Regulation Of Root Hair Development Research Articles

A cotton APS REDUCTASE represses root hair elongation via sulfur assimilation and hydrogen peroxide‐mediated cell wall damage

AbstractSulfur is a critical element for plant growth and development, and it is assimilated through a pathway called the sulfur assimilation pathway. APS REDUCTASE (APR) is a key rate‐limiting enzyme in this pathway, and it plays a vital role in regulating sulfur flow and plant development. However, the exact role and mechanism of APR in cotton fiber and root hair development are still not fully understood. This study used in vitro cultured ovules grown on a sulfur‐deficient medium to explore the role of sulfur in fiber development. Genetic approaches such as virus‐induced gene silencing (VIGS) was used to determine the role of target genes. In vitro assays revealed the role of sulfur in fiber development and identified GhAPR2 as a key candidate gene. The GhAPR2 negatively regulates fiber cell initiation and Arabidopsis root hair elongation. Further studies demonstrated that overexpression of GhAPR2 increased the contents of sulfite and GSH to enhance sulfur assimilation in Arabidopsis, leading to abnormal accumulation of ROS in root hair. This down‐regulates the transcription of proline‐rich cell wall protein‐coding genes such as AtEXTs and AtPRPs, reducing the content of hydroxyproline in the cell wall, disrupting the development of the root hair cell wall, and inhibiting root hair elongation. This study elucidated the functional pathway of APR2 in the regulation of root hair development. APR2 first affects sulfur assimilation, which affects the content of reactive oxygen species (ROS), and finally affects the content of hydroxyproline‐rich glycoprotein, which in turn affects the development of root hair. This model also contributes to the understanding of the role of sulfur in fiber development, as well as cell differentiation and development.

Arabidopsis ABRE-binding factors modulate salinity-induced inhibition of root hair growth by interacting with and suppressing RHD6

Soil salinity causes crop losses worldwide. Root hairs are the primary targets of salt stress, however, the signaling networks involved in the precise regulation of root hair growth and development by salinity are poorly understood. Here, we confirmed that salt stress inhibits the number and length of root hairs in Arabidopsis. We found that the master regulator of root hair development and growth, the RHD6 transcription factor, is involved in this process, as salt treatment largely compromised root hair overaccumulation in RHD6-overexpressing plants. Yeast-two-hybrid and co-immunoprecipitation analyses revealed that RHD6 physically interacts with ABF proteins, the master transcription factors in abscisic acid signaling, which is involved in tolerance to several stresses including salinity. Phenotypic analyses showed that ABF proteins, which function upstream of RHD6, positively modulate the salinity-induced inhibition of root hair development. Further analyses showed that ABF3 suppresses the transcriptional activation activity of RHD6, thereby regulating the expression of genes related to root hair development. Overexpression of ABF3 reduced the root hair-overgrowing phenotype of RHD6-overexpressing plants. Collectively, our results demonstrate an essential signaling module in which ABF proteins directly suppress the transcriptional activation activity of RHD6 to reduce the length and number of root hairs under salt stress conditions.

APOLO lncRNA, a self-calibrating switch of root development

APOLO lncRNA, a self-calibrating switch of root development

RSL Class II Transcription Factors Guide the Nuclear Localization of RHL1 to Regulate Root Hair Development.

