Primary Root Meristem Research Articles

Strigolactone signaling in root development and phosphate starvation

Strigolactones (SLs), have recently been recognized as phytohormone involve in orchestrating shoot and root architecture. In, roots SLs positively regulate root hair length and density, suppress lateral root formation and promote primary root meristem cell number. The biosynthesis and exudation of SLs increases under low phosphate level to regulate root responses. This hormonal response suggests an adaptation strategy of plant to optimize growth and development under nutrient limitations. However, little is known on signal-transduction pathways associated with SL activities. In this review, we outline the current knowledge on SL biology by describing their role in the regulation of root development. Also, we discuss the recent findings on the non-cell autonomous signaling of SLs, that involve PIN polarization, vesicle trafficking, changes in actin architecture and dynamic in response to phosphate starvation.

The CLE40 and CRN/CLV2 Signaling Pathways Antagonistically Control Root Meristem Growth in Arabidopsis

Differentiation processes in the primary root meristem are controlled by several signaling pathways that are regulated by phytohormones or by secreted peptides. Long-term maintenance of an active root meristem requires that the generation of new stem cells and the loss of these from the meristem due to differentiation are precisely coordinated. Via phenotypic and large-scale transcriptome analyses of mutants, we show that the signaling peptide CLE40 and the receptor proteins CLV2 and CRN act in two genetically separable pathways that antagonistically regulate cell differentiation in the proximal root meristem. CLE40 inhibits cell differentiation throughout the primary root meristem by controlling genes with roles in abscisic acid, auxin, and cytokinin signaling. CRN and CLV2 jointly control target genes that promote cell differentiation specifically in the transition zone of the proximal root meristem. While CRN and CLV2 are not acting in the CLE40 signaling pathway under normal growth conditions, both proteins are required when the levels of CLE40 or related CLE peptides increase. We show here that two antagonistically acting pathways controlling root meristem differentiation can be activated by the same peptide in a dosage-dependent manner.

MAIL1 is essential for development of the primary root but not of anchor roots

MAIN-LIKE1 (MAIL1) is a ubiquitously expressed nuclear protein, which has a crucial function during root development. We have recently described loss of function mutants for MAIL1, in which the organization and function of the primary root meristem is lost soon after germination. Moreover cell differentiation is impaired resulting in primary root growth arrest soon after emergence. Here we show that mail1 mutants form several anchor roots from the hypocotyl to root junction. These anchor roots show similar defects in the organization of the stem cell niche as the primary root. In contrast, differentiation processes are not impaired and thus anchor roots seem to be able to compensate for the loss of primary root function. Our data show that MAIL1 is essential for specification of cell fate in the primary root but not in anchor roots.

Long Noncoding RNA Modulates Alternative Splicing Regulators in Arabidopsis

Alternative splicing (AS) of pre-mRNA represents a major mechanism underlying increased transcriptome and proteome complexity. Here, we show that the nuclear speckle RNA-binding protein (NSR) and the AS competitor long noncoding RNA (or ASCO-lncRNA) constitute an AS regulatory module. AtNSR-GFP translational fusions are expressed in primary and lateral root (LR) meristems. Double Atnsr mutants and ASCO overexpressors exhibit an altered ability to form LRs after auxin treatment. Interestingly, auxin induces a major change in AS patterns of many genes, a response largely dependent on NSRs. RNA immunoprecipitation assays demonstrate that AtNSRs interact not only with their alternatively spliced mRNA targets but also with the ASCO-RNA in vivo. The ASCO-RNA displaces an AS target from an NSR-containing complex in vitro. Expression of ASCO-RNA in Arabidopsis affects the splicing patterns of several NSR-regulated mRNA targets. Hence, lncRNA can hijack nuclear AS regulators to modulate AS patterns during development.

Spatiotemporal changes in the role of cytokinin during root development

The root is a dynamic system whose structure is regulated by a complex network of interactions between hormones. The primary root meristem is specified in the embryo. After germination, the primary root meristem grows and then reaches a final size that will be maintained during the life of the plant. Subsequently, secondary structures such as lateral roots and root nodules form via the re-specification of differentiated cells. Cytokinin plays key roles in the regulation of root development. Down-regulation of the cytokinin response is required for the specification of a new stem cell niche, during both embryo and lateral root development. In the root meristem, cytokinin signalling regulates the longitudinal zonation of the meristem by controlling cell differentiation. Moreover, cytokinin regulates radial patterning of root vasculature by promoting protophloem cell identity and by spatially inhibiting protoxylem formation. In this review, an effort is made to describe the known details of the role of cytokinin during root development, taking into account also the interactions between cytokinin and other hormones. Attention is given on the dynamicity of cytokinin signalling output during different developmental events. Indeed, there is much evidence that the effects of cytokinin change as organs grow, underlining the importance of the spatiotemporal specificity of cytokinin signalling.

