Root Hair Development Research Articles

The MYB transcription factor TaPHR3-A1 is involved in phosphate signaling and governs yield-related traits in bread wheat.

Improved inorganic phosphate (Pi) use efficiency in crops will be important for sustainable agriculture. Exploring molecular mechanisms that regulate Pi uptake could provide useful information for breeding wheat with improved Pi use efficiency. Here, a TaPHR3-A1 (Gene ID: TraesCS7A02G415800) ortholog of rice OsPHR3 that functions in transcriptional regulation of Pi signaling was cloned from wheat chromosome 7A. Ectopic expression of TaPHR3-A1 in Arabidopsis and rice produced enhanced vegetative growth and more seeds. Overexpression in transgenic rice led to increased biomass, grain number, and primary panicle branching by 61.23, 42.12, and 36.34% compared with the wild type. Transgenic wheat lines with down-regulation of TaPHR3-A1 exhibited retarded growth and root hair development at the seedling stage, and showed yield-related effects at the adult stage when grown in both low- and sufficient Pi conditions, indicating that TaPHR3-A1 positively regulated tolerance to low Pi. Introgression lines further confirmed the effect of TaPHR3-A1 in improving grain number. The Chinese wheat mini core collection and a recombinant inbred line analysis demonstrated that the favorable allele TaPHR3-A1-A associated with higher grain number was positively selected in breeding. A TaPHR3-A1-derived cleaved amplified polymorphic sequence marker effectively identified haplotype TaPHR3-A1-A. Our results suggested that TaPHR3-A1 was a functional regulatory factor for Pi uptake and provided useful information for marker-assisted selection for high yield in wheat.

Heterologous overexpression of the GbTCP5 gene increased root hair length, root hair and stem trichome density, and lignin content in transgenic Arabidopsis.

Teosinte branched1/cycloidea/proliferating cell factor1 (TCP) is a plant-specific protein family member involved in plant growth and development. However, the functions of most members of the cotton TCP family are unknown. In this study, the GbTCP5 gene encodes a sea-island cotton class II TCP CIN subclass transcription factor. The GbTCP5 transcription factor is located in the nucleus, has transcriptional activation activity, and can bind to TCP II cis-acting elements. GbTCP5 was widely expressed in tissues with the highest transcript level in the calyx. GbTCP5 is expressed at different developmental stages of the fiber and has significantly high transcriptional level expression in the fibers at 20, 30 and 35days post anthesis (DPA). Heterologous overexpression of the GbTCP5 gene increased root hair length, root hair and stem trichome density, and stem lignin content in transgenic Arabidopsis compared to the wild type (WT). GbTCP5 binds the promoters of the GL3, EGL3, CPC, MYB46, LBD30, CesA4, VND7, CCOMT1, and CAD5 genes to upregulate their expression. Moreover, the homologous genes of these genes are expressed in the fibers of different developmental stages of the sea-island cotton fiber. These results indicate that GbTCP5 regulates root hair development and secondary wall formation in Arabidopsis and may be a candidate gene for improving cotton fiber quality.

Acyl-Homoserine Lactone from Plant-Associated Pseudomonas sp. Influences Solanum lycopersicum Germination and Root Growth.

It is known that plant and associated bacteria coevolved, but just now the roles of chemical signaling compounds in these intricate relationships have been systematically studied. Many Gram-negative bacteria produce N-acyl-L-homoserine lactones (AHL), chemical signals used in quorum-sensing bacterial communications mechanisms. In recent years, it has been shown that these compounds may also influence the development of plants, acting as allelochemicals, in still not well understood eukaryot-prokaryot interactions. In the present work, a quorum-sensing molecule produced by the tomato associated bacterium Pseudomonas sp. was characterized and its effects on germination and growth of tomato seedlings were accessed. The chemical study of the bacterium extract led to the identification of N-3-oxo-dodecanoyl-L-homoserine lactone (1), using gas chromatography coupled to electron impact mass spectrometry (GC-MS), and ultra-high resolution Qq-time-of-flight mass spectrometry (UHR-QqTOF-MS) equipped with an electrospray ionization source (ESI). The synthetic compound was tested at different concentrations in tomato to evaluate its effects on seed germination and seedlings root growth. Inhibition of tomato seed germination and root growth were observed in the presence of micromolar concentrations of the compound 1. Scanning electron microscopy evidenced morphological alterations on roots in the presence of thecompound, with reduction of growth, impaired root hairs development and cracks in the rhizodermis.