Root hairs are important for absorption of nutrients and water from the rhizosphere. The Root Hair Defective-Six Like (RSL) Class II family of transcription factors is expressed preferentially in root hairs and has a conserved role in root hair development in land plants. We functionally characterized the seven members of the RSL Class II subfamily in the rice (Oryza sativa) genome. In root hairs, six of these genes were preferentially expressed and four were strongly expressed. Phenotypic analysis of each mutant revealed that Os07g39940 plays a major role in root hair formation, based on observations of a short root hair phenotype in those mutants. Overexpression (OX) for each of four family members in rice resulted in an increase in the density and length of root hairs. These four members contain a transcription activation domain and are targeted to the nucleus. They interact with rice Root Hairless1 (OsRHL1), a key regulator of root hair development. When heterologously expressed in epidermal cells of Nicotiana benthamiana leaves, OsRHL1 was predominantly localized to the cytoplasm. When coexpressed with each of the four RSL Class II members, however, OsRLH1 was translocated to the nucleus. Transcriptome analysis using Os07g39940-OX plants revealed that 86 genes, including Class III peroxidases, were highly up-regulated. Furthermore, reactive oxygen species levels in the root hairs were increased in Os07g39940-OX plants but were drastically reduced in the os07g39940 and rhl1 mutants. Our results demonstrate that RSL Class II members function as essential regulators of root hair development in rice.

Arabidopsis FIM4 and FIM5 regulates the growth of root hairs in an auxin-insensitive way

ABSTRACTTip-growing cells provide a useful model system for studying the underlying mechanisms of plant cell growth. The apical growth of root hairs is dependent on the microfilament skeleton, and auxin is an important regulator of root hair development. We functionally characterized actin bundling proteins AtFIM4 and AtFIM5, which were preferentially expressed in tip-growing cells such as pollen tubes and root hairs. The morphology and length of root hairs in atfim4/atfim5 double mutant line had obvious defects. In addition, we found the growth of root hairs of atfim4/atfim5 double mutant was insensitive to exogenous IAA (indole-3-acetic acid) treatment. So we consider that AtFIM4 and AtFIM5 act together to regulate the growth of root hair in an auxin-insensitive way.

Adaptor proteins GIR1 and GIR2. II. Interaction with the co-repressor TOPLESS and promotion of histone deacetylation of target chromatin

Understanding how root hair development is controlled is important for understanding of many fundamental aspects of plant biology. Previously, we identified two plant-specific adaptor proteins GIR1 and GIR2 that interact with the major regulator of root hair development GL2 and suppress formation of root hair. Here, we show that GIR1 and GIR2 also interact with the co-repressor TOPLESS (TPL). This interaction required the GIR1 protein EAR motif, and was essential for the transcriptional repressor activity of GIR1. Both GIR1 and GIR2 promoted histone hypoacetylation of their target chromatin. Potentially, GIR1 and GIR2 might may link TPL to and participate in epigenetic regulation of root hair development.

Adaptor proteins GIR1 and GIR2. I. Interaction with the repressor GLABRA2 and regulation of root hair development

Plants use specialized root outgrowths, termed root hairs, to enhance acquisition of nutrients and water, help secure anchorage, and facilitate interactions with soil microbiome. One of the major regulators of this process is GLABRA2 (GL2), a transcriptional repressor of root hair differentiation. However, regulation of the GL2-function is relatively well characterized, it remains completely unknown whether GL2 itself functions in complex with other transcriptional regulators. We identified GIR1 and GIR2, a plant-specific two-member family of closely related proteins that interact with GL2. Loss-of-function mutants of GIR1 and GIR2 enhanced development of root hair whereas gain-of-function mutants repressed it. Thus, GIR1 and GIR2 might function as adaptor proteins that associate with GL2 and participate in control of root hair formation.

Complexity and specificity of the maize (Zea mays L.) root hair transcriptome

Root hairs are tubular extensions of epidermis cells. Transcriptome profiling demonstrated that the single cell-type root hair transcriptome was less complex than the transcriptome of multiple cell-type primary roots without root hairs. In total, 831 genes were exclusively and 5585 genes were preferentially expressed in root hairs [false discovery rate (FDR) ≤1%]. Among those, the most significantly enriched Gene Ontology (GO) functional terms were related to energy metabolism, highlighting the high energy demand for the development and function of root hairs. Subsequently, the maize homologs for 138 Arabidopsis genes known to be involved in root hair development were identified and their phylogenetic relationship and expression in root hairs were determined. This study indicated that the genetic regulation of root hair development in Arabidopsis and maize is controlled by common genes, but also shows differences which need to be dissected in future genetic experiments. Finally, a maize root view of the eFP browser was implemented including the root hair transcriptome of the present study and several previously published maize root transcriptome data sets. The eFP browser provides color-coded expression levels for these root types and tissues for any gene of interest, thus providing a novel resource to study gene expression and function in maize roots.