Unveiling Signaling Events in Root Responses to Strigolactone

Strigolactones (SLs), present in a wide variety of plant species, have been recently recognized as plant hormones. In the shoot, they suppress the outgrowth of preformed axillary buds (reviewed by Brewer et al., 2013 ); they are negative regulators of adventitious root formation ( Rasmussen et al., 2012 ) and positive regulators of shoot secondary growth ( Agusti et al., 2011 ). In roots, SLs positively regulate root-hair length, suppress lateral root formation ( Kapulnik et al., 2011 ), and promote cell number in the primary root meristem ( Ruyter-Spira et al., 2011 ). However, little is known on the signal-transduction pathways associated with SL activities. Here, we focus on roots and on the recent advances in understanding their SL-related signal transduction.

Impact of spatial constraints during seed germination on the evolution of angiosperm cotyledons: A case study from tropical Hydatellaceae (Nymphaeales)

A bipolar embryo with cotyledons is a characteristic feature that appeared early in the evolution of seed plants. Cotyledon number is an important character in angiosperm classification. We explore the links between functional aspects of seed germination and the number and location of the cotyledons, using as a model the early-divergent angiosperm family Hydatellaceae, in which seedlings are superficially monocot-like. • Seedlings of two species of tropical Hydatellaceae were studied using light and scanning electron microscopy. • Seedlings of Trithuria cowieana bear two free cotyledons. Each cotyledon possesses a green, filiform, vascularized blade that resembles subsequent leaves, and a basal, nonvascularized, haustorial outgrowth that remains in close contact with the endosperm. Seedlings of Trithuria konkanensis have two free cotyledonary haustoria inserted close to each other and a leaf blade probably belonging to one of the cotyledons. The cotyledonary node elongates between the haustoria and the leaf blade to form a mesocotyl. • To date, the absence or presence of a cotyledonary tube represents the only known qualitative morphological difference between the two major clades of Hydatellaceae. Cotyledons with a haustorium and leaf blade are unusual at the scale of seed plants and probably evolved due to homeosis. The mesocotyl of T. konkanensis resembles that of grasses and sedges. Seedling diversity in Hydatellaceae and other seed plants is linked with the principal physical and spatial constraint of their embryo structure, with the primary root and shoot apical meristems located at opposite poles, and haustorial cotyledon tips.

Overexpression of Arabidopsis Plasmodesmata Germin-Like Proteins Disrupts Root Growth and Development

In plants, a population of non-cell-autonomous proteins (NCAPs), including numerous transcription factors, move cell to cell through plasmodesmata (PD). In many cases, the intercellular trafficking of these NCAPs is regulated by their interaction with specific PD components. To gain further insight into the functions of this NCAP pathway, coimmunoprecipitation experiments were performed on a tobacco (Nicotiana tabacum) plasmodesmal-enriched cell wall protein preparation using as bait the NCAP, pumpkin (Cucurbita maxima) PHLOEM PROTEIN16 (Cm-PP16). A Cm-PP16 interaction partner, Nt-PLASMODESMAL GERMIN-LIKE PROTEIN1 (Nt-PDGLP1) was identified and shown to be a PD-located component. Arabidopsis thaliana putative orthologs, PDGLP1 and PDGLP2, were identified; expression studies indicated that, postgermination, these proteins were preferentially expressed in the root system. The PDGLP1 signal peptide was shown to function in localization to the PD by a novel mechanism involving the endoplasmic reticulum-Golgi secretory pathway. Overexpression of various tagged versions altered root meristem function, leading to reduced primary root but enhanced lateral root growth. This effect on root growth was corrected with an inability of these chimeric proteins to form stable PD-localized complexes. PDGLP1 and PDGLP2 appear to be involved in regulating primary root growth by controlling phloem-mediated allocation of resources between the primary and lateral root meristems.

Heat reduces nitric oxide production required for auxin-mediated gene expression and fate determination in tree tobacco guard cell protoplasts.

