Plant roots and rhizosphere
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Plant roots and rhizosphere
dedicated to mechanisms associated to root development, and adaptation to abiotic stresses but also to the relations between  roots  and their surrounding microbial communities. Involvement  of hormones in the regulation of root development is now reported in the "Plant hormones" site (http://www.scoop.it/t/plant-hormones-by-christophe-jacquet).
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3D deformation field in growing plant roots reveals both mechanical and biological responses to axial mechanical forces | Journal of Experimental Botany | Oxford Academic

3D deformation field in growing plant roots reveals both mechanical and biological responses to axial mechanical forces | Journal of Experimental Botany | Oxford Academic | Plant roots and rhizosphere | Scoop.it
Strong regions and physical barriers in soils may slow root elongation, leading to reduced water and nutrient uptake and decreased yield. In this study, the biomechanical responses of roots to axial mechanical forces were assessed by combining 3D live imaging, kinematics and a novel mechanical sensor. This system quantified Young’s elastic modulus of intact poplar roots (32MPa), a rapid <0.2 mN touch-elongation sensitivity, and the critical elongation force applied by growing roots that resulted in bending. Kinematic analysis revealed a multiphase bio-mechanical response of elongation rate and curvature in 3D. Measured critical elongation force was accurately predicted from an Euler buckling model, indicating that no biologically mediated accommodation to mechanical forces influenced bending during this short period of time. Force applied by growing roots increased more than 15-fold when buckling was prevented by lateral bracing of the root. The junction between the growing and the mature zones was identified as a zone of mechanical weakness that seemed critical to the bending process. This work identified key limiting factors for root growth and buckling under mechanical constraints. The findings are relevant to crop and soil sciences, and advance our understanding of root growth in heterogeneous structured soils.
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Formation and exudation of non-volatile products of the arabidiol triterpenoid degradation pathway in Arabidopsis roots

Formation and exudation of non-volatile products of the arabidiol triterpenoid degradation pathway in Arabidopsis roots | Plant roots and rhizosphere | Scoop.it
Triterpenoids produced by plants play important roles in the protection against biotic stress. Roots of Arabidopsis thaliana produce different triterpenoids, which include the tricyclic triterpene diol, arabidiol. In a degradation reaction induced by infection with the oomycete pathogen, Pythium irregulare, arabidiol is cleaved to the 11-carbon volatile homoterpene, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), and the 19-carbon ketone, apo-arabidiol. The arabidiol pathway and its volatile breakdown product DMNT have been implicated in the defense against P. irregulare infection. Here we show that the non-volatile breakdown product apo-arabidiol is further converted to the acetylated derivative α−14-acetyl-apo-arabidiol via a presumed epimerization and subsequent acetylation reaction. α−14-acetyl-apo-arabidiol and the detected intermediates in the derivatization pathway are partially exuded from the root indicating possible defensive activities of these molecules in the rhizosphere. The conversion steps of apo-arabidiol vary among different Arabidopsis accessions and are present in only rudimentary form in the close relative Arabidopsis lyrata, which supports an intra- and inter-specific modularity in triterpenoid metabolism.
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Frontiers | Volatile-Mediated Effects Predominate in Paraburkholderia phytofirmans Growth Promotion and Salt Stress Tolerance of Arabidopsis thaliana | Plant Biotic Interactions

Frontiers | Volatile-Mediated Effects Predominate in Paraburkholderia phytofirmans Growth Promotion and Salt Stress Tolerance of Arabidopsis thaliana | Plant Biotic Interactions | Plant roots and rhizosphere | Scoop.it
Abiotic stress has a growing impact on plant growth and agricultural activity worldwide. Specific plant growth promoting rhizobacteria have been reported to stimulate growth and tolerance to abiotic stress in plants, and molecular mechanisms like phytohormone synthesis and 1-aminocyclopropane-1-carboxylate deamination are usual candidates proposed to mediate these bacterial effects. Paraburkholderia phytofirmans PsJN is able to promote growth of several plant hosts, and improve their tolerance to chilling, drought and salinity. This work investigated bacterial determinants involved in PsJN stimulation of growth and salinity tolerance in Arabidopsis thaliana, showing bacteria enable plants to survive long-term salinity treatment, accumulating less sodium within leaf tissues relative to non-inoculated controls. Inactivation of specific bacterial genes encoding ACC deaminase, auxin catabolism, N-acyl-homoserine-lactone production, and flagellin synthesis showed these functions have little influence on bacterial induction of salinity tolerance. Volatile organic compound emission from strain PsJN was shown to reproduce the effects of direct bacterial inoculation of roots, increasing plant growth rate and tolerance to salinity evaluated both in vitro and in soil. Furthermore, early exposure to VOCs from P. phytofirmans was sufficient to stimulate long-term effects observed in Arabidopsis growth in the presence and absence of salinity. Organic compounds were analyzed in the headspace of PsJN cultures, showing production of 2-undecanone, 7-hexanol, 3-methylbutanol and dimethyl disulfide. Exposure of A. thaliana to different quantities of these molecules showed that they are able to influence growth in a wide range of added amounts. Exposure to a blend of the first three compounds was found to mimic the effects of PsJN on both general growth promotion and salinity tolerance. To our knowledge, this is the first report on volatile compound-mediated induction of plant abiotic stress tolerance by a Paraburkholderia species.
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The Plant Microbiota: Systems-Level Insights and Perspectives - Annual Review of Genetics, 50(1):211

