PHYTOCHROME A controls the DNA damage response and cell death tolerance within the Arabidopsis root meristem.

The DNA damage response avoids mutations into dividing cells. Here, we analysed the role of photoreceptors on the restriction of root growth imposed by genotoxic agents and its relationship with cell viability and performance of meristems. Comparison of root growth of Arabidopsis WT, phyA-211, phyB-9, and phyA-211phyB-9 double mutants unveiled a critical role for phytochrome A (PhyA) in protecting roots from genotoxic stress, regeneration and cell replenishment in the meristematic zone. PhyA was located on primary root tips, where it influences genes related to the repair of DNA, including ERF115 and RAD51. Interestingly, phyA-211 mutants treated with zeocin failed to induce the expression of the repressor of cell cycle MYB3R3, which correlated with expression of the mitotic cyclin CycB1, suggesting that PhyA is required for safeguarding the DNA integrity during cell division. Moreover, the growth of the primary roots of PhyA downstream component HY5 and root growth analyses in darkness suggest that cell viability and DNA damage responses within root meristems may act independently from light and photomorphogenesis. These data support novel roles for PhyA as a key player for stem cell niche maintenance and DNA damage responses, which are critical for proper root growth.

Blood Serum Stimulates the Virulence Potential of Mucorales through Enhancement in Mitochondrial Oxidative Metabolism and Rhizoferrin Production.

This study analyzed the role of blood serum in enhancing the mitochondrial metabolism and virulence of Mucorales through rhizoferrin secretion. We observed that the spores of clinically relevant Mucorales produced in the presence of serum exhibited higher virulence in a heterologous infection model of Galleria mellonella. Cell-free supernatants of the culture broth obtained from spores produced in serum showed increased toxicity against Caenorhabditis elegans, which was linked with the enhanced secretion of rhizoferrin. Spores from Mucoralean species produced or germinated in serum showed increased respiration rates and reactive oxygen species levels. The addition of non-lethal concentrations of potassium cyanide and N-acetylcysteine during the aerobic or anaerobic growth of Mucorales decreased the toxicity of the cell-free supernatants of the culture broth, suggesting that mitochondrial metabolism is important for serum-induced virulence. In support of this hypothesis, a mutant strain of Mucor lusitanicus that lacks fermentation and solely relies on oxidative metabolism exhibited virulence levels comparable to those of the wild-type strain under serum-induced conditions. Contrary to the lower virulence observed, even in the serum, the ADP-ribosylation factor-like 2 deletion strain exhibited decreased mitochondrial activity. Moreover, spores produced in the serum of M. lusitanicus and Rhizopus arrhizus that grew in the presence of a mitophagy inducer showed low virulence. These results suggest that serum-induced mitochondrial activity increases rhizoferrin levels, making Mucorales more virulent.

Loss-of-function of MEDIATOR 12 or 13 subunits causes the swelling of root hairs in response to sucrose and abscisic acid in Arabidopsis.

Root hairs are epidermal cell extensions that increase the root surface for water and nutrient acquisition. Thus, both the initiation and elongation of root hairs are critical for soil exploration and plant adaptation to ever changing growth conditions. Here, we describe the critical roles of two subunits of the Mediator complex, MED12 and MED13, in root hair growth in response to sucrose and abscisic acid, which are tightly linked to abiotic stress resistance. When compared to the WT, med12 and med13 mutants showed increased sensitivity to sucrose and ABA treatments on root meristem and elongation zones that were accompanied with alterations in root hair length and morphology, leading to the isodiametric growth of these structures. The swollen root hair phenotype appeared to be specific, since med8 or med16 mutants did not develop rounded hairs when supplied with 4.8% sucrose. Under standard growth medium, MED12 and MED13 were mainly expressed in root vascular tissues and cotyledons, and their expression was repressed by sucrose or ABA. Interestingly, med12 and med13 mutants manifested exacerbated levels of nitric oxide under normal growth conditions, and upon sucrose supplementation in trichoblast cells, which coincided with root hair deformation. Our results indicate that MED12 and MED13 play non-redundant functions for maintenance of root hair integrity in response to sucrose and ABA and involve nitric oxide as a cellular messenger in Arabidopsis thaliana.

Role of the nonribosomal peptide synthetase siderophore enzyme (Rfs) of Mucor lusitanicus in controlling the growth of fungal phytopathogens.