Plasma membrane H(+)-ATPase is involved in methyl jasmonate-induced root hair formation in lettuce (Lactuca sativa L.) seedlings.

Our results show that methyl jasmonate induces plasma membrane H (+) -ATPase activity and subsequently influences the apoplastic pH of trichoblasts to maintain a cell wall pH environment appropriate for root hair development. Root hairs, which arise from root epidermal cells, are tubular structures that increase the efficiency of water absorption and nutrient uptake. Plant hormones are critical regulators of root hair development. In this study, we investigated the regulatory role of the plasma membrane (PM) H(+)-ATPase in methyl jasmonate (MeJA)-induced root hair formation. We found that MeJA had a pronounced effect on the promotion of root hair formation in lettuce seedlings, but that this effect was blocked by the PM H(+)-ATPase inhibitor vanadate. Furthermore, MeJA treatment increased PM H(+)-ATPase activity in parallel with H(+) efflux from the root tips of lettuce seedlings and rhizosphere acidification. Our results also showed that MeJA-induced root hair formation was accompanied by hydrogen peroxide accumulation. The apoplastic acidification acted in concert with reactive oxygen species to modulate root hair formation. Our results suggest that the effect of MeJA on root hair formation is mediated by modulation of PM H(+)-ATPase activity.

Interaction between MYC2 and ETHYLENE INSENSITIVE3 Modulates Antagonism between Jasmonate and Ethylene Signaling inArabidopsis

Plants have evolved sophisticated mechanisms for integration of endogenous and exogenous signals to adapt to the changing environment. Both the phytohormones jasmonate (JA) and ethylene (ET) regulate plant growth, development, and defense. In addition to synergistic regulation of root hair development and resistance to necrotrophic fungi, JA and ET act antagonistically to regulate gene expression, apical hook curvature, and plant defense against insect attack. However, the molecular mechanism for such antagonism between JA and ET signaling remains unclear. Here, we demonstrate that interaction between the JA-activated transcription factor MYC2 and the ET-stabilized transcription factor ETHYLENE-INSENSITIVE3 (EIN3) modulates JA and ET signaling antagonism in Arabidopsis thaliana. MYC2 interacts with EIN3 to attenuate the transcriptional activity of EIN3 and repress ET-enhanced apical hook curvature. Conversely, EIN3 interacts with and represses MYC2 to inhibit JA-induced expression of wound-responsive genes and herbivory-inducible genes and to attenuate JA-regulated plant defense against generalist herbivores. Coordinated regulation of plant responses in both antagonistic and synergistic manners would help plants adapt to fluctuating environments.

The protein expression landscape of the Arabidopsis root

Because proteins are the major functional components of cells, knowledge of their cellular localization is crucial to gaining an understanding of the biology of multicellular organisms. We have generated a protein expression map of the Arabidopsis root providing the identity and cell type-specific localization of nearly 2,000 proteins. Grouping proteins into functional categories revealed unique cellular functions and identified cell type-specific biomarkers. Cellular colocalization provided support for numerous protein–protein interactions. With a binary comparison, we found that RNA and protein expression profiles are weakly correlated. We then performed peak integration at cell type-specific resolution and found an improved correlation with transcriptome data using continuous values. We performed GeLC-MS/MS (in-gel tryptic digestion followed by liquid chromatography-tandem mass spectrometry) proteomic experiments on mutants with ectopic and no root hairs, providing complementary proteomic data. Finally, among our root hair-specific proteins we identified two unique regulators of root hair development.