Tree tobacco (Nicotiana glauca) is an equatorial perennial with a high basal thermotolerance. Cultured tree tobacco guard cell protoplasts (GCPs) are useful for studying the effects of heat stress on fate-determining hormonal signaling. At lower temperatures (32°C or less), exogenous auxin (1-naphthalene acetic acid) and cytokinin (6-benzylaminopurine) cause GCPs to expand 20- to 30-fold, regenerate cell walls, dedifferentiate, reenter the cell cycle, and divide. At higher temperatures (34°C or greater), GCPs expand only 5- to 6-fold; they do not regenerate walls, dedifferentiate, reenter the cell cycle, or divide. Heat (38°C) suppresses activation of the BA auxin-responsive transgene promoter in tree tobacco GCPs, suggesting that inhibition of cell expansion and cell cycle reentry at high temperatures is due to suppressed auxin signaling. Nitric oxide (NO) has been implicated in auxin signaling in other plant systems. Here, we show that heat inhibits NO accumulation by GCPs and that L-N(G)-monomethyl arginine, an inhibitor of NO production in animals and plants, mimics the effects of heat by limiting cell expansion and preventing cell wall regeneration; inhibiting cell cycle reentry, dedifferentiation, and cell division; and suppressing activation of the BA auxin-responsive promoter. We also show that heat and L-N(G)-monomethyl arginine reduce the mitotic indices of primary root meristems and inhibit lateral root elongation similarly. These data link reduced NO levels to suppressed auxin signaling in heat-stressed cells and seedlings of thermotolerant plants and suggest that even plants that have evolved to withstand sustained high temperatures may still be negatively impacted by heat stress.

A cellular expression map of the Arabidopsis AUXIN RESPONSE FACTOR gene family

The plant hormone auxin triggers a wide range of developmental and growth responses throughout a plant's life. Most well-known auxin responses involve changes in gene expression that are mediated by a short pathway involving an auxin-receptor/ubiquitin-ligase, DNA-binding auxin response factor (ARF) transcription factors and their interacting auxin/indole-3-acetic acid (Aux/IAA) transcriptional inhibitors. Auxin promotes the degradation of Aux/IAA proteins through the auxin receptor and hence releases the inhibition of ARF transcription factors. Although this generic mechanism is now well understood, it is still unclear how developmental specificity is generated and how individual gene family members of response components contribute to local auxin responses. We have established a collection of transcriptional reporters for the ARF gene family and used these to generate a map of expression during embryogenesis and in the primary root meristem. Our results demonstrate that transcriptional regulation of ARF genes generates a complex pattern of overlapping activities. Genetic analysis shows that functions of co-expressed ARFs converge on the same biological processes, but can act either antagonistically or synergistically. Importantly, the existence of an 'ARF pre-pattern' could explain how cell-type-specific auxin responses are generated. Furthermore, this resource can now be used to probe the functions of ARF in other auxin-dependent processes.

Image cytometric determination of nuclear and chromosomal DNA contents in Allium cepa L.

Estimation of DNA content in individual chro- mosomes and their arms was carried out in the primary root meristems of Allium cepa L. var Local, (2n=16) using an Image Analysis System following Feulgen staining. Re- gression analysis revealed that fifth and seventh chromo- somes had higher DNA contents relative to their chromosomal lengths while the 1st, 2nd and 4th chromo- somes had less DNA contents than those expected on the basis of length.

A Versatile Set of Ligation-Independent Cloning Vectors for Functional Studies in Plants

With plant molecular biology entering the omics era, there is a need for simple cloning strategies that allow high throughput to systematically study the expression and function of large numbers of genes. Such strategies would facilitate the analysis of gene (sub)families and/or sets of coexpressed genes identified by transcriptomics. Here, we provide a set of 34 ligation-independent cloning (LIC) binary vectors for expression analysis, protein localization studies, and misexpression that will be made freely available. This set of plant LIC vectors offers a fast alternative to standard cloning strategies involving ligase or recombination enzyme technology. We demonstrate the use of this strategy and our new vectors by analyzing the expression domains of genes belonging to two subclades of the basic helix-loop-helix transcription factor family. We show that neither the closest homologs of TARGET OF MONOPTEROS7 (TMO7/ATBS1) nor the members of the ATBS1 INTERACTING FACTOR subclade of putative TMO7 interactors are expressed in the embryo and that there is very limited coexpression in the primary root meristem. This suggests that these basic helix-loop-helix transcription factors are most likely not involved in TMO7-dependent root meristem initiation.