Dimorphic species of Mucor, which are cosmopolitan fungi belonging to subphylum Mucoromycotina, are metabolically versatile. Some species of Mucor are sources of biotechnological products, such as biodiesel from Mucor circinelloides and expression of heterologous proteins from Mucor lusitanicus. Furthermore, Mucor lusitanicus has been described as a model for understanding mucormycosis infections. However, little is known regarding the relationship between Mucor lusitanicus and other soil inhabitants. In this study, we investigated the potential use of Mucor lusitanicus as a biocontrol agent against fungal phytopathogens, namely Fusarium oxysporum f. sp. lycopersici, Fusarium solani, and Alternaria solani, which destroy economically important crops. Results showed that aerobic cell-free supernatants of the culture broth (SS) from Mucor lusitanicus inhibited the growth of the fungal phytopathogens in culture, soil, and tomato fruits. The SS obtained from a strain of Mucor lusitanicus carrying the deletion of rfs gene, which encodes an enzyme involved in the synthesis of siderophore rhizoferrin, had a decreased inhibitory effect against the growth of the phytopathogens. Contrarily, this inhibitory effect was more evident with the SS from an rfs-overexpressing strain compared to the wild-type. This study provides a framework for the potential biotechnological use of the molecules secreted from Mucor lusitanicus in the biocontrol of fungal phytopathogens.

MITOGEN-ACTIVATED PROTEIN KINASE PHOSPHATASE 1 mediates root sensing of serotonin through jasmonic acid signaling and modulating reactive oxygen species.

Serotonin (5-hydroxytryptamine) acts as a neurotransmitter in mammals and is widely distributed in the plant kingdom, where it influences root growth and defense. Mitogen-Activated Protein Kinases (MAPKs) and MAPK phosphatases (MKPs) play critical functions in decoding hormonal signalling, but their possible roles in mediating serotonin responses await investigation. In this report, we unveiled positive roles for the MITOGEN-ACTIVATED PROTEIN KINASE PHOSPHATASE1 (MKP1) in the inhibition of the primary root growth, cell division, meristem structure, and differentiation events in Arabidopsis seedlings. mkp1 mutants were less sensitive to jasmonic acid applications that halted primary root growth in wild-type (WT) plants, and consistently, the neurotransmitter activated the expression of the JASMONATE ZIM-domain (JAZ) proteins JAZ1 and JAZ10, two critical proteins orchestrating jasmonic acid signalling. This effect correlated with exacerbated production of endogenous reactive oxygen species (ROS) in the WT, a process constitutively manifested in mkp1 mutants. These data help to clarify the relationship between serotonin and growth/defense trade-offs, and reveal the importance of the MAPK pathway in root development through ROS production.

The RNA polymerase II subunit NRPB2 is required for indeterminate root development, cell viability, stem cell niche maintenance, and de novo root tip regeneration in Arabidopsis.

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.

Mutation of MEDIATOR16 promotes plant biomass accumulation and root growth by modulating auxin signaling.

The MEDIATOR complex influences the transcription of genes acting as a RNA pol II co-activator. The MED16 subunit has been related to low phosphate sensing in roots, but how it influences the overall plant growth and root development remains unknown. In this study, we compared the root growth of Arabidopsis wild-type (WT), and two alleles of MED16 (med16-2 and med16-3) mutants in vitro. The MED16 loss-of-function seedlings showed longer primary roots with higher cell division capacity of meristematic cells, and an increased number of lateral roots than WT plants, which correlated with improved biomass accumulation. The auxin response reported by DR5:GFP fluorescence was comparable in WT and med16-2 root tips, but strongly decreased in pericycle cells and lateral root primordia in the mutants. Dose-response analysis supplementing indole-3-acetic acid (IAA), or the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA), indicated normal responses to auxin in the med16-2 and med16-3 mutants regarding primary root growth and lateral root formation, but strong resistance to NPA in primary roots, which could be correlated with cell division and elongation. Expression analysis of pPIN1::PIN1::GFP, pPIN3::PIN3::GFP, pIAA14:GUS, pIAA28:GUS and 35S:MED16-GFP suggests that MED16 could mediate auxin signaling. Our data imply that an altered auxin response in the med16 mutants is not necessarily deleterious for overall growth and developmental patterning and may instead directly regulate basic cellular programmes.

Micrococcus luteus LS570 promotes root branching in Arabidopsis via decreasing apical dominance of the primary root and an enhanced auxin response.

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.

N,N-dimethyl-hexadecylamine modulates Arabidopsis root growth through modifying the balance between stem cell niche and jasmonic acid-dependent gene expression.