MYB Superfamily in Brassica napus: Evidence for Hormone-Mediated Expression Profiles, Large Expansion, and Functions in Root Hair Development.

MYB proteins are involved in diverse important biological processes in plants. Herein, we obtained the MYB superfamily from the allotetraploid Brassica napus, which contains 227 MYB-related (BnMYBR/Bn1R-MYB), 429 R2R3-MYB (Bn2R-MYB), 22 R1R2R3-MYB (Bn3R-MYB), and two R1R2R2R1/2-MYB (Bn4R-MYB) genes. Phylogenetic analysis classified the Bn2R-MYBs into 43 subfamilies, and the BnMYBRs into five subfamilies. Sequence characteristics and exon/intron structures within each subfamily of the Bn2R-MYBs and BnMYBRs were highly conserved. The whole superfamily was unevenly distributed on 19 chromosomes and underwent unbalanced expansion in B. napus. Allopolyploidy between B. oleracea and B. rapa mainly contributed to the expansion in their descendent B. napus, in which B. rapa-derived genes were more retained. Comparative phylogenetic analysis of 2R-MYB proteins from nine Brassicaceae and seven non-Brassicaceae species identified five Brassicaceae-specific subfamilies and five subfamilies that are lacking from the examined Brassicaceae species, which provided an example for the adaptive evolution of the 2R-MYB gene family alongside angiosperm diversification. Ectopic expression of four Bn2R-MYBs under the control of the viral CaMV35S and/or native promoters could rescue the lesser root hair phenotype of the Arabidopsis thaliana wer mutant plants, proving the conserved negative roles of the 2R-MYBs of the S15 subfamily in root hair development. RNA-sequencing data revealed that the Bn2R-MYBs and BnMYBRs had diverse transcript profiles in roots in response to the treatments with various hormones. Our findings provide valuable information for further functional characterizations of B. napus MYB genes.

Flavonols regulate root hair development by modulating accumulation of reactive oxygen species in the root epidermis.

Reactive oxygen species (ROS) are signaling molecules produced by tissue-specific respiratory burst oxidase homolog (RBOH) enzymes to drive development. In Arabidopsis thaliana, ROS produced by RBOHC was previously reported to drive root hair elongation. We identified a specific role for one ROS, H2O2, in driving root hair initiation and demonstrated that localized synthesis of flavonol antioxidants control the level of H2O2 and root hair formation. Root hairs form from trichoblast cells that express RBOHC and have elevated H2O2 compared with adjacent atrichoblast cells that do not form root hairs. The flavonol-deficient tt4 mutant has elevated ROS in trichoblasts and elevated frequency of root hair formation compared with the wild type. The increases in ROS and root hairs in tt4 are reversed by genetic or chemical complementation. Auxin-induced root hair initiation and ROS accumulation were reduced in an rbohc mutant and increased in tt4, consistent with flavonols modulating ROS and auxin transport. These results support a model in which localized synthesis of RBOHC and flavonol antioxidants establish patterns of ROS accumulation that drive root hair formation.