Global expression pattern comparison between low phosphorus insensitive 4 and WT Arabidopsis reveals an important role of reactive oxygen species and jasmonic acid in the root tip response to phosphate starvation

Plants are exposed to several biotic and abiotic stresses. A common environmental stress that plants have to face both in natural and agricultural ecosystems that impacts both its growth and development is low phosphate (Pi) availability. There has been an important progress in the knowledge of the molecular mechanisms by which plants cope with Pi deficiency. However, the mechanisms that mediate alterations in the architecture of the Arabidopsis root system responses to Pi starvation are still largely unknown. One of the most conspicuous developmental effects of low Pi on the Arabidopsis root system is the inhibition of primary root growth that is accompanied by loss of root meristematic activity. To identify signalling pathways potentially involved in the Arabidpsis root meristem response to Pi-deprivation, here we report the global gene expression analysis of the root tip of wild type and low phosphorus insensitive4 (lpi4) mutant grown under Pi limiting conditions. Differential gene expression analysis and physiological experiments show that changes in the redox status, probably mediated by jasmonic acid and ethylene, play an important role in the primary root meristem exhaustion process triggered by Pi-starvation.

ABA promotes quiescence of the quiescent centre and suppresses stem cell differentiation in the Arabidopsis primary root meristem

It is well known that abscisic acid (ABA) can halt meristems for long periods without loss of meristem function, and can also promote root growth at low concentrations, but the mechanisms underlying such regulation are largely unknown. Here we show that ABA promotes stem cell maintenance in Arabidopsis root meristems by both promoting the quiescence of the quiescent centre (QC) and suppressing the differentiation of stem cells and their daughters. We demonstrate that these two mechanisms of regulation by ABA involve distinct pathways, and identify components in each pathway. Our findings demonstrate a cellular mechanism for a positive role for ABA in promoting root meristem maintenance and root growth in Arabidopsis.

Environmental Regulation of Lateral Root Emergence inMedicago truncatulaRequires the HD-Zip I Transcription Factor HB1

The adaptation of root architecture to environmental constraints is a major agricultural trait, notably in legumes, the third main crop worldwide. This root developmental plasticity depends on the formation of lateral roots (LRs) emerging from primary roots. In the model legume Medicago truncatula, the HD-Zip I transcription factor HB1 is expressed in primary and lateral root meristems and induced by salt stress. Constitutive expression of HB1 in M. truncatula roots alters their architecture, whereas hb1 TILLING mutants showed increased lateral root emergence. Electrophoretic mobility shift assay, promoter mutagenesis, and chromatin immunoprecipitation-PCR assays revealed that HB1 directly recognizes a CAATAATTG cis-element present in the promoter of a LOB-like (for Lateral Organ Boundaries) gene, LBD1, transcriptionally regulated by auxin. Expression of these genes in response to abscisic acid and auxin and their behavior in hb1 mutants revealed an HB1-mediated repression of LBD1 acting during LR emergence. M. truncatula HB1 regulates an adaptive developmental response to minimize the root surface exposed to adverse environmental stresses.

MONOPTEROS controls embryonic root initiation by regulating a mobile transcription factor

Acquisition of cell identity in plants relies strongly on positional information, hence cell-cell communication and inductive signalling are instrumental for developmental patterning. During Arabidopsis embryogenesis, an extra-embryonic cell is specified to become the founder cell of the primary root meristem, hypophysis, in response to signals from adjacent embryonic cells. The auxin-dependent transcription factor MONOPTEROS (MP) drives hypophysis specification by promoting transport of the hormone auxin from the embryo to the hypophysis precursor. However, auxin accumulation is not sufficient for hypophysis specification, indicating that additional MP-dependent signals are required. Here we describe the microarray-based isolation of MP target genes that mediate signalling from embryo to hypophysis. Of three direct transcriptional target genes, TARGET OF MP 5 (TMO5) and TMO7 encode basic helix-loop-helix (bHLH) transcription factors that are expressed in the hypophysis-adjacent embryo cells, and are required and partially sufficient for MP-dependent root initiation. Importantly, the small TMO7 transcription factor moves from its site of synthesis in the embryo to the hypophysis precursor, thus representing a novel MP-dependent intercellular signal in embryonic root specification.

