Here, we report the aluminum (Al)-induced organ-specific expression of a WAK1 (cell wall-associated receptor kinase 1) gene and cell type-specific localization of WAK proteins in Arabidopsis. WAK1-specific reverse transcriptase-polymerase chain reaction analysis revealed an Al-induced WAK1 gene expression in roots. Short-and long-term analysis of gene expression in root fractions showed a typical "on" and "off" pattern with a first peak at 3 h of Al exposure followed by a sharp decline at 6 h and a complete disappearance after 9 h of Al exposure, suggesting the WAK1 is a further representative of Al-induced early genes. In shoots, upon root Al exposure, an increased but stable WAK1 expression was observed. Using confocal microscopy, we visualized Al-induced closure of leaf stomata, consistent with previous suggestions that the Al stress primarily experienced in roots associated with the transfer of root-shoot signals. Elevated levels of WAK protein in root cells were observed through western blots after 6 h of Al exposure, indicating a lag time between the Al-induced WAK transcription and translation. WAK proteins are localized abundantly to peripheries of cortex cells within the elongation zone of the root apex. In these root cells, disintegration of cortical microtubules was observed after Al treatment but not after the Al analog lanthanum treatments. Tip-growing control root hairs, stem stomata, and leaf stomatal pores are characterized with high amounts of WAKs, suggesting WAKs are accumulating at plasma membrane domains, which suffer from mechanical stress and lack dense arrays of supporting cortical microtubules. Further, transgenic plants overexpressing WAK1 showed an enhanced Al tolerance in terms of root growth when compared with the wild-type plants, making the WAK1 one of the important candidates for plant defense against Al toxicity.

The Arabidopsis cell wall-associated kinase (WAK) and WAK-like kinase (WAKL) family of receptor-like kinase genes encodes transmembrane proteins with a cytoplasmic serine/threonine kinase domain and an extracellular region containing epidermal growth factor-like repeats. Previous studies have suggested that some WAK members are involved in plant defense and heavy metal responses, whereas others are required for cell elongation and plant development. The WAK/ WAKL gene family consists of 26 members in Arabidopsis and can be divided into four groups. Here, we describe the characterization of group 2 members that are composed of a cluster of seven tandemly arrayed WAKL genes. The predicted WAKL proteins are highly similar in their cytoplasmic region but are more divergent in their predicted extracellular ligand-binding region. WAKL7 encodes a truncated WAKL isoform that is predicted to be secreted from the cytoplasm. Ratios of nonsynonymous to synonymous substitutions suggest that the extracellular region is subject to diversifying selection. Comparison of the WAKL and WAK gene clusters suggests that they arose independently. Protein gel-blot and immunolocalization analyses suggest that WAKL6 is associated with the cell wall. Histochemical analyses of WAKL promoters fused with the ␤-glucuronidase reporter gene have shown that the expressions of WAKL members are developmentally regulated and tissue specific. Unlike WAK members whose expressions were found predominately in green tissues, WAKL genes are highly expressed in roots and flowers. The expression of WAKL5 and WAKL7 can be induced by wounding stress and by the salicylic acid analog 2,6-dichloroisonicotinic acid in an nonexpressor of pathogenesis-related gene 1-dependent manner, suggesting that they, like some WAK members, are wound inducible and can be defined as pathogenesis-related genes.The plant cell wall, or extracellular matrix (ECM), is a complex array of carbohydrates, proteins, and proteoglygans surrounding the cell (Carpita and Gibeaut, 1993). The ECM provides structural support, determines cell shape, protects the cell against environmental insults, and mediates cell growth and differentiation (Cosgrove, 1999). The ECM is closely associated with the plasma membrane and functions in mediating communication between neighboring cells and between cells and the environment (Pennell, 1998; Kohorn, 2000). Interactions between ECM components and the plasma membrane are undoubtedly important for the efficient relay of environmental signals to the cytoplasm (Kohorn, 1999). In animal cells, a class of plasma membrane receptors called integrins mediates the attachment of the ECM to the plasma membrane via interactions with adhesive glycoproteins, such as fibronectin, vitronectin, and collagen (Hay, 1981; Hynes, 1981 Hynes, , 1987. Interactions between these molecules and ECM ligands result in the translation of environmental cues into signals that can affect cellular functions (Quaranta and Jones, 1991). In plant cells, proteins immunologically related to vi...