Single MYB-type transcription factor AtCAPRICE: A new efficient tool to engineer the production of anthocyanin in tobacco

The single MYB-type protein, CAPRICE has long been thought to be a positive regulator of root hair development in Arabidopsis thaliana. To further probe the role of AtCPC and how conserved this role is across plant species, the AtCPC gene was constitutively overexpressed in tobacco ( Nicotiana tabacum). Here, we show that flowers of transformants overexpressing AtCPC displayed an unexpected defection in pigmentation. A reduction in the level of anthocyanin was detected and the severity of abnormal flavonoid phenotype correlates well with the increased expression level of AtCPC gene. We also found that transcripts of three late biosynthetic genes were substantially down-regulated. Furthermore, AtCPC could interact with other known anthocyanin regulators. Our data suggest that in a heterologous host, AtCPC acts as a repressor of anthocyanin production, from which a new evidence for better understanding of the transcriptional regulation of flavonoid biosynthesis and, an implication for the development of new varieties of tobacco and other commercially important ornamental plants are provided.

Interactions between jasmonates and ethylene in the regulation of root hair development in Arabidopsis

Root hair formation is an important model with which to study cell patterning and differentiation in higher plants. Ethylene and auxin are critical regulators of root hair development. The role of jasmonates (JAs) was examined in Arabidopsis root hair development as well as their interactions with ethylene in this process. The results have shown that both methyl jasmonate (MeJA) and jasmonic acid (JA) have a pronounced effect on promoting root hair formation. However, the effect of MeJA and JA on root hair formation was blocked by ethylene inhibitors Ag+ or aminoethoxyvinylglycine (AVG). The stimulatory effects of MeJA and JA were also diminished in ethylene-insensitive mutants etr1-1 and etr1-3. Furthermore, the JA biosynthesis inhibitors ibuprofen and salicylhydroxamic acid (SHAM) suppressed 1-aminocyclopropane-1-carboxylic acid (ACC)-induced root hair formation, and decreased the root hairs in seedlings of the ethylene over-producing mutant eto1-1. These results suggested that JAs promote root hair formation, through an interaction with ethylene.

Nitric Oxide Functions as a Positive Regulator of Root Hair Development

The root epidermis is composed by two cell types: trichoblasts (or hair cells) and atrichoblasts (or non-hair cells). In lettuce (Lactuca sativa cv. Grand Rapids var. Rapidmor oscura) plants grown hydroponically in water, the root epidermis did not form root hairs. The addition of 10 µM sodium nitroprusside (SNP), a nitric oxide (NO) donor, resulted in almost all rhizodermal cells differentiated into root hairs. Treatment with the synthetic auxin 1-naphthyl acetic acid (NAA) displayed a significant increase of root hair formation (RHF) that was prevented by the specific NO scavenger carboxy-PTIO (cPTIO). In Arabidopsis, two mutants have been shown to be defective in NO production and to display altered phenotypes in which NO is implicated. Arabidopsis nos1 has a mutation in an NO synthase structural gene (NOS1), and the nia1 nia2 double mutant is null for nitrate reductase (NR) activity. We observed that both mutants were affected in their capacity of developing root hairs. Root hair elongation was significantly reduced in nos1 and nia1 nia2 mutants as well as in cPTIO-treated wild type plants. A correlation was found between endogenous NO level in roots detected by the fluorescent probe DAF-FM DA and RHF. In Arabidopsis, as well as in lettuce, cPTIO blocked the NAA-induced root hair elongation. Taken together, these results indicate that: (i) NO is a critical molecule in the process leading to RHF, and (ii) NO is involved in the auxin-signaling cascade leading to RHF.