The Plant Microbiota: Systems-Level Insights and Perspectives - Annual Review of Genetics, 50(1):211 | Plant roots and rhizosphere | Scoop.it
Plants do not grow as axenic organisms in nature, but host a diverse community of microorganisms, termed the plant microbiota. There is an increasing awareness that the plant microbiota plays a role in plant growth and can provide protection from invading pathogens. Apart from intense research on crop plants, Arabidopsis is emerging as a valuable model system to investigate the drivers shaping stable bacterial communities on leaves and roots and as a tool to decipher the intricate relationship among the host and its colonizing microorganisms. Gnotobiotic experimental systems help establish causal relationships between plant and microbiota genotypes and phenotypes and test hypotheses on biotic and abiotic perturbations in a systematic way. We highlight major recent findings in plant microbiota research using comparative community profiling and omics analyses, and discuss these approaches in light of community establishment and beneficial traits like nutrient acquisition and plant health.
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Michael Fokin's comment, December 8, 7:14 PM
pls correct the link. in goes to annualreview through! your institute proxy
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Cell-Type-Specific H+-ATPase Activity in Root Tissues Enables K+ Retention and Mediates Acclimation of Barley (Hordeum vulgare) to Salinity Stress

Cell-Type-Specific H+-ATPase Activity in Root Tissues Enables K+ Retention and Mediates Acclimation of Barley (Hordeum vulgare) to Salinity Stress | Plant roots and rhizosphere | Scoop.it
While the importance of cell type specificity in plant adaptive responses is widely accepted, only a limited number of studies have addressed this issue at the functional level. We have combined electrophysiological, imaging, and biochemical techniques to reveal the physiological mechanisms conferring higher sensitivity of apical root cells to salinity in barley (Hordeum vulgare). We show that salinity application to the root apex arrests root growth in a highly tissue- and treatment-specific manner. Although salinity-induced transient net Na+ uptake was about 4-fold higher in the root apex compared with the mature zone, mature root cells accumulated more cytosolic and vacuolar Na+, suggesting that the higher sensitivity of apical cells to salt is not related to either enhanced Na+ exclusion or sequestration inside the root. Rather, the above differential sensitivity between the two zones originates from a 10-fold difference in K+ efflux between the mature zone and the apical region (much poorer in the root apex) of the root. Major factors contributing to this poor K+ retention ability are (1) an intrinsically lower H+-ATPase activity in the root apex, (2) greater salt-induced membrane depolarization, and (3) a higher reactive oxygen species production under NaCl and a larger density of reactive oxygen species-activated cation currents in the apex. Salinity treatment increased (2- to 5-fold) the content of 10 (out of 25 detected) amino acids in the root apex but not in the mature zone and changed the organic acid and sugar contents. The causal link between the observed changes in the root metabolic profile and the regulation of transporter activity is discussed.
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Fungal and plant gene expression in the Tulasnella calospora–Serapias vomeracea symbiosis provides clues about nitrogen pathways in orchid mycorrhizas

Fungal and plant gene expression in the Tulasnella calospora–Serapias vomeracea symbiosis provides clues about nitrogen pathways in orchid mycorrhizas | Plant roots and rhizosphere | Scoop.it
Orchids are highly dependent on their mycorrhizal fungal partners for nutrient supply, especially during early developmental stages. In addition to organic carbon, nitrogen (N) is probably a major nutrient transferred to the plant because orchid tissues are highly N-enriched. We know almost nothing about the N form preferentially transferred to the plant or about the key molecular determinants required for N uptake and transfer.
We identified, in the genome of the orchid mycorrhizal fungus Tulasnella calospora, two functional ammonium transporters and several amino acid transporters but found no evidence of a nitrate assimilation system, in agreement with the N preference of the free-living mycelium grown on different N sources.
Differential expression in symbiosis of a repertoire of fungal and plant genes involved in the transport and metabolism of N compounds suggested that organic N may be the main form transferred to the orchid host and that ammonium is taken up by the intracellular fungus from the apoplatic symbiotic interface.
This is the first study addressing the genetic determinants of N uptake and transport in orchid mycorrhizas, and provides a model for nutrient exchanges at the symbiotic interface, which may guide future experiments.
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Control of Arabidopsis lateral root primordium boundaries by MYB36 - Fernández-Marcos - 2016 - New Phytologist -

