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

Vitamin B6 (vitB6) serves as an essential cofactor for more than 140 enzymes. Pyridoxal 5'-phosphate (PLP), active cofactor form of vitB6, can be photolytically destroyed by trace amounts of ultraviolet-B (UV-B). How sun-exposed organisms cope with PLP photosensitivity and modulate vitB6 homeostasis is currently unknown. We previously reported on two Arabidopsis mutants, rus1 and rus2, that are hypersensitive to trace amounts of UV-B light. We performed mutagenesis screens for second-site suppressors of the rus mutant phenotype and identified mutations in the ASPARTATE AMINOTRANSFERASE2 (ASP2) gene. ASP2 encodes for cytosolic aspartate aminotransferase (AAT), a PLP-dependent enzyme that plays a key role in carbon and nitrogen metabolism. Genetic analyses have shown that specific amino acid substitutions in ASP2 override the phenotypes of rus1 and rus2 single mutants as well as rus1 rus2 double mutant. These substitutions, all shown to reside at specific positions in the PLP-binding pocket, resulted in no PLP binding. Additional asp2 mutants that abolish AAT enzymatic activity, but which alter amino acids outside of the PLP-binding pocket, fail to suppress the rus phenotype. Furthermore, exogenously adding vitB6 in growth media can rescue both rus1 and rus2. Our data suggest that AAT plays a role in vitB6 homeostasis in Arabidopsis.

Perfluorooctanesulfonate (PFOS) as an accumulative emerging persistent organic pollutant in crops poses severe threats to human health. Lettuce varieties that accumulate a lower amount of PFOS (low-accumulating crop variety, LACV) have been identified, but the regarding mechanisms remain unsolved. Here, rhizospheric activation, uptake, translocation, and compartmentalization of PFOS in LACV were investigated in comparison with those of high-accumulating crop variety (HACV) in terms of rhizospheric forms, transporters, and subcellular distributions of PFOS. The enhanced PFOS desorption from the rhizosphere soils by dissolved organic matter from root exudates was observed with weaker effect in LACV than in HACV. PFOS root uptake was controlled by a transporter-mediated passive process in which low activities of aquaporins and rapid-type anion channels were corrected with low expression levels of PIPs (PIP1–1 and PIP2–2) and ALMTs (ALMT10 and ALMT13) genes in LACV roots. Higher PFOS proportions in root cell walls and trophoplasts caused lower root-to-shoot transport in LACV. The ability to cope with PFOS toxicity to shoot cells was poorer in LACV relative to HACV since PFOS proportions were higher in chloroplasts but lower in vacuoles. Our findings provide novel insights into PFOS accumulation in lettuce and further understanding of multiprocess mechanisms of LACV.