Cell-to-cell movement of the CAPRICE protein inArabidopsisroot epidermal cell differentiation

CAPRICE (CPC), a small, R3-type Myb-like protein, is a positive regulator of root hair development in Arabidopsis. Cell-to-cell movement of CPC is important for the differentiation of epidermal cells into trichoblasts (root hair cells). CPC is transported from atrichoblasts (hairless cells), where it is expressed, to trichoblasts, and generally accumulates in their nuclei. Using truncated versions of CPC fused to GFP, we identified a signal domain that is necessary and sufficient for CPC cell-to-cell movement. This domain includes the N-terminal region and a part of the Myb domain. Amino acid substitution experiments indicated that W76 and M78 in the Myb domain are critical for targeted transport, and that W76 is crucial for the nuclear accumulation of CPC:GFP. To evaluate the tissue-specificity of CPC movement, CPC:GFP was expressed in the stele using the SHR promoter and in trichoblasts using the EGL3 promoter. CPC:GFP was able to move from trichoblasts to atrichoblasts but could not exit from the stele, suggesting the involvement of tissue-specific regulatory factors in the intercellular movement of CPC. Analyses with a secretion inhibitor, Brefeldin A, and with an rhd3 mutant defective in the secretion process in root epidermis suggested that intercellular CPC movement is mediated through plasmodesmata. Furthermore, the fusion of CPC to tandem-GFPs defined the capability of CPC to increase the size exclusion limit of plasmodesmata.

Ethylene and phosphorus availability have interacting yet distinct effects on root hair development.

The hypothesis that ethylene participates in the regulation of root hair development by phosphorus availability in Arabidopsis thaliana was tested by chemically manipulating ethylene synthesis and response and with ethylene-insensitive mutants. Low phosphorus-induced root hair development could be mimicked by adding the ethylene precursor, 1-aminocyclopropane-1-carboxylate (ACC), to high phosphorus media, and inhibited by adding ethylene inhibitors to low phosphorus media. Ethylene-insensitive mutants showed a reduced response to low phosphorus, indicating ethylene involvement in root hair responses to phosphorus deficiency. To dissect the nature of this involvement, the morphological and anatomical changes associated with increased root hair density were investigated. Growth in low phosphorus resulted in smaller, more numerous cortical cells, resulting in a larger number of root hair-bearing epidermal cell files. Cortical cell number was not affected by ethylene inhibitors, ACC, or mutations reducing ethylene sensitivity in roots grown with low phosphorus, indicating that ethylene does not participate in this response. The exception was the eir1 mutation, which strongly reduced this change in radial anatomy, supporting a role for polar auxin transport in this process. Trichoblast cell length was reduced by low phosphorus availability in all genotypes, but even more so for ein2-1 and ein4. The proportion of epidermal cells forming hairs and root hair length were reduced in ethylene-insensitive mutants, especially in the presence of low phosphorus. These results demonstrate multiple effects of low phosphorus from the earliest stages of root hair development, and cross-talk between ethylene and phosphorus in the control of a subset of the low phosphorus effects, concentrating on those later in development.

The control of root hair formation: suggested mechanisms

Little is known about the physiological regulation of root hair development. Though the literature on the topic is fragmentary, with many gaps, it has been used to put forward some proposals for a central regulatory mechanism, taking note especially of the strong physiological influence of environmental factors on root hair formation. A hypothesis which implicates ethylene (ET) in this mechanism is proposed. This hypothesis is supported by numerous indications and observations in the literature about the effect of ET on cell wall formation and cell wall properties during the phase of root expansion and hair formation. It is assumed that environmental conditions control the effective ET concentration in epidermal hair-forming cells; they influence root development and ET production, while, through affecting the rhizosphere, they modify gas exchange at the root surface, accelerating removal of ET thereby limiting its function in hair formation. In such a way the effects of various specific factors (e.g. soil temperature, bulk density, gas exchange at the root surface or the effect of nutrients) might be manifestations of a central regulatory mechanism. The hope is that this hypothesis will stimulate further research and some suggestions are made as to the direction such research might take.

