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Expansins are non-hydrolytic cell wall-loosening proteins that are involved in the cell wall modifications that underlie many plant developmental processes. Root hair growth requires the accumulation of cell wall materials and dynamic cell wall modification at the tip region. Although several lines of indirect evidence support the idea that expansin-mediated wall modification occurs during root hair growth, the involvement of these proteins remains to be demonstrated in vivo. In this study, we used RNA interference (RNAi) to examine the biological function of Arabidopsis thaliana EXPANSIN A7 (AtEXPA7), which is expressed specifically in the root hair cell. The root hairspecific AtEXPA7 promoter was used to drive RNAi expression, which targeted two independent regions in the AtEXPA7 transcript. Quantitative reverse transcriptase-PCR analyses were used to examine AtEXPA7 transcript levels. In four independent RNAi transformant lines, RNAi expression reduced AtEXPA7 transcript levels by 25-58% compared to controls. Accordingly, the root hairs of RNAi transformant lines were 25-48% shorter than control plants and exhibited a broader range of lengths than the controls. Our results provide in vivo evidence that expansins are required for root hair tip growth.

Cell differentiation ultimately relies on the regulation of cell type-specific genes. For a root hair cell to undergo morphogenesis, diverse cellular processes including cell-wall loosening must occur in a root hair cell-specific manner. Previously, we identified and characterized root hair-specific cis-elements (RHE) from the genes encoding the cell wall-loosening protein EXPANSIN A (EXPA) which functions preferentially on dicot cell walls. This study reports two root hair-specific grass EXPB genes that contain RHEs. These genes are thought to encode proteins that function more efficiently on grass cell walls. The proximal promoter regions of two orthologous EXPB genes from rice (Oryza sativa; OsEXPB5) and barley (Hordeum vulgare; HvEXPB1) included RHE motifs. These promoters could direct root hair-specific expression of green fluorescent protein (GFP) in the roots of rice and Arabidopsis (Arabidopsis thaliana). Promoter deletion analyses demonstrated that the RHE motifs are necessary for root hair-specific expression of these EXPB promoters. Phylogenetic analysis of EXP protein sequences indicated that grass EXPBs are the only orthologs to these root hair-specific EXPBs, separating dicot EXPBs to distal branches of the tree. These results suggest that RHE-containing root hair-specific EXPB genes have evolved for grass-specific cell wall modification during root hair morphogenesis.

The roots of plants normally carry small hairs arranged in a regular pattern. Transfer DNA-tagged lines of Arabidopsis thaliana included a mutant with few, randomly distributed root hairs. The mutated gene CAPRICE (CPC) encoded a protein with a Myb-like DNA binding domain typical of transcription factors involved in animal and plant development. Analysis in combination with other root hair mutations showed that CPC may work together with the TTG gene and upstream of the GL2 gene. Transgenic plants overexpressing CPC had more root hairs and fewer trichomes than normal. Thus, the CPC gene determines the fate of epidermal cell differentiation in Arabidopsis

Root hair development is controlled by environmental signals. Studies on root hair plasticity in Arabidopsis thaliana have mainly focused on phosphate and iron deficiency. Root hair growth and development and their physiological role in response to salt stress are largely unknown. Here, we show that root epidermal cell types and root hair development are highly regulated by salt stress. Root hair length and density decreased significantly in a dose-dependent manner on both primary roots and junction sites between roots and shoots. The root hair growth and development were sensitive to inhibition by ions but not to osmotic stress. High salinity also alters anatomical structure of roots, leading to a decrease in cell number in N positions and enlargement of the cells. Moreover, analysis of the salt overly sensitive mutants indicated that salt-induced root hair response is caused by ion disequilibrium and appears to be an adaptive mechanism that reduces excessive ion uptake. Finally, we show that genes WER, GL3, EGL3, CPC, and GL2 might be involved in cell specification of root epidermis in stressed plants. Taken together, data suggests that salt-induced root hair plasticity represents a coordinated strategy for early stress avoidance and tolerance as well as a morphological sign of stress adaptation.

