose-6-phosphate dehydrogenase (pentose phosphate pathway) and neutral trehalase (hydrolysis of trehalose) were only marginally or not affected by root exudates. • Root exudates have an effect on both membrane activity and gene expression and the results are discussed in relation to the catabolic and anabolic metabolism of spores during presymbiotic growth. [ABSTRACT FROM AUTHOR]

• Physical obstructions that reduce space for root growth can profoundly affect plant performance. The aim of this study was to investigate the ability of roots to avoid obstructions and forage for usable space, and to reveal the mechanism involved. • Eight grass species from four genera were examined. Each genus included species characteristic of habitats with high and low nutrient availability. The ability to limit root mass and to adjust morphology within substrate containing obstructions in the form of gravel was investigated. A treatment with activated carbon, which adsorbs organic compounds, was used to examine the possible involvement of root exudates in responses to obstructions. • Only species characteristic of nutrient-poor habitats restricted placement of root mass in substrate containing obstructions, and this response disappeared in the presence of activated carbon. Root morphological responses to obstructions differed from those shown in response to nutrient-poor conditions or compacted soil. • These results suggest that the ability to avoid obstructions is dependent on the sensitivity of roots to their own exudates accumulating in the vicinity of obstructions. This is similar to other behavioural responses in which cues or signals are used to adjust growth before stressful conditions are encountered. [ABSTRACT FROM AUTHOR]

• Microbes detoxify arsenate by reduction and efflux of arsenite. Plants have a high capacity to reduce arsenate, but arsenic efflux has not been reported. • Tomato ( Lycopersicon esculentum) and rice ( Oryza sativa) were grown hydroponically and supplied with 10 µm arsenate or arsenite, with or without phosphate, for 1–3 d. The chemical species of As in nutrient solutions, roots and xylem sap were monitored, roles of microbes and root exudates in As transformation were investigated and efflux of As species from tomato roots was determined. • Arsenite remained stable in the nutrient solution, whereas arsenate was rapidly reduced to arsenite. Microbes and root exudates contributed little to the reduction of external arsenate. Arsenite was the predominant species in roots and xylem sap. Phosphate inhibited arsenate uptake and the appearance of arsenite in the nutrient solution, but the reduction was near complete in 24 h in both –P- and +P-treated tomato. Phosphate had a greater effect in rice than tomato. Efflux of both arsenite and arsenate was observed; the former was inhibited and the latter enhanced by the metabolic inhibitor carbonylcyanide m-chlorophenylhydrazone. • Tomato and rice roots rapidly reduce arsenate to arsenite, some of which is actively effluxed to the medium. The study reveals a new aspect of As metabolism in plants. [ABSTRACT FROM AUTHOR]

• Fertilizer-induced reductions in CO2 flux from soil ( ) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to reductions in fertilized forest soils. • Fertilization reduced by 16–19% in 65-yr-old plantations of northern red oak ( Quercus rubra) and sugar maple ( Acer saccharum), and in a natural 85-yr-old yellow birch ( Betula allegheniensis) stand. • In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. • In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to . [ABSTRACT FROM AUTHOR]

Although the overwhelming majority of non-aquatic vascular plants form vesicular--arbuscular (VA) mycorrhizal associations, the extent of colonization of the host root by any given fungal symbiont varies considerably depending on host and environmental factors. Because VA mycorrhizal fungi are obligate biotrophs, transfer of photosynthate from host to fungus may be an important factor in regulating the extent of VA mycorrhizal formation. Host metabolites must cross the plasma membrane before becoming available to the fungus. Several studies on rates of root exudation under various environmental conditions show a strong correlation between rates of root exudation and percent of root length colonized by VA mycorrhizal fungi. However, passive leakage of simple metabolites from roots as the sole means of regulating fungal colonization seems improbable for an obligate biotroph which has not yet been successfully cultured on any artificial medium. So far there has been insufficient investigation of hormone interactions between symbionts, and of the interference by the fungus in host cell wall synthesis, to evaluate the possible role of these factors in controlling growth of VA mycorrhizal fungi. Cytochemical studies of the host-fungus interface suggest modification of host plasma membrane ATPase activity as arbuscules develop, but the function of this altered activity remains unresolved. The presence of a linked Pi-photosynthate exchange mechanism on the host plasma membrane analogous to the Pl-photosynthate translocator known to exist in the outer membrane of chloroplasts remains an uninvestigated possible mechanism for balancing photosynthate demand by the fungus with enhanced P uptake. [ABSTRACT FROM AUTHOR]