Genetic Regulation of Root Hair Development in Arabidopsis Thaliana: A Network Model

The root epidermis of Arabidopsis thaliana is formed by alternate files of hair and non-hair cells. Epidermal cells overlying two cortex cells eventually develop a hair, while those overlying only one cortex cell do not. Here we propose a network model that integrates most of the available genetic and molecular data on the regulatory and signaling pathways underlying root epidermal differentiation. The network architecture includes two pathways; one formed by the genesTTG , R homolog, GL2 and CPC, and the other one by the signal transduction proteins ETR1 and CTR1. Both parallel pathways regulate the activity of AXR2 and RHD6, which in turn control the development of root hairs. The regulatory network was simulated as a dynamical system of eight discrete state variables. The distinction between epidermal cells contacting one or two cortical cells was accounted for by fixing the initial states of CPC and ETR1 proteins. The model allows for predictions of mutants and pharmacological effects because it includes the ethylene receptor. The dynamical system reaches one of the six stable states depending upon the initial state of the CPC variable and the ethylene receptor. Two of the stable states describe the activation patterns observed in mature trichoblasts (hair cells) and atrichoblasts (non-hair cells) in the wild-type phenotype and under normal ethylene availability. The other four states correspond to changes in the number of hair cells due to experimentally induced changes in ethylene availability. This model provides a hypothesis on the interactions among genes that encode transcription factors that regulate root hair development and the proteins involved in the ethylene transduction pathway. This is the first effort to use a dynamical system to understand the complex genetic regulatory interactions that ruleArabidopsis primary root development. The advantages of this type of models over static schematic representations are discussed.

The role of ethylene in the development of plant form

Ethylene is a gaseous growth factor involved in a diverse array of cellular, developmental and stress-related processes in plants. A number of examples of the role played by ethylene in the development of form in plants are described; reaction wood formation, floral induction, sex determination, flooding-induced shoot elongation, and leaf abscission. Recent advances in the understanding of the molecular mechanism underpinning post-pollination perianth wilting in orchids is reviewed. This study indicates that the process of post-pollination perianth wilting involves an early increase in sensitivity to endogenous levels of ethylene which set in motion a chain of events in which ethylene autocatalytically induces its own synthesis in the pistil. Ethylene also induces the expression of ACO in the perianth which converts pistil-derived ACC into ethylene which drives the wilting process. Concepts drawn from this system are then applied to the Arabidopsis root epidermis in which ethylene is a positive regulator of root hair development in an effort to come to a mechanistic understanding of the process of pattern formation in this system. Understanding the molecular basis of the role of ethylene in these model systems will provide useful paradigms for examining the part played by ethylene in the diverse array of processes in which this growth factor is involved.

The Arabidopsis Athb‐10 (GLABRA2) is an HD‐Zip protein required for regulation of root hair development

Homeodomain-leucine zipper (HD-Zip) proteins are putative transcription factors identified only in plants. Related Arabidopsis homeobox genes, isolated by virtue of sequence conservation within the helix-3 region of the homeodomain, fall into four families based on sequence similarity. This paper reports the characterization of Athb-10, a 747 amino acid protein belonging to the fourth HD-ZIP family. The studies indicate that, although less conserved, the leucine zipper of Athb-10 can functionally replace that of Athb-2 in an in vitro DNA-binding assay. Gene mapping experiments and sequence comparison analysis revealed that Athb-10 corresponds to GLABRA2, a homeodomain protein involved in trichome development. The mRNA expression analysis revealed that Athb-10/GLABRA2 is expressed not only in trichome-bearing organs, but also in the root. The analysis of wild-type and mutant plants showed that the Athb-10/GLABRA2 gene expression in the aerial part of the plant and in the root is affected by mutations at the TTG locus. Morphological analysis of the g/2-1 mutant revealed that the gene is necessary not only for local outgrowth of the trichome, but also for the regulation of root hair development in a subset of epidermal cells. Interestingly, the development of root hair cells in a position normally occupied by non-hair cells is dependent upon the ethylene regime in which the gl2-1 plants are grown. Sequence analysis of the gl2-1 allele revealed that the mutant gene encodes a truncated protein that might still retain a partial activity responsible for the formation of aborted trichomes and for the ethylene-dependent regulation of root hair formation.