The Arabidopsis root produces a position-dependent pattern of hair-bearing and hairless cell types during epidermis development. Five loci (TRANSPARENT TESTA GLABRA [TTG], GLABRA2 [GL2], ROOT HAIR DEFECTIVE6 [RHD6], CONSTITUTIVE TRIPLE RESPONSE1 [CTR1], and AUXIN RESISTANT2 [AXR2]) and the plant hormones ethylene and auxin have been reported to affect the production of root hair and hairless cells in the Arabidopsis root. In this study, genetic, molecular, and physiological tests were employed to define the roles of these loci and hormones. Epistasis tests and reporter gene studies indicated that the hairless cell-promoting genes TTG and GL2 are likely to act early to negatively regulate the ethylene and auxin pathways. Studies of the developmental timing of the hormone effects indicated that ethylene and auxin pathways promote root hair outgrowth after cell-type differentiation has been initiated. The genetic analysis of ethylene- and auxin-related mutations showed that root hair formation is influenced by a network of hormone pathways, including a partially redundant ethylene signaling pathway. A model is proposed in which the patterning of root epidermal cells in Arabidopsis is regulated by the cell position-dependent action of the TTG/GL2 pathway, and the ethylene and auxin hormone pathways act to promote root hair outgrowth at a relatively late stage of differentiation

Root hair development is orchestrated by nutritional factors and plant hormones. We investigated the action of ammonium (NH₄+) and its interactions with methyl jasmonate (MeJA) and ethylene in Arabidopsis root hair growth. The formation of root hair branches was dramatically stimulated in media containing 1.25 to 20 mM NH₄+ at pH values of 4.0 to 6.5. The NH₄+-treated root hairs showed a very short tip growth stage and swells on the sides that indicated the emergence of branches. MeJA (0.08 to 10 μM) worked in synergism with NH₄+ to enhance hair branching. In contrast, ethylene had an antagonistic effect; the stimulation of hair branching by NH₄+ was suppressed by the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and was diminished in ethylene-overproducing mutant eto1-1 seedlings. Moreover, the application of Ag+, an ethylene inhibitor, reduced the ACC-induced inhibition of NH₄+-stimulated hair branching and restored NH₄+-stimulated hair branching in eto1-1 seedlings. Thus, the actions of jasmonate and ethylene appear to be dependent on nutritional conditions such as available nitrogen.

Root-hair initiation in Arabidopsis thaliana provides a model for studying cell polarity and its role in plant morphogenesis. Root hairs normally emerge at the apical end of root epidermal cells, implying that these cells are polarized. We have identified a mutant, rhd6, that displays three defects: (a) a reduction in the number of root hairs, (b) an overall basal shift in the site of root-hair emergence, and (c) a relatively high frequency of epidermal cells with multiple root hairs. These defects implicate the RHD6 gene in root-hair initiation and indicate that RHD6 is normally associated with the establishment of, or response to, root epidermal cell polarity. Similar alterations in the site of root-hair emergence, although less extreme, were also discovered in roots of the auxin-, ethylene-, abscisic acid-resistant mutant axr2 and the ethylene-resistant mutant etr1. All three rhd6 mutant phenotypes were rescued when either auxin (indoleacetic acid) or an ethylene precursor (1-aminocyclopropane-1-carboxylic acid) was included in the growth medium. The rhd6 root phenotypes could be phenocopied by treating wild-type seedlings with an inhibitor of the ethylene pathway (aminoethoxyvinylglycine). These results indicate that RHD6 is normally involved in directing the selection or assembly of the root-hair initiation site through a process involving auxin and ethylene