Grapevine ABA receptor VvPYL1 regulates root hair development in Transgenic Arabidopsis

Root architecture is very important for plant growth. In this study, we characterized the process of root formation in grapevine (Vitis vinifera L.). Continuous observation of root morphology during development revealed that the establishment of root system could be divided into five stages: initial cultivation (stage I), preliminary development (stage II), even change (stage III), root system formation (stage IV), and root architecture stability (stage V). The level of abscisic acid (ABA) increased from stages II to IV and was stable at stage V. Quantitative expression analysis of 11 genes encoding ABA-related rate-limiting enzymes in different tissues showed that the expression of VvPYL1 was the highest in roots. Spatiotemporal expression analysis showed that VvPYL1 was highly expressed during stages II and III. Furthermore, VvPYL1 was highly expressed in lateral roots of grapevine seedlings in tissue culture. Overexpression of VvPYL1 in Arabidopsis thaliana resulted in longer root hairs compared with wild-type plants. Moreover, the root hair length of transgenic lines was hypersensitive to exogenously applied ABA. Additionally, VvPYL1 overexpressing plants showed greater drought tolerance and longer root hairs than wild-type plants under osmotic stress. These results suggest that VvPYL1 may play a key role in root development and drought resistance.

A phosphorylation-based switch controls TAA1-mediated auxin biosynthesis in plants

Auxin determines the developmental fate of plant tissues, and local auxin concentration is precisely controlled. The role of auxin transport in modulating local auxin concentration has been widely studied but the regulation of local auxin biosynthesis is less well understood. Here, we show that TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA1), a key enzyme in the auxin biosynthesis pathway in Arabidopsis thaliana is phosphorylated at Threonine 101 (T101). T101 phosphorylation status can act as an on/off switch to control TAA1-dependent auxin biosynthesis and is required for proper regulation of root meristem size and root hair development. This phosphosite is evolutionarily conserved suggesting post-translational regulation of auxin biosynthesis may be a general phenomenon. In addition, we show that auxin itself, in part via TRANS-MEMBRANE KINASE 4 (TMK4), can induce T101 phosphorylation of TAA1 suggesting a self-regulatory loop whereby local auxin signalling can suppress biosynthesis. We conclude that phosphorylation-dependent control of TAA1 enzymatic activity may contribute to regulation of auxin concentration in response to endogenous and/or external cues.

Hormonal regulation of root hair growth and responses to the environment in Arabidopsis.

The main functions of plant roots are water and nutrient uptake, soil anchorage, and interaction with soil-living biota. Root hairs, single cell tubular extensions of root epidermal cells, facilitate or enhance these functions by drastically enlarging the absorptive surface. Root hair development is constantly adapted to changes in the root's surroundings, allowing for optimization of root functionality in heterogeneous soil environments. The underlying molecular pathway is the result of a complex interplay between position-dependent signalling and feedback loops. Phytohormone signalling interconnects this root hair signalling cascade with biotic and abiotic changes in the rhizosphere, enabling dynamic hormone-driven changes in root hair growth, density, length, and morphology. This review critically discusses the influence of the major plant hormones on root hair development, and how changes in rhizosphere properties impact on the latter.

Arabidopsis JAZ Proteins Interact with and Suppress RHD6 Transcription Factor to Regulate Jasmonate-Stimulated Root Hair Development.

Root hairs arise from trichoblasts and are crucial for plant anchorage, nutrient acquisition, and environmental interactions. The phytohormone jasmonate is known to regulate root hair development in Arabidopsis (Arabidopsis thaliana), but little is known about the molecular mechanism underlying jasmonate modulation in this process. Here, we show that the application of exogenous jasmonate significantly stimulated root hair elongation, but, on the contrary, blocking the perception or signaling of jasmonate resulted in defective root hairs. Jasmonate consistently elevated the expression levels of several crucial genes positively involved in root hair growth. Mechanistic investigation revealed that JASMONATE ZIM-DOMAIN (JAZ) proteins, critical repressors of jasmonate signaling, physically interacted with ROOT HAIR DEFECTIVE 6 (RHD6) and RHD6 LIKE1 (RSL1), two transcription factors that are essential for root hair development. JAZ proteins inhibited the transcriptional function of RHD6 and interfered with the interaction of RHD6 with RSL1. Genetic analysis indicated that jasmonate promoted root hair growth in a RHD6/RSL1-dependent manner. Moreover, overexpression of RHD6 largely rescued the root hair defects of JAZ-accumulating plants. Collectively, our study reveals a key signaling module in which JAZ repressors of the jasmonate pathway directly modulate RHD6 and RSL1 transcription factors to integrate jasmonate signaling and the root hair developmental process.