N,N-dimethyl-hexadecylamine (DMHDA) is released as part of volatile blends emitted by plant probiotic bacteria and affects root architecture, defense and nutrition of plants. Here, we investigated the changes in gene expression of transcription factors responsible of maintenance of the root stem cell niche and jasmonic acid signaling in Arabidopsis seedlings in response to this volatile. Concentrations of DMHDA that repress primary root growth were found to alter cell size and division augmenting cell tissue layers in the meristem and causing root widening. DMHDA triggered the division of quiescent center cells, which correlated with repression of SHORT ROOT (SHR), SCARECROW (SCR), and PLETHORA 1 (PLT1) proteins and induction of WUSCHEL-RELATED HOMEOBOX 5 (WOX5) transcription factor. Interestingly, an activation of the expression of the jasmonic acid-related reporter genes JAZ1/TIFY10A-GFP and JAZ10pro::JAZ10-GFP suggests that the halted growth of the primary root inversely correlated with expression patterns underlying the defense reaction, which may be of adaptive importance to protect roots against biotic stress. Our data help to unravel the gene expression signatures upon sensing of a highly active bacterial volatile in Arabidopsis seedlings.

Trichoderma atroviride-emitted volatiles improve growth of Arabidopsis seedlings through modulation of sucrose transport and metabolism.

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.

The bacterial volatile N,N-dimethyl-hexadecylamine promotes Arabidopsis primary root elongation through cytokinin signaling and the AHK2 receptor.

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.

MEDIATOR16 orchestrates local and systemic responses to phosphate scarcity in Arabidopsis roots.

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.

Two Oleosins Expressed in the Mesocarp of Native Mexican Avocado, Key Genes in the Oil Content.

Intracellular lipid droplets (LD) provide the oil storage mechanism of plants. They are found within seeds as individual structures, even under conditions of cold stress and dehydration, due to the protein that covers them. This protein, called oleosin, is found exclusively in plants and has been widely studied in seeds. Avocado fruits (Persea americana Mill.) are rich in oil, which is stored in the mesocarp, not in the seeds. The presence of oleosin in the mesocarp tissue of avocadoes has been reported, but its physiological role is still unknown. In this study, we identify two genes that code for oleosin in the mesocarp of the native Mexican avocado. These sequences are very different from those of seed oleosins. Both genes are expressed during fruit ripening, while one, PaOle1, has the highest expression in the green fruit stage. The protein of PaOle1 is stable during the fruit ripening process and covers all the mesocarp LDs. The expression of PaOle1 gene and protein is organ specific to avocado mesocarp. Among avocadoes varieties oleosin abundance is directly related to oil content.

The growth of Arabidopsis primary root is repressed by several and diverse amino acids through auxin-dependent and independent mechanisms and MPK6 kinase activity.

Amino acids serve as structural monomers for protein synthesis and are considered important biostimulants for plants. In this report, the effects of all 20-L amino acids in Arabidopsis primary root growth were evaluated. 15 amino acids inhibited growth, being l-leucine (l-Leu), l-lysine (l-Lys), l-tryptophan (l-Trp), and l-glutamate (l-Glu) the most active, which repressed both cell division and elongation in primary roots. Comparisons of DR5:GFP expression and growth of WT Arabidopsis seedlings and several auxin response mutants including slr, axr1 and axr2 single mutants, arf7/arf19 double mutant and tir1/afb2/afb3 triple mutant, treated with inhibitory concentrations of l-Glu, l-Leu, l-Lys and l-Trp revealed gene-dependent, specific changes in auxin response. In addition, l- isomers of Glu, Leu and Lys, but not l-Trp diminished the GFP fluorescence of pPIN1::PIN1:GFP, pPIN2::PIN2:GFP, pPIN3::PIN3:GFP and pPIN7::PIN7:GFP constructs in root tips. MPK6 activity in roots was enhanced by amino acid treatment, being greater in response to l-Trp while mpk6 mutants supported cell division and elongation at high doses of l-Glu, l-Leu, l-Lys and l-Trp. We conclude that independently of their auxin modulating properties, amino acids signals converge in MPK6 to alter the Arabidopsis primary root growth.

Mutation of MEDIATOR 18 and chromate trigger twinning of the primary root meristem in Arabidopsis.

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.

YUCCA4 overexpression modulates auxin biosynthesis and transport and influences plant growth and development via crosstalk with abscisic acid in Arabidopsis thaliana.

Auxin regulates a plethora of events during plant growth and development, acting in concert with other phytohormones. YUCCA genes encode flavin monooxygenases that function in tryptophan-dependent auxin biosynthesis. To understand the contribution of the YUCCA4 (YUC4) gene on auxin homeostasis, plant growth and interaction with abscisic acid (ABA) signaling, 35S::YUC4 seedlings were generated, which showed elongated hypocotyls with hyponastic leaves and changes in root system architecture that correlate with enhanced auxin responsive gene expression. Differential expression of PIN1, 2, 3 and 7 auxin transporters was detected in roots of YUC4 overexpressing seedlings compared to the wild-type: PIN1 was down-regulated whereas PIN2, PIN3 and PIN7 were up-regulated. Noteworthy, 35S::YUC4 lines showed enhanced sensitivity to ABA on seed germination and post-embryonic root growth, involving ABI4 transcription factor. The auxin reporter genes DR5::GUS, DR5::GFP and BA3::GUS further revealed that abscisic acid impairs auxin responses in 35S::YUC4 seedlings. Our results indicate that YUC4 overexpression influences several aspects of auxin homeostasis and reveal the critical roles of ABI4 during auxin-ABA interaction in germination and primary root growth.