All sun-exposed organisms are affected by UV-B [(UVB) 280 -320 nm], an integral part of sunlight. UVB can cause stresses or act as a developmental signal depending on its fluence levels. In plants, the mechanism by which high-fluence-rate UVB causes damages and activates DNA-repair systems has been extensively studied. However, little is known about how nondamaging low-fluencerate UVB is perceived to regulate plant morphogenesis and development. Here, we report the identification of an Arabidopsis mutant, root UVB sensitive 1 (rus1), whose primary root is hypersensitive to very low-fluence-rate (VLF) UVB. Under standard growth-chamber fluorescent white light, rus1 displays stunted root growth and fails to form postembryonic leaves. Experiments with different monochromatic light sources showed that rus1 phenotypes can be rescued if the seedlings are allowed to grow in light conditions with minimum UVB. We determined that roots, not other organs, perceive the UVB signal. Genetic and molecular analyses confirmed that the root light-sensitive phenotypes are independent of all other known plant photoreceptors. Three rus1 alleles have been identified and characterized. A map-based approach was used to identify the RUS1 locus. RUS1 encodes a protein that contains DUF647 (domain of unknown function 647), a domain highly conserved in eukaryotes. Our results demonstrate a root VLF UVB-sensing mechanism that is involved in Arabidopsis early seedling morphogenesis and development.

Ultraviolet B light (UV-B; 280-320 nm) perception and signaling are well-known phenomena in plants, although no specific UV-B photoreceptors have yet been identified. We previously reported on the root UV-B sensitive1 (rus1) mutants in Arabidopsis (Arabidopsis thaliana), which display a block to development under very-low-fluence-rate UV-B (,0.1 mmol m 22 s 21 ) after the seedling emerges from the seed. Here, we report the analysis and cloning of the rus2-1 mutation in Arabidopsis. The phenotype of rus2-1 mutant seedlings is virtually indistinguishable from the phenotype of rus1 seedlings. A map-based approach was used to clone RUS2. RUS2 encodes a domain of unknown function (DUF647)-containing protein that is homologous to the RUS1 protein. rus1-2 rus2-1 double mutant seedlings have the same phenotype as both rus1 and rus2 single mutants, suggesting that the two genes work in the same pathway. RUS2-Green Fluorescent Protein shows a similar expression pattern as that of RUS1-Green Fluorescent Protein, and RUS1 and RUS2 proteins interact physically in yeast. This proteinprotein interaction depends on the DUF647 domain, and site-directed mutagenesis identified specific residues in DUF647 that are required for both protein-protein interaction and physiological function. Six RUS genes are found in Arabidopsis, rice (Oryza sativa), and moss (Physcomitrella patens), and one RUS member, RUS3, is conserved in plants and animals. Our results demonstrate that RUS2 works with RUS1 in a root UV-B-sensing pathway that plays a vital role in Arabidopsis early seedling morphogenesis and development.

The cell wall-associated receptor kinase (WAK) and WAK-like kinase (WAKL) gene family members are good candidates for physical linkers that signal between the cell wall and the cytoplasmic compartment. Previous studies have suggested that while some WAK/WAKL members play a role in bacterial pathogen and heavy-metal aluminum responses, others are involved in cell elongation and plant development. Here, we report a functional role for the WAKL4 gene in Arabidopsis (Arabidopsis thaliana) mineral responses. Confocal microscopic studies localized WAKL4-green fluorescent protein fusion proteins on the cell surfaces suggesting that, like other WAK/WAKL proteins, WAKL4 protein is associated with the cell wall. Histochemical analyses of the WAKL4 promoter fused with the b-glucuronidase reporter gene have shown that WAKL4 expression is induced by Na