Control of Arabidopsis lateral root primordium boundaries by MYB36 - Fernández-Marcos - 2016 - New Phytologist - | Plant roots and rhizosphere | Scoop.it
Root branching in plants relies on the de novo formation of lateral roots. These are initiated from founder cells, triggering new formative divisions that generate lateral root primordia (LRP). The LRP size and shape depends on the balance between positive and negative signals that control cell proliferation.
The mechanisms controlling proliferation potential of LRP cells remains poorly understood. We found that Arabidopsis thaliana MYB36, which have been previously shown to regulate genes required for Casparian strip formation and the transition from proliferation to differentiation in the primary root, plays a new role in controlling LRP development at later stages.
We found that MYB36 is a novel component of LR development at later stages. MYB36 was expressed in the cells surrounding LRP where it controls a set of peroxidase genes, which maintain reactive oxygen species (ROS) balance. This was required to define the transition between proliferating and arrested cells inside the LRP, coinciding with the change from flat to dome-shaped primordia. Reducing the levels of hydrogen peroxide (H2O2) in myb36-5 significantly rescues the mutant phenotype.
Our results uncover a role for MYB36 outside the endodermis during LRP development through a mechanism analogous to regulating the proliferation/differentiation transition in the root meristem.
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Frontiers | Accumulation and Secretion of Coumarinolignans and other Coumarins in Arabidopsis thaliana Roots in Response to Iron Deficiency at High pH | Plant Physiology

Frontiers | Accumulation and Secretion of Coumarinolignans and other Coumarins in Arabidopsis thaliana Roots in Response to Iron Deficiency at High pH | Plant Physiology | Plant roots and rhizosphere | Scoop.it
Root secretion of coumarin-phenolic type compounds has been recently shown to be related to Arabidopsis thaliana tolerance to Fe deficiency at high pH. Previous studies revealed the identity of a few simple coumarins occurring in roots and exudates of Fe-deficient A. thaliana plants, and left open the possible existence of other unknown phenolics. We used HPLC-UV/VIS/ESI-MS(TOF), HPLC/ESI-MS(ion trap) and HPLC/ESI-MS(Q-TOF) to characterize (identify and quantify) phenolic-type compounds accumulated in roots or secreted into the nutrient solution of A. thaliana plants in response to Fe deficiency. Plants grown with or without Fe and using nutrient solutions buffered at pH 5.5 or 7.5 enabled to identify an array of phenolics. These include several coumarinolignans not previously reported in A. thaliana (cleomiscosins A, B, C, and D and the 5′-hydroxycleomiscosins A and/or B), as well as some coumarin precursors (ferulic acid and coniferyl and sinapyl aldehydes), and previously reported cathecol (fraxetin) and non-cathecol coumarins (scopoletin, isofraxidin and fraxinol), some of them in hexoside forms not previously characterized. The production and secretion of phenolics were more intense when the plant accessibility to Fe was diminished and the plant Fe status deteriorated, as it occurs when plants are grown in the absence of Fe at pH 7.5. Aglycones and hexosides of the four coumarins were abundant in roots, whereas only the aglycone forms could be quantified in the nutrient solution. A comprehensive quantification of coumarins, first carried out in this study, revealed that the catechol coumarin fraxetin was predominant in exudates (but not in roots) of Fe-deficient A. thaliana plants grown at pH 7.5. Also, fraxetin was able to mobilize efficiently Fe from a Fe(III)-oxide at pH 5.5 and pH 7.5. On the other hand, non-catechol coumarins were much less efficient in mobilizing Fe and were present in much lower concentrations, making unlikely that they could play a role in Fe mobilization. The structural features of the array of coumarin type-compounds produced suggest some can mobilize Fe from the soil and others can be more efficient as allelochemicals.
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SMALL ACIDIC PROTEIN 1 (SMAP1) and SCFTIR1 ubiquitin proteasome pathway act in concert to induce 2,4‐dichlorophenoxyacetic acid‐mediated alteration of actin in Arabidopsis roots