ALTERED MERISTEM PROGRAM 1 promotes growth and biomass accumulation influencing guard cell aperture and photosynthetic efficiency in Arabidopsis.

ALTERED MERISTEM PROGRAM 1 (AMP1) encodes a putative glutamate-carboxypeptidase important for plant growth and development. In this study, by comparing the growth of Arabidopsis wild-type, amp1-10 and amp1-13 mutants, and AMP1-GFP/OX2- and AMP1-GFP/OX7-overexpressing seedlings in vitro and in soil, we uncover the role of AMP1 in biomass accumulation in Arabidopsis. AMP1-overexpressing plants had longer primary roots and increased lateral root number and density than the WT, which correlated with improved root, shoot, and total biomass accumulation. AMP1-overexpressing seedlings had an enhanced rate of growth of primary roots, and accordingly, sucrose supplementation restored primary root growth and promoted lateral root formation in amp1 mutants, while reproductive development, fruit size, and seed content were also modified according to disruption or overexpression of AMP1. We further found that AMP1 plays an important role for stomatal development, guard cell functioning, and carbon assimilation. These data help explain the pleiotropic functions of AMP1 in both root and shoot system development, possibly acting in a sugar-dependent manner for regulation of root architecture, biomass accumulation, and seed production.

CONSTITUTIVE TRIPLE RESPONSE1 and PIN2 act in a coordinate manner to support the indeterminate root growth and meristem cell proliferating activity in Arabidopsis seedlings.

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.

The bacterial volatile dimethyl-hexa-decylamine reveals an antagonistic interaction between jasmonic acid and cytokinin in controlling primary root growth of Arabidopsis seedlings.

Chemical communication underlies major adaptive traits in plants and shapes the root microbiome. An increasing number of diffusible and/or volatile organic compounds released by bacteria have been identified, which play phytostimulant or protective functions, including dimethyl-hexa-decylamine (DMHDA), a volatile biosynthesized by Arthrobacter agilis UMCV2 that induces jasmonic acid (JA) signaling in Arabidopsis. Here, he found that the growth repressing effects of both DMHDA and JA are antagonized by kinetin and correlated with an inhibition of cytokinin-related ARR5::GUS and TCS::GFP expression in Arabidopsis primary roots. Moreover, we demonstrate that shoot supplementation of JA triggers JAZ1 expression both locally and systemically and represses cytokinin-dependent promoter activity in roots. A similar effect was observed after cotyledon wounding, in which an increase of JA-inducible LOX2:GUS expression represses root growth, which correlates with the loss of TCS::GFP detection at the very root tip. Our data demonstrate that the bacterial volatile DMHDA crosstalks with cytokinin signaling and reveals the downstream antagonistic interaction between JA and cytokinin in controlling root growth.

Mitogen activated protein kinase 6 and MAP kinase phosphatase 1 are involved in the response of Arabidopsis roots to L-glutamate.

KEY MESSAGE: The function and components of L-glutamate signaling pathways in plants have just begun to be elucidated. Here, using a combination of genetic and biochemical strategies, we demonstrated that a MAPK module is involved in the control of root developmental responses to this amino acid. Root system architecture plays an essential role in plant adaptation to biotic and abiotic factors via adjusting signal transduction and gene expression. L-Glutamate (L-Glu), an amino acid with neurotransmitter functions in animals, inhibits root growth, but the underlying genetic mechanisms are poorly understood. Through a combination of genetic analysis, in-gel kinase assays, detailed cell elongation and division measurements and confocal analysis of expression of auxin, quiescent center and stem cell niche related genes, the critical roles of L-Glu in primary root growth acting through the mitogen-activated protein kinase 6 (MPK6) and the dual specificity serine-threonine-tyrosine phosphatase MKP1 could be revealed. In-gel phosphorylation assays revealed a rapid and dose-dependent induction of MPK6 and MPK3 activities in wild-type Arabidopsis seedlings in response to L-Glu. Mutations in MPK6 or MKP1 reduced or increased root cell division and elongation in response to L-Glu, possibly modulating auxin transport and/or response, but in a PLETHORA1 and 2 independent manner. Our data highlight MPK6 and MKP1 as components of an L-Glu pathway linking the auxin response, and cell division for primary root growth.