Phospholipase D‐derived phosphatidic acid promotes root hair development under phosphorus deficiency by suppressing vacuolar degradation of PIN‐FORMED2

Summary Root hair development is crucial for phosphate absorption, but how phosphorus deficiency affects root hair initiation and elongation remains unclear.We demonstrated the roles of auxin efflux carrier PIN‐FORMED2 (PIN2) and phospholipase D (PLD)‐derived phosphatidic acid (PA), a key signaling molecule, in promoting root hair development in Arabidopsis thaliana under a low phosphate (LP) condition.Root hair elongation under LP conditions was greatly suppressed in pin2 mutant or under treatment with a PLDζ2‐specific inhibitor, revealing that PIN2 and polar auxin transport and PLDζ2‐PA are crucial in LP responses. PIN2 was accumulated and degraded in the vacuole under a normal phosphate (NP) condition, whereas its vacuolar accumulation was suppressed under the LP or NP plus PA conditions. Vacuolar accumulation of PIN2 was increased in pldζ2 mutants under LP conditions. Increased or decreased PIN2 vacuolar accumulation is not observed in sorting nexin1 (snx1) mutant, indicating that vacuolar accumulation of PIN2 is mediated by SNX1 and the relevant trafficking process. PA binds to SNX1 and promotes its accumulation at the plasma membrane, especially under LP conditions, and hence promotes root hair development by suppressing the vacuolar degradation of PIN2.We uncovered a link between PLD‐derived PA and SNX1‐dependent vacuolar degradation of PIN2 in regulating root hair development under phosphorus deficiency.

Nitrate regulation of lateral root and root hair development in plants.

Nitrogen (N) is one of the most important macronutrients for plant growth and development. However, the concentration and distribution of N varies in soil due to a variety of environmental factors. In response, higher plants have evolved a developmentally flexible root system to efficiently take up N under N-limited conditions. Over the past decade, significant progress has been made in understanding this form of plant ‘root-foraging’ behavior, which is controlled by both a local and a long-distance systemic nitrate signaling pathway. In this review, we focus on the key components of nitrate perception, signaling, and transduction and its role in lateral root development. We also highlight recent findings on the molecular mechanisms of the nitrate systemic signaling pathway, including small signaling peptides involved in long-distance shoot–root communication. Furthermore, we summarize the transcription factor networks responsible for nitrate-dependent lateral root and root hair development.

Zinc finger protein 5 (ZFP5) associates with ethylene signaling to regulate the phosphate and potassium deficiency-induced root hair development in Arabidopsis.

Zinc finger protein transcription factor ZFP5 positively regulates root hair elongation in response to Pi and potassium deficiency by mainly activating the expression of EIN2 in Arabidopsis. Phosphate (Pi) and potassium (K+) are major plant nutrients required for plant growth and development, and plants respond to low-nutrient conditions via metabolic and morphology changes. The C2H2 transcription factor ZFP5 is a key regulator of trichome and root hair development in Arabidopsis. However, its role in regulating root hair development under nutrient deprivations remains unknown. Here, we show that Pi and potassium deficiency could not restore the short root hair phenotype of zfp5 mutant and ZFP5 RNAi lines to wild type level. The deprivation of either of these nutrients also induced the expression of ZFP5 and the activity of an ethylene reporter, pEBS:GUS. The significant reduction of root hair length in ein2-1 and ein3-1 as compared to wild-type under Pi and potassium deficiency supports the involvement of ethylene in root hair elongation. Furthermore, the application of 1-aminocyclopropane-1-carboxylic acid (ACC) significantly enhanced the expression level of ZFP5 while the application of 2-aminoethoxyvinyl glycine (AVG) had the opposite effect when either Pi or potassium was deprived. Further experiments reveal that ZFP5 mainly regulates transcription of ETHYLENE INSENSITIVE 2 (EIN2) to control deficiency-mediated root hair development through ethylene signaling. Generally, these results suggest that ZFP5 regulates root hair elongation by interacting with ethylene signaling mainly through regulates the expression of EIN2 in response to Pi and potassium deficiency in Arabidopsis.