SMALL ACIDIC PROTEIN 1 (SMAP1) and SCFTIR1 ubiquitin proteasome pathway act in concert to induce 2,4‐dichlorophenoxyacetic acid‐mediated alteration of actin in Arabidopsis roots | Plant roots and rhizosphere | Scoop.it
2,4-dichlorophanoxyacetic acid (2,4-D), a functional analogue of auxin, is used as an exogenous source of auxin as it evokes physiological responses like the endogenous auxin, Indole-3-acetic acid (IAA). Previous molecular analyses of the auxin response pathway revealed that IAA and 2,4-D share a common mode of action to elicit downstream physiological responses. However, recent findings with 2,4-D specific mutants suggested that 2,4-D and IAA might also use distinct pathways to modulate Arabidopsis root growth. Using genetic and cellular approaches, we demonstrate that the distinct effects of 2,4-D and IAA on actin filament organization partly dictate the differential responses of roots to these two auxin analogues. 2,4-D but not IAA altered the actin structure in long term and short term assays. Analysis of the 2,4-D specific mutant aar1-1 revealed that Small Acidic Protein 1 (SMAP1) functions positively to facilitate the 2,4-D-induced depolymerization of actin. The ubiquitin proteasome mutants, tir1-1 and axr1-12, which show enhanced resistance to 2,4-D compared with IAA for root growth inhibition, were also found to have less disrupted actin filament networks after 2,4-D exposure. Consistently, chemical inhibitor of the ubiquitin proteasome pathway mitigated the disrupting effects of 2,4-D on the organization of actin filaments. Roots of the double mutant aar1-1 tir1-1 also showed enhanced resistance to 2,4-D-induced root growth inhibition and actin degradation compared with their respective parental lines. Collectively, these results suggest that the effects of 2,4-D on actin filament organization and root growth is mediated through synergistic interactions between SMAP1 and SCFTIR1 ubiquitin proteasome components.
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Phosphorus uptake, partitioning and redistribution during grain filling in rice

Phosphorus uptake, partitioning and redistribution during grain filling in rice | Plant roots and rhizosphere | Scoop.it
Backgrounds and Aims In cultivated rice, phosphorus (P) in grains originates from two possible sources, namely exogenous (post-flowering root P uptake from soil) or endogenous (P remobilization from vegetative parts) sources. This study investigates P partitioning and remobilization in rice plants throughout grain filling to resolve contributions of P sources to grain P levels in rice.

Methods Rice plants (Oryza sativa ‘IR64’) were grown under P-sufficient or P-deficient conditions in the field and in hydroponics. Post-flowering uptake, partitioning and re-partitioning of P was investigated by quantifying tissue P levels over the grain filling period in the field conditions, and by employing 33P isotope as a tracer in the hydroponic study.

Key Results Post-flowering P uptake represented 40–70 % of the aerial plant P accumulation at maturity. The panicle was the main P sink in all studies, and the amount of P potentially remobilized from vegetative tissues to the panicle during grain filling was around 20 % of the total aerial P measured at flowering. In hydroponics, less than 20 % of the P tracer taken up at 9 d after flowering (DAF) was found in the above-ground tissues at 14 DAF and half of it was partitioned to the panicle in both P treatments.

Conclusions The results demonstrate that P uptake from the soil during grain filling is a critical contributor to the P content in grains in irrigated rice. The P tracer study suggests that the mechanism of P loading into grains involves little direct transfer of post-flowering P uptake to the grain but rather substantial mobilization of P that was previously taken up and stored in vegetative tissues.
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ROOT HAIR DEFECTIVE SIX-LIKE4 (RSL4) promotes root hair elongation by transcriptionally regulating the expression of genes required for cell growth - Vijayakumar - 2016 - New Phytologist -

ROOT HAIR DEFECTIVE SIX-LIKE4 (RSL4) promotes root hair elongation by transcriptionally regulating the expression of genes required for cell growth - Vijayakumar - 2016 - New Phytologist - | Plant roots and rhizosphere | Scoop.it
ROOT HAIR DEFECTIVE SIX-LIKE4 (RSL4) is necessary and sufficient for root hair elongation in Arabidopsis thaliana. Root hair length is determined by the duration for which RSL4 protein is present in the developing root hair. The aim of this research was to identify genes regulated by RSL4 that affect root hair growth.
To identify genes regulated by RSL4, we identified genes whose expression was elevated by induction of RSL4 activity in the presence of an inhibitor of translation.
Thirty-four genes were identified as putative targets of RSL transcriptional regulation, and the results suggest that the activities of SUPPRESSOR OF ACTIN (SAC1), EXOCSYT SUBUNIT 70A1 (EXO70A1), PEROXIDASE7 (PRX7) and CALCIUM-DEPENDENT PROTEIN KINASE11 (CPK11) are required for root hair elongation.
These data indicate that RSL4 controls cell growth by controlling the expression of genes encoding proteins involved in cell signalling, cell wall modification and secretion.
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NIN Is Involved in the Regulation of Arbuscular Mycorrhizal Symbiosis

