Aux͞IAA gene family members were first identified by their rapid transcriptional increase in response to auxin. Auxin͞indole-3-acetic acid protein (Aux͞IAA) luciferase (LUC) fusions expressed in Arabidopsis under control of a non-auxin-responsive promoter were used to monitor the effect of auxin on protein abundance independent of transcriptional regulation by auxin. After 2 hr in the presence of 1 M exogenous dichlorophenoxyacetic acid (2,4D), a synthetic auxin, the levels of pea IAA6 (PSIAA6) and Arabidopsis IAA1 LUC activity were 35% and 67%, respectively, of mocktreated genetically identical seedlings, whereas the activity of LUC alone from equivalently treated seedlings remained unaltered. The steady-state level of an Aux͞IAA fusion protein lacking domain II, one of the conserved domains found in all Aux͞IAA proteins, was not reduced in the presence of auxin. Higher levels of exogenous auxin were required to affect the steady-state level of the PSIAA6::LUC fusion with a point mutation in domain II. A 13-aa consensus sequence from domain II fused to LUC created an auxin-responsive fusion protein. The change in steady-state levels in response to auxin is extremely rapid, with a decrease in LUC activity detectable by 2 min after auxin application. Direct half-life measurements show that the decrease caused by exogenous auxin is due to the decrease in fusion protein half-life. These results suggest that auxin rapidly modulates the degradation rate of Aux͞IAA proteins, with higher levels of auxin increasing the proteolytic rate of Aux͞IAA family members.

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.

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.

Since the discovery of Systemin in tomatoes in 1991, many additional peptide hormones have been discovered in plants. Plant peptide hormones typically start as small precursor proteins (~100 a.a.), which include N-terminal secretory or anchor sequences, and are post-translationally processed into a smaller hormonally active peptide (on the range of 5-18 a.a.). To date, peptide hormones have been shown to have diverse functions in plants, including nearly every aspect of development and morphogenesis. Here, we discuss recent reports regarding the CLV3/ESR-RELATED (CLE) and ROOT GROWTH FACTOR (RGF) families of peptide hormones.