A putative transporter is essential for integrating nutrient and hormone signaling with lateral root growth and nodule development inMedicago truncatula

Legume root architecture involves not only elaboration of the root system by the formation of lateral roots but also the formation of symbiotic root nodules in association with nitrogen-fixing soil rhizobia. The Medicago truncatula LATD/NIP gene plays an essential role in the development of both primary and lateral roots as well as nodule development. We have cloned the LATD/NIP gene and show that it encodes a member of the NRT1(PTR) transporter family. LATD/NIP is expressed throughout the plant. pLATD/NIP-GFP promoter-reporter fusions in transgenic roots establish the spatial expression of LATD/NIP in primary root, lateral root and nodule meristems and the surrounding cells. Expression of LATD/NIP is regulated by hormones, in particular by abscisic acid which has been previously shown to rescue the primary and lateral root meristem arrest of latd mutants. latd mutants respond normally to ammonium but have defects in responses of the root architecture to nitrate. Taken together, these results suggest that LATD/NIP may encode a nitrate transporter or transporter of another compound.

A role for redox factors in shaping root architecture under Phosphorus deficiency

The developmental response of the Arabidopsis root system to low phosphorus (P) availability involves the reduction in primary root elongation accompanied by the formation of numerous lateral roots. We studied the roles of selected redox metabolites, namely, radical oxygen species (ROS) and ascorbic acid (ASC) in the regulation of root system architecture by different P availability. Rapidly growing roots of plants grown on P-sufficient medium synthesize ROS in root elongation zone and quiescent centre. We have demonstrated that the arrest of root elongation at low P medium coincides with the disappearance of ROS from the elongation zone. P-starvation resulted in a decrease in ascorbic acid level in roots. This correlated with a decrease in cell division activity. On the other hand, feeding P-deficient plants with ASC, stimulated mitotic activity in the primary root meristem and partly reversed the inhibition of root growth imposed by low P conditions. In this paper, we discuss the idea of the involvement of redox agents in the regulation of root system architecture under low P availability.

Arabidopsis plant homeodomain finger proteins operate downstream of auxin accumulation in specifying the vasculature and primary root meristem

In Arabidopsis thaliana, auxin is a key regulator of tissue patterning in the developing embryo. We have identified a group of proteins that act downstream of auxin accumulation in auxin-mediated root and vascular development in the embryo. Combined mutations in OBERON1 (OBE1) and OBERON2 (OBE2) give rise to obe1 obe2 double mutant seedlings that closely phenocopy the monopteros (mp) mutant phenotype, with an absence of roots and defective development of the vasculature. We show that, in contrast to the situation in mp mutants, obe1 obe2 double mutant embryos show auxin maxima at the root pole and in the provascular region, and that the SCF(TIR1) pathway, which translates auxin accumulation into transcriptional activation of auxin-responsive genes, remains intact. Although we focus on the impact of obe mutations on aspects of embryo development, the effect of such mutations on a broad range of auxin-related gene expression and the tissue expression patterns of OBE genes in seedlings suggest that OBE proteins have a wider role to play in growth and development. We suggest that OBE1 and OBE2 most likely control the transcription of genes required for auxin responses through the action of their PHD finger domains.

Reactive oxygen species localization in roots of Arabidopsis thaliana seedlings grown under phosphate deficiency

Arabidopsis plants responding to phosphorus (P) deficiency increase lateral root formation and reduce primary root elongation. In addition the number and length of root hairs increases in response to P deficiency. Here we studied the patterns of radical oxygen species (ROS) in the roots of Arabidopsis seedlings cultured on media supplemented with high or low P concentration. We found that P availability affected ROS distribution in the apical part of roots. If plants were grown on high P medium, ROS were located in the root elongation zone and quiescent centre. At low P ROS were absent in the elongation zone, however, their synthesis was detected in the primary root meristem. The proximal part of roots was characterized by ROS production in the lateral root primordia and in elongation zones of young lateral roots irrespective of P concentration in the medium. On the other hand, plants grown at high or low P differed in the pattern of ROS distribution in older lateral roots. At high P, the elongation zone was the primary site of ROS production. At low P, ROS were not detected in the elongation zone. However, they were present in the proximal part of the lateral root meristem. These results suggest that P deficiency affects ROS distribution in distal parts of Arabidopsis roots. Under P-sufficiency ROS maximum was observed in the elongation zone, under low P, ROS were not synthesized in this segment of the root, however, they were detected in the apical root meristem.