Expansin gene loss is a common occurrence during adaptation to an aquatic environment.

Expansins comprise a superfamily of plant cell wall loosening proteins that can be divided into four individual families (EXPA, EXPB, EXLA and EXLB). Aside from inferred roles in a variety of plant growth and developmental traits, little is known regarding the function of specific expansin clades, for which there are at least 16 in flowering plants (angiosperms); however, there is evidence to suggest that some expansins have cell-specific functions, in root hair and pollen tube development, for example. Recently, two duckweed genomes have been sequenced (Spirodela polyrhiza strains 7498 and 9509), revealing significantly reduced superfamily sizes. We hypothesized that there would be a correlation between expansin loss and morphological reductions seen among highly adapted aquatic species. In order to provide an answer to this question, we characterized the expansin superfamilies of the greater duckweed Spirodela, the marine eelgrass Zostera marina and the bladderwort Utricularia gibba. We discovered rampant expansin gene and clade loss among the three, including a complete absence of the EXLB family and EXPA-VII. The most convincing correlation between morphological reduction and expansin loss was seen for Utricularia and Spirodela, which both lack root hairs and the root hair expansin clade EXPA-X. Contrary to the pattern observed in other species, four Utricularia expansins failed to branch within any clade, suggesting that they may be the result of neofunctionalization. Last, an expansin clade previously discovered only in eudicots was identified in Spirodela, allowing us to conclude that the last common ancestor of monocots and eudicots contained a minimum of 17 expansins.

Arabidopsis ZINC FINGER PROTEIN1 Acts Downstream of GL2 to Repress Root Hair Initiation and Elongation by Directly Suppressing bHLH Genes.

Cys2His2-like fold group (C2H2)-type zinc finger proteins promote root hair growth and development by regulating their target genes. However, little is known about their potential negative roles in root hair initiation and elongation. Here, we show that the C2H2-type zinc finger protein named ZINC FINGER PROTEIN1 (AtZP1), which contains an ERF-associated amphiphilic repression (EAR) motif, negatively regulates Arabidopsis (Arabidopsis thaliana) root hair initiation and elongation. Our results demonstrate that AtZP1 is highly expressed in root hairs and that AtZP1 inhibits transcriptional activity during root hair development. Plants overexpressing AtZP1 lacked root hairs, while loss-of-function mutants had longer and more numerous root hairs than the wild type. Transcriptome analysis indicated that AtZP1 downregulates genes encoding basic helix-loop-helix (bHLH) transcription factors associated with root hair cell differentiation and elongation. Mutation or deletion of the EAR motif substantially reduced the inhibitory activity of AtZP1. Chromatin immunoprecipitation assays, AtZP1:glucocorticoid receptor (GR) induction experiments, electrophoretic mobility shift assays, and yeast one-hybrid assays showed that AtZP1 directly targets the promoters of bHLH transcription factor genes, including the key root hair initiation gene ROOT HAIR DEFECTIVE6 (RHD6) and root hair elongation genes ROOT HAIR DEFECTIVE 6-LIKE 2 (RSL2) and RSL4, and suppresses root hair development. Our findings suggest that AtZP1 functions downstream of GL2 and negatively regulates root hair initiation and elongation, by suppressing RHD6, RSL4, and RSL2 transcription via the GL2/ZP1/RSL pathway.

Developmental Roles of AUX1/LAX Auxin Influx Carriers in Plants.

