Summary
Root hair growth requires intensive cell‐wall modification. This study demonstrates that root hair‐specific expansin As, a sub‐clade of the cell wall‐loosening expansin proteins, are required ...for root hair elongation in rice (Oryza sativa L.). We identified a gene encoding EXPA17 (OsEXPA17) from a rice mutant with short root hairs. Promoter::reporter transgenic lines exhibited exclusive OsEXPA17 expression in root hair cells. The OsEXPA17 mutant protein (OsexpA17) contained a point mutation, causing a change in the amino acid sequence (Gly104→Arg). This amino acid alteration is predicted to disrupt a highly conserved disulfide bond in the mutant. Suppression of OsEXPA17 by RNA interference further confirmed requirement for the gene in root hair elongation. Complementation of the OsEXPA17 mutant with other root hair EXPAs (OsEXPA30 and Arabidopsis EXPA7) can restore root hair elongation, indicating functional conservation of these root hair EXPAs in monocots and dicots. These results demonstrate that members of the root hair EXPA sub‐clade play a crucial role in root hair cell elongation in Graminaceae.
Molecular link between auxin and ROS-mediated polar growth Mangano, Silvina; Denita-Juarez, Silvina Paola; Choi, Hee-Seung ...
Proceedings of the National Academy of Sciences - PNAS,
05/2017, Letnik:
114, Številka:
20
Journal Article
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Odprti dostop
Root hair polar growth is endogenously controlled by auxin and sustained by oscillating levels of reactive oxygen species (ROS). These cells extend several hundred-fold their original size toward ...signals important for plant survival. Although their final cell size is of fundamental importance, the molecular mechanisms that control it remain largely unknown. Here we show that ROS production is controlled by the transcription factor RSL4, which in turn is transcriptionally regulated by auxin through several auxin response factors (ARFs). In this manner, auxin controls ROS-mediated polar growth by activating RSL4, which then up-regulates the expression of genes encoding NADPH oxidases (also known as RESPIRATORY BURST OXIDASE HOMOLOG proteins) and class III peroxidases, which catalyze ROS production. Chemical or genetic interference with ROS balance or peroxidase activity affects root hair final cell size. Overall, our findings establish amolecular link between auxin and ROS-mediated polar root hair growth.
PIN-FORMED (PIN)-mediated polar auxin transport (PAT) is involved in key developmental processes in plants. Various internal and external cues influence plant development via the modulation of ...intracellular PIN polarity and, thus, the direction of PAT, but the mechanisms underlying these processes remain largely unknown. PIN proteins harbor a hydrophilic loop (HL) that has important regulatory functions; here, we used the HL as bait in protein pulldown screening for modulators of intracellular PIN trafficking in Arabidopsis thaliana. Calcium-dependent protein kinase 29 (CPK29), a Ca2+-dependent protein kinase, was identified and shown to phosphorylate specific target residues on the PIN-HL that were not phosphorylated by other kinases. Furthermore, loss of CPK29 or mutations of the phospho-target residues in PIN-HLs significantly compromised intracellular PIN trafficking and polarity, causing defects in PIN-mediated auxin redistribution and biological processes such as lateral root formation, root twisting, hypocotyl gravitropism, phyllotaxis, and reproductive development. These findings indicate that CPK29 directly interprets Ca2+ signals from internal and external triggers, resulting in the modulation of PIN trafficking and auxin responses.
PIN-FORMEDs (PINs) are auxin efflux carriers that asymmetrically target the plasma membrane (PM) and are critical for forming local auxin gradients and auxin responses. While the cytoplasmic ...hydrophilic loop domain of PIN (PIN-HL) is known to include some molecular cues (e.g., phosphorylation) for the modulation of PIN's intracellular trafficking and activity, the complexity of auxin responses suggests that additional regulatory modules may operate in the PIN-HL domain. Here, we have identified and characterized a PIN-HL-interacting protein (PIP) called FORMATION OF APLOID AND BINUCLEATE CELL 1C (FAB1C), a phosphatidylinositol-3-phosphate 5-kinase, which modulates PIN's lytic trafficking. FAB1C directly interacts with PIN-HL and is required for the polarity establishment and vacuolar trafficking of PINs. Unphosphorylated forms of PIN2 interact more readily with FAB1C and are more susceptible to vacuolar lytic trafficking compared to phosphorylated forms. FAB1C also affected lateral root formation by modulating the abundance of periclinally localized PIN1 and auxin maximum in the growing lateral root primordium. These findings suggest that a membrane-lipid modifier can target the cargo-including vesicle by directly interacting with the cargo and modulate its trafficking depending on the cargo's phosphorylation status.
AtHMA1 is a member of the heavy metal-transporting ATPase family. It exhibits amino acid sequence similarity to two other Zn(II) transporters, AtHMA2 and AtHMA4, and contains poly-His motifs that are ...commonly found in Zn(II)-binding proteins, but lacks some amino acids that are typical for this class of transporters. AtHMA1 localizes to the chloroplast envelope. In comparison with wild-type plants, we observed a more pronounced sensitivity in the presence of high Zn(II) concentrations, and increased accumulation of Zn in the chloroplast of T-DNA insertional mutants in AtHMA1. The Zn(II)-sensitive phenotype of AtHMA1 knock-out plants was complemented by the expression of AtHMA1 under the control of its own promoter. The Zn(II)-transporting activity of AtHMA1 was confirmed in a heterologous expression system, Saccharomyces cerevisiae. The sensitivity of yeast to high concentrations of Zn(II) was altered by the expression of AtHMA1 lacking its N-terminal chloroplast-targeting signal. Taken together, these results suggest that under conditions of excess Zn(II), AtHMA1 contributes to Zn(II) detoxification by reducing the Zn content of Arabidopsis thaliana plastids.