summary: Nodulation is reviewed in terms of the phenotypes proposed by Vincent (1980). Individual legumes may be infectible by one or more of the three bacterial genera (collectively known as rhizobia) Rhizobium, Bradyrhizobium, or Azorhizobium. The type of infection process by which rhizobia gain entry is largely governed by the host genotype. In addition to the widely studied root‐hair pathway, infections may be associated with lateral root emergence or occur between root epidermal cells. The exact chemical and physical nature of the root hair/epidermal cell wall is likely to be a critical factor in determining whether infections can proceed. In addition to differing with species, wall composition may be influenced by soil chemical (e.g. Ca2+) and biotic factors (e.g. bacteria). Rhizobial features essential for infection include particular surface polysaccharides and the induction of nodulation genes by plant root exudates. Neither of these is likely to be a major barrier to the extension of nodulation to new hosts. Dissemination of rhizobia within developing nodules may be intercellular, via infection threads or by division of a small number of infected cells. All functional symbioses eventually have 'intracellular' bacteria, in the sense that rhizobia are geographically located within the boundary of the host cell walls. However, they remain extracellular in the sense that they are always confined by a membrane which is largely of host cell origin. In some genera they are also surrounded by infection thread walls, probably modified forms of 'invasive' infection thread walls, which allow differentiation of rhizobia into the nitrogen‐fixing form. Thus, natural, functional, symbioses may (a) never involve a stage in which bacteria are confined within tubular infection threads or (b) never release bacteria from infection threads. These features are determined by host genotype. The one feature of legume nodules so far found never to vary is the stem‐like character of a peripheral vascular system. This contrasts with the central vascular system of actinorhizas and the rhizobial‐induced nodules on the Ulmaceous genus Parasponia. Although of great intrinsic interest, this character is unlikely to present an insurmountable barrier to the extension of nodulation to new species. Other features, such as the ability to produce haemoglobin are now known to the in the genetic makeup of many higher plants. The discovery of the wide range of nodule structures occurring in nature, together with work on mutant rhizobia which may bypass critical stages in the nodulation process, suggest various ways in which the extension of nodulation to non‐nodulated legumes and to other (initially at least, dicotyledonous) plants may be engineered. CONTENTSSummary129I.Introduction130II.The symbionts130III.Stages in nodulation132IV.Stems and nodules143V.Prospects for finding/making new symbioses144VI.Conclusions145Acknowledgements147References147 [ABSTRACT FROM AUTHOR]

The ectomycorrhizal fungus Paxillus involutus Fr. was grown in the presence of root exudate of Pinus resinosa Ait. for two weeks. The effect of root exudate on the synthesis of ethanol-soluble antifungal compounds by P. involutus was determined by a bioassay based on microconidial germination of Fusarium oxysporum Schlecht emend, Sny. & Hans f.sp. pini. Cultivation of P. involutus on root exudate led to a greater than two-fold increase in fungitoxic activity when compared to cultivation on modified Melin-Norkrans medium in Petri dishes. Inoculation of P. resinosa root exudate with both P. involutus and a spore suspension of F. oxysporun: led to an 80% suppression of the sporulation of F. oxysporum when compared to controls lacking P. involutus. The increased resistance of P. resinosa P. involutus associations to Fusarium root rot may, therefore, be the result of biosynthesis of antifungal compounds by P. involutus which is stimulated by P. resinosa root exudate. [ABSTRACT FROM AUTHOR]

Nineteenth century studies described Lysimachia cavities as schizogenous, but Guttenberg (1928) reported them as only being schizogenous initially. Inner epithelial cell walls then swelled and dissolved in mucilage secreted into the cavity. Later, as a gradually solidifying resin accumulated, epithelial cells reportedly formed a new and cellulosic cell wall. My re-examination of cavity development in Lysimachia nummularia L., using a resin embedding method, revealed only typical schizogenous development, thus refuting Guttenberg's interpretation and supporting earlier workers. [ABSTRACT FROM AUTHOR]

The stimulatory effect of ABA on fluid exudation from excised maize roots has been confirmed. The maximum stimulation of water flow by ABA occurred at about 20 °C, and was enhanced by low concentration of the bathing medium. ABA increased the hydraulic conductivity of the root and the activation energy of water permeability was greater in the presence of ABA. It is believed that ABA acts directly at the membrane level, where it forms pores or channels in the membrane; these increase permeability to ions and water. [ABSTRACT FROM AUTHOR]

The effects of photoperiod on cytokinin levels in Xanthium strumarium L. were examined using a single photoperiodic cycle. Increasing the night length from 6 to 16 h resulted in a progressive decline in cytokinin levels in leaf laminae and buds. Interruption of a 16-h dark period midway by either a 5- or 60-min light-break at least partially negated the depressive effect of the dark period on cytokinin levels. All peaks of cytokinin activity were similarly affected by short day treatment. The reduction of cytokinin levels in response to a long dark period was dependent upon the presence of leaves greater than 1.0 cm in length but not upon the presence of buds. While exposure of the entire plant to a single long dark period led to a marked reduction in cytokinin levels in the leaf laminae, no such reduction was apparent in leaves which were exposed individually to a long night (with the rest of the plant, including the majority of the leaves, left in long day). The latter treatment did, however, lead to a partial reduction in the cytokinin flux from the detopped root system. Bark ringing of the stem between roots and leaves prior to photoperiodic treatment largely prevented the short day-induced decline in cytokinin levels in both leaf laminae and root exudate. These results are taken to indicate the existence of a shoot-to-root signal capable of moderating cytokinin production in the root system. [ABSTRACT FROM AUTHOR]