NIN Is Involved in the Regulation of Arbuscular Mycorrhizal Symbiosis | Plant roots and rhizosphere | Scoop.it
Arbuscular mycorrhizal (AM) symbiosis is an intimate and ancient symbiosis found between most of terrestrial plants and fungi from the Glomeromycota family. Later during evolution, the establishment of the nodulation between legume plants and soil bacteria known as rhizobia, involved several genes of the signaling pathway previously implicated for AM symbiosis. For the past years, the identification of the genes belonging to this Common Symbiotic Signaling Pathway have been mostly done on nodulation. Among the different genes already well identified as required for nodulation, we focused our attention on the involvement of Nodule Inception (NIN) in AM symbiosis. We show here that NIN expression is induced during AM symbiosis, and that the Medicago truncatula nin mutant is less colonized than the wild-type M. truncatula strain. Moreover, nin mutant displays a defect in the ability to be infected by the fungus Rhizophagus irregularis. This work brings a new evidence of the common genes involved in overlapping signaling pathways of both nodulation and in AM symbiosis.


Via Jean-Michel Ané
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A nonnative and a native fungal plant pathogen similarly stimulate ectomycorrhizal development but are perceived differently by a fungal symbiont

A nonnative and a native fungal plant pathogen similarly stimulate ectomycorrhizal development but are perceived differently by a fungal symbiont | Plant roots and rhizosphere | Scoop.it
The effects of plant symbionts on host defence responses against pathogens have been extensively documented, but little is known about the impact of pathogens on the symbiosis and if such an impact may differ for nonnative and native pathogens. Here, this issue was addressed in a study of the model system comprising Pinus pinea, its ectomycorrhizal symbiont Tuber borchii, and the nonnative and native pathogens Heterobasidion irregulare and Heterobasidion annosum, respectively. In a 6-month inoculation experiment and using both in planta and gene expression analyses, we tested the hypothesis that H. irregulare has greater effects on the symbiosis than H. annosum. Although the two pathogens induced the same morphological reaction in the plant−symbiont complex, with mycorrhizal density increasing exponentially with pathogen colonization of the host, the number of target genes regulated in T. borchii in plants inoculated with the native pathogen (i.e. 67% of tested genes) was more than twice that in plants inoculated with the nonnative pathogen (i.e. 27% of genes). Although the two fungal pathogens did not differentially affect the amount of ectomycorrhizas, the fungal symbiont perceived their presence differently. The results may suggest that the symbiont has the ability to recognize a self/native and a nonself/nonnative pathogen, probably through host plant-mediated signal transduction.

Via Francis Martin
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Dynamic control of lateral root positioning

Dynamic control of lateral root positioning | Plant roots and rhizosphere | Scoop.it
Highlights



The root cap contains an auxin source that modulates lateral root patterning.


Recurrent programmed cell death controls the regular spacing of lateral roots.


Periodic release of auxin by dying root cap may trigger prebranch site formation.


Tropic responses control the LR sidedness.

In dicot root systems, lateral roots are in general regularly spaced along the longitudinal axis of the primary root to facilitate water and nutrient uptake. Recently, recurrent programmed cell death in the root cap of the growing root has been implicated in lateral root spacing. The root cap contains an auxin source that modulates lateral root patterning. Periodic release of auxin by dying root cap cells seems to trigger lateral root specification at regular intervals. However, it is currently unclear through which molecular mechanisms auxin restricts lateral root specification to specific cells along the longitudinal and radial axes of the root, or how environmental signals impact this process.
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Receptor kinase complex transmits RALF peptide signal to inhibit root growth in Arabidopsis

Receptor kinase complex transmits RALF peptide signal to inhibit root growth in Arabidopsis | Plant roots and rhizosphere | Scoop.it
Significance

Receptor-like kinase FERONIA (FER) is a versatile regulator of cell growth under both normal and stress environments. FER binds its peptide ligand, rapid alkalinization factor 1 (RALF1), and triggers downstream events to inhibit cell growth in primary roots. However, the mechanism of RALF1 reception by FER is still largely unknown. In this study, we identified a receptor-like cytoplasmic kinase (RPM1-induced protein kinase, RIPK) that directly interacts with and is phosphorylated by FER in a RALF1 peptide-dependent manner. The defects of fer-4 mutant in RALF1 response and root hair development are mimicked by ripk loss-of-function but partially compensated by RIPK overexpression. These and other data suggest that formation of the FER–RIPK complex serves as a crucial step in the RALF1 signaling pathway.