Plant hormone auxin regulates several aspects of plant growth and development. Auxin is predominantly synthesized in the shoot apex and developing leaf primordia and from there it is transported to the target tissues e.g. roots. Auxin transport is polar in nature and is carrier-mediated. AUXIN1/LIKE-AUX1 (AUX1/LAX) family members are the major auxin influx carriers whereas PIN-FORMED (PIN) family and some members of the P-GLYCOPROTEIN/ATP-BINDING CASSETTE B4 (PGP/ABCB) family are major auxin efflux carriers. AUX1/LAX auxin influx carriers are multi-membrane spanning transmembrane proteins sharing similarity to amino acid permeases. Mutations in AUX1/LAX genes result in auxin related developmental defects and have been implicated in regulating key plant processes including root and lateral root development, root gravitropism, root hair development, vascular patterning, seed germination, apical hook formation, leaf morphogenesis, phyllotactic patterning, female gametophyte development and embryo development. Recently AUX1 has also been implicated in regulating plant responses to abiotic stresses. This review summarizes our current understanding of the developmental roles of AUX1/LAX gene family and will also briefly discuss the modelling approaches that are providing new insight into the role of auxin transport in plant development.

Heterologous Expression of GbTCP4, a Class II TCP Transcription Factor, Regulates Trichome Formation and Root Hair Development in Arabidopsis.

Two class I family teosinte branched1/cycloidea/proliferating cell factor1 (TCP) proteins from allotetraploid cotton are involved in cotton fiber cell differentiation and elongation and root hair development. However, the biological function of most class II TCP proteins is unclear. This study sought to reveal the characteristics and functions of the sea-island cotton class II TCP gene GbTCP4 by biochemical, genetic, and molecular biology methods. GbTCP4 protein localizes to nuclei, binding two types of TCP-binding cis-acting elements, including the one in its promoter. Expression pattern analysis revealed that GbTCP4 is widely expressed in tissues, with the highest level in flowers. GbTCP4 is expressed at different fiber development stages and has high transcription in fibers beginning at 5 days post anthesis (DPA). GbTCP4 overexpression increases primary root hair length and density and leaf and stem trichomes in transgenic Arabidopsis relative to wild-type plants (WT). GbTCP4 binds directly to the CAPRICE (CPC) promoter, increasing CPC transcript levels in roots and reducing them in leaves. Compared with WT plants, lignin content in the stems of transgenic Arabidopsis overexpressing GbTCP4 increased, and AtCAD5 gene transcript levels increased. These results suggest that GbTCP4 regulates trichome formation and root hair development in Arabidopsis and may be a candidate gene for regulating cotton fiber elongation.

Effect of Salt Stress on Cytosine Methylation within GL2, An Arabidopsis thaliana Gene Involved in Root Epidermal Cell Differentiation. Absence of Inheritance in the Unstressed Progeny.

Methylation/demethylation of cytosines is an epigenetic strategy for transcriptional regulation, allowing organisms to rapidly respond and adapt to different stimuli. In this context, and using Arabidopsis thaliana as a plant model, we explored whether an environmental stress is sufficient to trigger a change in the methylation status of Glabra-2, a master gene associated with root epidermal cell differentiation. As this gene acts mainly in the epidermis in the root, we examined the stress-driven methylation levels specifically in that tissue. We focused on the stress caused by different salt concentrations in the growth medium. When testing the effect of 20 and 75 mM NaCl, we found that there is a significant decrease in the CG methylation level of the analyzed genomic region within the epidermis. Whereas this reduction was 23% in mildly stressed plants, it turned out to be more robust (33%) in severely stressed ones. Notably, this latter epigenetic change was accompanied by an increase in the number of trichoblasts, the epidermal cell type responsible for root hair development. Analysis of an eventual inheritance of epigenetic marks showed that the non-stressed progeny (F1) of stressed plants did not inherit—in a Lamarckian fashion—the methylation changes that had been acquired by the parental individuals.