Root-soil contact is an important factor for uptake of a less mobile soil nutrient such as phosphorus (P) by crop plants. Root hairs can substantially increase root-soil contact. Identification of crop cultivars with more and longer root hairs can, therefore, be useful for increasing P uptake in low input agriculture. We studied the root hairs of wheat (Triticum aestivum L. cvs. Kosack, Foreman, Kraka) and barley cultivars (Hordeum vulgare L. cvs. Angora, Hamu, Alexis, Canut) in relation to P depletion from the rhizosphere in three soils of different P levels (0.45, 1.1 and 1.6 mmoles P kg⁻¹ soil; extracted with 0.5 M NaHCO₃). Root hairs were measured in solution culture having nutrients and concentration similar to soil solution. Root hairs of Kraka were much longer (1.27 ± 0.26 mm) and denser (38 ± 3) hairs mm⁻¹ root) than those of Kosack which had shorter (0.49 ± 0.2 mm) and fewer (24 ± 3) hairs mm⁻¹ root) root hairs. Root hairs increased root surface area (RSA) of Kraka by 341%. The increase with Foreman was 142% and with Kosack it was 95%. For winter barley, the length (1.1 ± 0.3 mm) and density (30 ± 1 hairs mm⁻¹ root) of root hairs of Hamu differed from root hair length (0.52 ± 0.18 mm) and density (27 ± 1 hairs mm⁻¹ root) of Angora. Root hairs of spring barley cultivars differed in length (Canut 1.0 ± 0.24 mm; Alexis 0.64 ± 0.19 mm) but not in density (Canut 31 ± 1, Alexis 30 ± 2 hairs mm⁻¹ root). Root hair diameter (12 ± 1µm) did not differ among the cultivars. Root hairs increased RSA of Canut by 245%, Hamu by 237%, Alexis by 143% and Angora 112%. The variation in root hair parameters of the cultivars was related to quantity of P depleted from rhizosphere. The correlation (R²) between the root hair lengths of wheat cultivars and the quantity of P depleted from the rhizosphere soil (Q) was (0.99***) in low-P, (0.85***) in medium-P and (0.78**) in high-P soil. The values of (R²) between the root hair surface areas of wheat cultivars and Q were (1.00***) in low-P, (0.74**) in medium-P and (0.66**) in high-P soil. Similar high values of R² were found for barley. These results show that the variation in root hairs of cereal cultivars can be considerable and it can play a significant role in P acquisition, especially in low-P soils.

Rhizobium meliloti produces lipochitooligosaccharide nodulation NodRm factors that are required for nodulation of legume hosts. NodRm factors are O-acetylated and N-acylated by specific C16-unsaturated fatty acids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhibited a significantly reduced capacity to elicit infection thread (IT) formation in alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodF/nodL mutants were unable to penetrate into legume hosts and to form ITs. Nevertheless, these mutants induced widespread cell wall tip growth in trichoblasts and other epidermal cells and were also able to elicit cortical cell activation at a distance. NodRm factor structural requirements are thus clearly more stringent for bacterial entry than for the elicitation of developmental plant responses

Root hairs as specialized epidermal cells represent part of the outermost interface between a plant and its soil environment. They make up to 70% of the root surface and, therefore, are likely to contribute significantly to nutrient uptake. To study uptake systems for mineral nitrogen, three genes homologous to Arabidopsis nitrate and ammonium transporters (AtNrt1 and AtAmt1) were isolated from a root hair-specific tomato cDNA library. Accumulation of LeNrt1-1, LeNrt1-2, and LeAmt1 transcripts was root-specific, with no detectable transcripts in stems or leaves. Expression was root cell type-specific and regulated by nitrogen availability. LeNrt1-2 mRNA accumulation was restricted to root hairs that had been exposed to nitrate. In contrast, LeNrt1-1 transcripts were detected in root hairs as well as other root tissues under all nitrogen treatments applied. Analogous to LeNrt1-1, the gene LeAmt1 was expressed under all nitrogen conditions tested, and root hair-specific mRNA accumulation was highest following exposure to ammonium. Expression of LeAMT1 in an ammonium uptake-deficient yeast strain restored growth on low ammonium medium, confirming its involvement in ammonium transport. Root hair specificity and characteristics of substrate regulation suggest an important role of the three genes in uptake of mineral nitrogen.