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Abstract

A number of hormones work together to control plant cell growth. Rapid Alkalinization Factor 1 (RALF1), a plant-derived small regulatory peptide, inhibits cell elongation through suppression of rhizosphere acidification in plants. Although a receptor-like kinase, FERONIA (FER), has been shown to act as a receptor for RALF1, the signaling mechanism remains unknown. In this study, we identified a receptor-like cytoplasmic kinase (RPM1-induced protein kinase, RIPK), a plasma membrane-associated member of the RLCK-VII subfamily, that is recruited to the receptor complex through interacting with FER in response to RALF1. RALF1 triggers the phosphorylation of both FER and RIPK in a mutually dependent manner. Genetic analysis of the fer-4 and ripk mutants reveals RIPK, as well as FER, to be required for RALF1 response in roots. The RALF1–FER–RIPK interactions may thus represent a mechanism for peptide signaling in plants.
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Plenty Is No Plague: Streptomyces Symbiosis with Crops

Plenty Is No Plague: Streptomyces Symbiosis with Crops | Plant roots and rhizosphere | Scoop.it
Streptomyces spp. constitute a major clade of the phylum Actinobacteria. These Gram-positive, filamentous prokaryotes are ubiquitous in soils and marine sediments, and are commonly found in the rhizosphere or inside plant roots. Plant-interacting Streptomyces have received limited attention, in contrast to Streptomyces spp. extensively investigated for decades in medicine given their rich potential for secondary metabolite biosynthesis. Recent genomic, metabolomic, and biotechnological advances have produced key insights into Streptomyces spp., paving the way to the use of their metabolites in agriculture. In this Opinion article we propose how Streptomyces spp. could dominate future aspects of crop nutrition and protection. Risks and benefits of the use of these microorganisms in agriculture are also discussed.
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Transcription Factors WOX11/12 Directly Activate WOX5/7 to Promote Root Primordia Initiation and Organogenesis

Transcription Factors WOX11/12 Directly Activate WOX5/7 to Promote Root Primordia Initiation and Organogenesis | Plant roots and rhizosphere | Scoop.it
De novo organogenesis, which gives rise to adventitious roots and shoots, is a type of plant regeneration for survival after wounding. In Arabidopsis (Arabidopsis thaliana), two main cell fate transition steps are required to establish the root primordium during de novo root organogenesis from leaf explants. The first step from regeneration-competent cells to root founder cells involves activation of WUSCHEL-RELATED HOMEOBOX11 (WOX11) and WOX12 (WOX11/12) expression by auxin. However, the molecular mechanism controlling the second step of fate transition from root founder cells to root primordium is poorly understood. In this study, we show that the expression levels of WOX11/12 decrease while those of WOX5 and 7 (WOX5/7) increase during the transition from root founder cells to the root primordium. WOX11/12 function genetically upstream of WOX5/7, and the WOX11/12 proteins directly bind to the promoters of WOX5/7 to activate their transcription. Mutations in WOX5/7 result in defective primordium formation. Overall, our data indicate that the expression switch from WOX11/12 to WOX5/7 is critical for initiation of the root primordium during de novo root organogenesis.
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Arbuscular mycorrhizal fungi communities from tropical Africa reveal strong ecological structure

Arbuscular mycorrhizal fungi communities from tropical Africa reveal strong ecological structure | Plant roots and rhizosphere | Scoop.it
Understanding the distribution and diversity of arbuscular mycorrhizal fungi (AMF) and the rules that govern AMF assemblages has been hampered by a lack of data from natural ecosystems. In addition, the current knowledge on AMF diversity is biased towards temperate ecosystems, whereas little is known about other habitats such as dry tropical ecosystems.
We explored the diversity and structure of AMF communities in grasslands, savannas, dry forests and miombo in a protected area under dry tropical climate (Gorongosa National Park, Mozambique) using 454 pyrosequencing.
In total, 147 AMF virtual taxa (VT) were detected, including 22 VT new to science. We found a high turnover of AMF with ˂ 12% of VT present in all vegetation types. Forested areas supported more diverse AMF communities than savannas and grassland. Miombo woodlands had the highest AMF richness, number of novel VT, and number of exclusive and indicator taxa.
Our data reveal a sharp differentiation of AMF communities between forested areas and periodically flooded savannas and grasslands. This marked ecological structure of AMF communities provides the first comprehensive landscape-scale evidence that, at the background of globally low endemism of AMF, local communities are shaped by regional processes including environmental filtering by edaphic properties and natural disturbance.
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RSL class I genes positively regulate root hair development in Oryza sativa