MEDIATOR16 Orchestrates the STOP1-ALMT1 Signaling Pathway for Plant Adaptation to Phosphate Scarcity in Arabidopsis

Phosphate (Pi) is a critical macronutrient for the biochemical and molecular functions of cells. Under Pi limitation, plants manifest adaptive strategies to increase Pi scavenging. However, the molecular mechanisms underlying these responses remain unclear and only a few components have been identified to date. Here, we show that under Pi-scarcity, the MEDIATOR (MED) transcriptional co-activator, through its MED16 subunit, plays a critical role in Arabidopsis root system architecture remodeling, including primary root growth, lateral root formation, and root hair development. Transcriptomic and genetic analyses further revealed that MED16, via a direct interaction with SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1), is required for the transcriptional activation of ALUMINUM ACTIVATED MALATE TRANSPORT 1 (ALMT1), a membrane permease, to increase malate exudation in response to low Pi. Our results unravel a critical transcriptional component for understanding how plant cells orchestrate gene expression with the STOP1-ALMT1 module for survival under deficiency of a critical macronutrient.

SMAX1/SMXL2 regulate root and root hair development downstream of KAI2-mediated signalling in Arabidopsis.

Karrikins are smoke-derived compounds presumed to mimic endogenous signalling molecules (KAI2-ligand, KL), whose signalling pathway is closely related to that of strigolactones (SLs), important regulators of plant development. Both karrikins/KLs and SLs are perceived by closely related α/β hydrolase receptors (KAI2 and D14 respectively), and signalling through both receptors requires the F-box protein MAX2. Furthermore, both pathways trigger proteasome-mediated degradation of related SMAX1-LIKE (SMXL) proteins, to influence development. It has previously been suggested in multiple studies that SLs are important regulators of root and root hair development in Arabidopsis, but these conclusions are based on phenotypes observed in the non-specific max2 mutants and by use of racemic-GR24, a mixture of stereoisomers that activates both D14 and KAI2 signalling pathways. Here, we demonstrate that the majority of the effects on Arabidopsis root development previously attributed to SL signalling are actually mediated by the KAI2 signalling pathway. Using mutants defective in SL or KL synthesis and/or perception, we show that KAI2-mediated signalling alone regulates root hair density and root hair length as well as root skewing, straightness and diameter, while both KAI2 and D14 pathways regulate lateral root density and epidermal cell length. We test the key hypothesis that KAI2 signals by a non-canonical receptor-target mechanism in the context of root development. Our results provide no evidence for this, and we instead show that all effects of KAI2 in the root can be explained by canonical SMAX1/SMXL2 activity. However, we do find evidence for non-canonical GR24 ligand-receptor interactions in D14/KAI2-mediated root hair development. Overall, our results demonstrate that the KAI2 signalling pathway is an important new regulator of root hair and root development in Arabidopsis and lay an important basis for research into a molecular understanding of how very similar and partially overlapping hormone signalling pathways regulate different phenotypic outputs.

Inhibition of Pre-mRNA Splicing Promotes Root Hair Development in Arabidopsis thaliana.

Root hairs protruding from epidermal cells increase the surface area for water absorption and nutrient uptake. Various environmental factors including light, oxygen concentration, carbon dioxide concentration, calcium and mycorrhizal associations promote root hair formation in Arabidopsis thaliana. Light regulates the expression of a large number of genes at the transcriptional and post-transcriptional levels; however, there is little information linking the light response to root hair development. In this study, we describe a novel mutant, light-sensitive root-hair development 1 (lrh1), that displays enhanced root hair development in response to light. Hypocotyl and root elongation was inhibited in the lrh1 mutant, which had a late flowering phenotype. We identified the gene encoding the p14 protein, a putative component of the splicing factor 3b complex essential for pre-mRNA splicing, as being responsible for the lrh1 phenotype. Indeed, regulation of alternative splicing was affected in lrh1 mutants and treatment with a splicing inhibitor mimicked the lrh1 phenotype. Genome-wide alterations in pre-mRNA splicing patterns including differential splicing events of light signaling- and circadian clock-related genes were found in lrh1 as well as a difference in transcriptional regulation of multiple genes including upregulation of essential genes for root hair development. These results suggest that pre-mRNA splicing is the key mechanism regulating root hair development in response to light signals.