ROOT HAIR SPECIFIC (RHS) genes, which contain the root hair-specific cis-element (RHE) in their regulatory regions, function in root hair morphogenesis. Here, we demonstrate that an Arabidopsis ...thaliana basic helix-loop-helix transcription factor, ROOT HAIR DEFECTVE SIX-LIKE4 (RSL4), directly binds to the RHE in vitro and in vivo, upregulates RHS genes, and stimulates root hair formation in Arabidopsis. Orthologs of RSL4 from a eudicot (poplar Populus trichocarpa), a monocot (rice Oryza sativa), and a lycophyte (Selaginella moellendorffii) each restored root hair growth in the Arabidopsis rsl4 mutant. In addition, the rice and S. moellendorffii RSL4 orthologs bound to the RHE in in vitro and in vivo assays. The RSL4 orthologous genes contain RHEs in their promoter regions, and RSL4 was able to bind to its own RHEs in vivo and amplify its own expression. This process likely provides a positive feedback loop for sustainable root hair growth. When RSL4 and its orthologs were expressed in cells in nonroot- hair positions, they induced ectopic root hair growth, indicating that these genes are sufficient to specify root hair formation. Our results suggest that RSL4 mediates root hair formation by regulating RHS genes and that this mechanism is conserved throughout the tracheophyte (vascular plant) lineage.
ATP binding cassette (ABC) transporters transport diverse substrates across membranes in various organisms. However, plant ABC transporters have only been scantily characterized. By taking advantage ...of the auxin-sensitive Arabidopsis thaliana root hair cell and tobacco (Nicotiana tabacum) suspension cell systems, we show here that Arabidopsis P-glycoprotein4 (PGP4) displays auxin efflux activity in plant cells. Root hair cell-specific overexpression of PGP4 (PGP4ox) and known auxin efflux transporters, such as PGP1, PGP19, and PIN-FORMEDs, decreased root hair elongation, whereas overexpression of the influx transporter AUXIN-RESISTANT1 enhanced root hair length. PGP4ox-mediated root hair shortening was rescued by the application of auxin or an auxin efflux inhibitor. These results indicate that the increased auxin efflux activity conferred by PGP4 reduces auxin levels in the root hair cell and consequently inhibits root hair elongation. PGP4ox in tobacco suspension cells also increased auxin efflux. PGP4 proteins were targeted to the plasma membrane of Arabidopsis root hair cells and tobacco cells without any clear subcellular polarity. Brefeldin A partially interfered with the trafficking of PGP4 reversibly, and this was rescued by pretreatment with auxin. These results suggest that PGP4 is an auxin efflux transporter in plants and that its trafficking to the plasma membrane involves both BFA-sensitive and -insensitive pathways.
Root hairs form a substantial portion of the root surface area. Compared with their nutritional function, the physical function of root hairs has been poorly characterised. This study investigates ...the physical role of root hairs of Arabidopsis thaliana seedlings in interaction of the root with water and soil and in plant survival upon soil disruption. Five transgenic lines with different root hair lengths were used to assess the physical function of root hairs. Upon soil disruption by water falling from a height (mimicking rainfall), long-haired lines showed much higher anchorage rates than short-haired lines. The root-pulling test revealed that a greater amount of soil adhered to long-haired roots than to short-haired roots. When seedlings were pulled out and laid on the soil surface for 15 d, survival rates of long-haired seedlings were higher than those of short-haired seedlings. Moreover, the water holding capacity of roots was much greater among long-haired seedlings than short-haired seedlings. These results suggest that root hairs play a significant role in plant survival upon soil disruption which could be fatal for young seedlings growing on thin soil surface with a short primary root and root hairs as the only soil anchoring system.
Different PIN-FORMED proteins (PINs) contribute to intercellular and intracellular auxin transport, depending on their distinctive subcellular localizations. Arabidopsis thaliana PINs with a long ...hydrophilic loop (HL) (PIN1 to PIN4 and PIN7; long PINs) localize predominantly to the plasma membrane (PM), whereas short PINs (PIN5 and PIN8) localize predominantly to internal compartments. However, the subcellular localization of the short PINs has been observed mostly for PINs ectopically expressed in different cell types, and the role of the HL in PIN trafficking remains unclear. Here, we tested whether a long PINHL can provide its original molecular cues to a short PIN by transplanting the HL. The transplanted long PIN2-HL was sufficient for phosphorylation and PM trafficking of the chimeric PIN5:PIN2-HL but failed to provide the characteristic polarity of PIN2. Unlike previous observations, PIN5 showed clear PM localization in diverse cell types where PIN5 is natively or ectopically expressed and even polar PM localization in one cell type. Furthermore, in the root epidermis, the subcellular localization of PIN5 switched from PM to internal compartments according to the developmental stage. Our results suggest that the long PIN-HL is partially modular for the trafficking behavior of PINs and that the intracellular trafficking of PIN is plastic depending on cell type and developmental stage.
Plant ATP-binding cassette (ABC) transporters consist of largest family members among many other membrane transporters and have been implicated in various functions such as detoxification, disease ...resistance and transport of diverse substrates. Of the ABC-B/multi-drug resistance/P-glycoprotein (ABCB/MDR/PGP) subfamily, at least five members have been reported to mediate cellular transport of auxin or auxin derivatives. Although single mutant phenotypes of these genes are milder than PIN-FORMED (PIN) mutants, those ABCBs significantly contribute for the directional auxin movement in the tissue-level auxin-transporting assay. Uniformly localized ABCB proteins in the plasma membrane (PM) are generaly found in different plant species and stably retained regardless of internal and external signals. This implies that these ABCB proteins may play as basal auxin transporters.
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