RSL class I genes positively regulate root hair development in Oryza sativa | Plant roots and rhizosphere | Scoop.it
Root hairs are filamentous protuberances from superficial cells of plant roots that are critical for nutrient uptake. Genes encoding ROOT HAIR DEFECTIVE-SIX LIKE (RSL) class I basic helix–loop–helix proteins are expressed in future root hair cells (trichoblasts) of the Arabidopsis thaliana root where they positively regulate root hair cell development.
We characterized the function of class I genes in Oryza sativa root development.
We show that there are three RSL class I genes in O. sativa and that each is expressed in developing root hair cells. Reduction of RSL class I function results in the development of shorter root hairs than in wild-type. Ectopic overexpression results in the development of ectopic root hair cells.
These data suggest that expression of individual RSL class I proteins is sufficient for root hair development in the cereal O. sativa (rice). Therefore RSL class I genes have been conserved since O. sativa and A. thaliana last shared a common ancestor. However, given that RSL class I genes are not sufficient for root hair development in A. thaliana, it suggests that there are differences in the mechanisms repressing RSL class I gene activity between members of the Poaceae and Brassicaceae.
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Nitrogen transport in the orchid mycorrhizal symbiosis – further evidence for a mutualistic association

Nitrogen transport in the orchid mycorrhizal symbiosis – further evidence for a mutualistic association | Plant roots and rhizosphere | Scoop.it
Mycorrhizas are symbioses integral to the health of plant-based ecosystems (Smith & Read, 2008). In a typical mycorrhizal association, fungi in, or on, plant roots pass soil-acquired inorganic nutrients and water to the plant host. In return, the host transfers excess photosynthate to the fungus. Orchid mycorrhizas were considered to be unusual symbioses in that during initial colonization of the young, nonphotosynthetic host, the fungus was thought to provide both inorganic and organic nutrition to the plant and to receive nothing in return for its services. That is until a significant new study by Fochi et al., in this issue of New Phytologist (pp. 365–379), investigating expression of fungal and plant nitrogen (N) transport and assimilation genes in mycorrhizas formed between the fungus Tulasnella calospora and the photosynthetic orchid, Serapias vomeracea. The research suggests, for the first time, flow of nutrients back to the fungal partner from the nonphotosynthetic orchid host. Thus orchid mycorrhizas now appear to represent a true mutualism in both the early and mature stages of plant development (Cameron et al., 2006) and they thus join the physiological ranks of the more extensively studied arbuscular mycorrhizal and ectomycorrhizal associations.
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Genome-wide association mapping and agronomic impact of cowpea root architecture

Genome-wide association mapping and agronomic impact of cowpea root architecture | Plant roots and rhizosphere | Scoop.it
Key message

Genetic analysis of data produced by novel root phenotyping tools was used to establish relationships between cowpea root traits and performance indicators as well between root traits and Striga tolerance.
Abstract

Selection and breeding for better root phenotypes can improve acquisition of soil resources and hence crop production in marginal environments. We hypothesized that biologically relevant variation is measurable in cowpea root architecture. This study implemented manual phenotyping (shovelomics) and automated image phenotyping (DIRT) on a 189-entry diversity panel of cowpea to reveal biologically important variation and genome regions affecting root architecture phenes. Significant variation in root phenes was found and relatively high heritabilities were detected for root traits assessed manually (0.4 for nodulation and 0.8 for number of larger laterals) as well as repeatability traits phenotyped via DIRT (0.5 for a measure of root width and 0.3 for a measure of root tips). Genome-wide association study identified 11 significant quantitative trait loci (QTL) from manually scored root architecture traits and 21 QTL from root architecture traits phenotyped by DIRT image analysis. Subsequent comparisons of results from this root study with other field studies revealed QTL co-localizations between root traits and performance indicators including seed weight per plant, pod number, and Striga (Striga gesnerioides) tolerance. The data suggest selection for root phenotypes could be employed by breeding programs to improve production in multiple constraint environments.
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The nutrient preference of plants influences their rhizosphere microbiome

The nutrient preference of plants influences their rhizosphere microbiome | Plant roots and rhizosphere | Scoop.it
Highlights



Plant nutrient preference participates in determining plant rhizosphere microbiome.


Soil nitrite-N, available K and Mn, and total P and Fe are key factors.


Cucumber nutrient preference results in a low diversity of fungal community.


Tomato nutrient preference leads to a low diversity of bacterial community.

Abstract

Many studies in recent decades have shown the signature effect of the host plant in determining the plant-associated microbiome in the soil. However, the important question as to the factors contributing to the selective enrichment of microorganisms in the plant rhizosphere has not been fully addressed. In this study, the role of the nutrient preferences of two plant species, tomato and cucumber, in variations in the soil microbiome were investigated using a five-season continuous pot experiment. The results of MiSeq sequencing showed that these two plants assembled specific bacterial and fungal communities in their rhizospheres, and the soil nutrient status resulting from the plant nutrient preference was identified as a key driver in the development of a plant-specific microbiome.
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Low levels of strigolactones in roots as a component of the systemic signal of drought stress in tomato - Visentin - 2016 - New Phytologist -

Low levels of strigolactones in roots as a component of the systemic signal of drought stress in tomato - Visentin - 2016 - New Phytologist - | Plant roots and rhizosphere | Scoop.it
Strigolactones (SL) contribute to drought acclimatization in shoots, because SL-depleted plants are hypersensitive to drought due to stomatal hyposensitivity to abscisic acid (ABA). However, under drought, SL biosynthesis is repressed in roots, suggesting organ specificity in their metabolism and role. Because SL can be transported acropetally, such a drop may also affect shoots, as a systemic indication of stress.
We investigated this hypothesis by analysing molecularly and physiologically wild-type (WT) tomato (Solanum lycopersicum) scions grafted onto SL-depleted rootstocks, compared with self-grafted WT and SL-depleted genotypes, during a drought time-course.
Shoots receiving few SL from the roots behaved as if under mild stress even if irrigated. Their stomata were hypersensitive to ABA (likely via a localized enhancement of SL synthesis in shoots). Exogenous SL also enhanced stomata sensitivity to ABA.
As the partial shift of SL synthesis from roots to shoots mimics what happens under drought, a reduction of root-produced SL might represent a systemic signal unlinked from shootward ABA translocation, and sufficient to prime the plant for better stress avoidance.
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Too many partners in root–shoot signals. Does hydraulics qualify as the only signal that feeds back over time for reliable stomatal control? - Tardieu - 2016 - New Phytologist -

Too many partners in root–shoot signals. Does hydraulics qualify as the only signal that feeds back over time for reliable stomatal control? - Tardieu - 2016 - New Phytologist - | Plant roots and rhizosphere | Scoop.it
Chemical messages originating from roots, and able to contribute to stomatal control, have been the object of debate since the 1980s (Kramer, 1988). At that time, the dogma was that hydraulics control transpiration, but a series of experiments showed that it was not sufficient to explain stomatal behaviour under drought, at least in some cases (Schulze et al., 1988). Since then, several compounds have been shown to contribute to chemical messages from roots to shoots, namely abscisic acid (ABA), sap pH, precursors of ethylene and others, but hydraulic signals continue to be considered as essential. In this issue of New Phytologist, Visentin et al. (pp. 954–963) bring an important contribution to this debate and show that strigolactones (SLs) in roots are a component of the systemic signal in tomato.
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Frontiers | Root System Architecture and Abiotic Stress Tolerance: Current Knowledge in Root and Tuber Crops | Crop Science and Horticulture

Frontiers | Root System Architecture and Abiotic Stress Tolerance: Current Knowledge in Root and Tuber Crops | Crop Science and Horticulture | Plant roots and rhizosphere | Scoop.it
The challenge to produce more food for a rising global population on diminishing agricultural land is complicated by the effects of climate change on agricultural productivity. Although great progress has been made in crop improvement, so far most efforts have targeted above-ground traits. Roots are essential for plant adaptation and productivity, but are less studied due to the difficulty of observing them during the plant life cycle. Root system architecture (RSA), made up of structural features like root length, spread, number, and length of lateral roots, among others, exhibits great plasticity in response to environmental changes, and could be critical to developing crops with more efficient roots. Much of the research on root traits has thus far focused on the most common cereal crops and model plants. As cereal yields have reached their yield potential in some regions, understanding their root system may help overcome these plateaus. However, root and tuber crops (RTCs) such as potato, sweetpotato, cassava, and yam may hold more potential for providing food security in the future, and knowledge of their root system additionally focuses directly on the edible portion. Root-trait modeling for multiple stress scenarios, together with high-throughput phenotyping and genotyping techniques, robust databases, and data analytical pipelines, may provide a valuable base for a truly inclusive ‘green revolution.’ In the current review, we discuss RSA with special reference to RTCs, and how knowledge on genetics of RSA can be manipulated to improve their tolerance to abiotic stresses.
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