A Putative Leucine-Rich Repeat Receptor Kinase Involved in Brassinosteroid Signal Transduction

AbstractBrassinosteroids are a class of growth-promoting regulators that play a key role throughout plant development. Despite their importance, nothing is known of the mechanism of action of these steroid hormones. We describe the identification of 18 Arabidopsis dwarf mutants that are unable to respond to exogenously added brassinosteroid, a phenotype that might be expected for brassinosteroid signaling mutants. All 18 mutations define alleles of a single previously described gene, BRI1. We cloned BRI1 and examined its expression pattern. It encodes a ubiquitously expressed putative receptor kinase. The extracellular domain contains 25 tandem leucine-rich repeats that resemble repeats found in animal hormone receptors, plant disease resistance genes, and genes involved in unknown signaling pathways controlling plant development

ASKθ, a group-III Arabidopsis GSK3, functions in the brassinosteroid signalling pathway

Brassinosteroids (BRs) are plant hormones that regulate many processes including cell elongation, leaf development, pollen tube growth and xylem differentiation. GSK3/shaggy-like kinases (GSK) are critical regulators of intracellular signalling initiated by the binding of BR to the BRI1 receptor complex. Three GSKs have already been shown to relay BR responses, including phosphorylation of the transcriptional regulator BES1. However, recent studies indicate that one or more yet unidentified protein kinases are involved in BR signalling. Here, we show that the in vivo protein kinase activity of the group-III GSK, ASKθ, was negatively regulated by BRI1. Arabidopsis thaliana plants with enhanced ASKθ activity displayed a bri1-like phenotype. ASKθ overexpressors accumulated high levels of brassinolide, castasterone and typhasterol, and were insensitive to BR. ASKθ localized to the nucleus and directly phosphorylated BES1 and BZR1. Moreover, the BES1/BZR1-like transcription factor BEH2 was isolated as an ASKθ interaction partner in a yeast two-hybrid screen. ASKθ phosphorylated BEH2 both in vitro and in vivo. Overall, these data provide strong evidence that ASKθ is a novel component of the BR signalling cascade, targeting the transcription factors BES1, BZR1 and BEH2

New genomic resources for switchgrass: a BAC library and comparative analysis of homoeologous genomic regions harboring bioenergy traits

<p>Abstract</p> <p>Background</p> <p>Switchgrass, a C4 species and a warm-season grass native to the prairies of North America, has been targeted for development into an herbaceous biomass fuel crop. Genetic improvement of switchgrass feedstock traits through marker-assisted breeding and biotechnology approaches calls for genomic tools development. Establishment of integrated physical and genetic maps for switchgrass will accelerate mapping of value added traits useful to breeding programs and to isolate important target genes using map based cloning. The reported polyploidy series in switchgrass ranges from diploid (2X = 18) to duodecaploid (12X = 108). Like in other large, repeat-rich plant genomes, this genomic complexity will hinder whole genome sequencing efforts. An extensive physical map providing enough information to resolve the homoeologous genomes would provide the necessary framework for accurate assembly of the switchgrass genome.</p> <p>Results</p> <p>A switchgrass BAC library constructed by partial digestion of nuclear DNA with <it>Eco</it>RI contains 147,456 clones covering the effective genome approximately 10 times based on a genome size of 3.2 Gigabases (~1.6 Gb effective). Restriction digestion and PFGE analysis of 234 randomly chosen BACs indicated that 95% of the clones contained inserts, ranging from 60 to 180 kb with an average of 120 kb. Comparative sequence analysis of two homoeologous genomic regions harboring orthologs of the rice <it>OsBRI1 </it>locus, a low-copy gene encoding a putative protein kinase and associated with biomass, revealed that orthologous clones from homoeologous chromosomes can be unambiguously distinguished from each other and correctly assembled to respective fingerprint contigs. Thus, the data obtained not only provide genomic resources for further analysis of switchgrass genome, but also improve efforts for an accurate genome sequencing strategy.</p> <p>Conclusions</p> <p>The construction of the first switchgrass BAC library and comparative analysis of homoeologous harboring <it>OsBRI1 </it>orthologs present a glimpse into the switchgrass genome structure and complexity. Data obtained demonstrate the feasibility of using HICF fingerprinting to resolve the homoeologous chromosomes of the two distinct genomes in switchgrass, providing a robust and accurate BAC-based physical platform for this species. The genomic resources and sequence data generated will lay the foundation for deciphering the switchgrass genome and lead the way for an accurate genome sequencing strategy.</p

Genetic Evidence for an Indispensable Role of Somatic Embryogenesis Receptor Kinases in Brassinosteroid Signaling

The authors are grateful to the Arabidopsis Biological Resource Center for providing the T-DNA insertion lines discussed in this work. We thank Dr. Yanhai Yin (Iowa State University) for providing anti-BES1 antibody, Dr. Jiayang Li (Institute of Genetics and Developmental Biology, Chinese Academy of Sciences) for bri1-301 seeds, and Dr. Xing-wang Deng (Yale University) for cop1-4 and cop1-6 seeds as controls.Author Summary Brassinosteroids (BRs) are a group of plant hormones critical for plant growth and development. BRs are perceived by a cell-surface receptor complex including two distinctive receptor kinases, BRI1 and BAK1. Whereas BRI1 is a true BR-binding receptor, BAK1 does not appear to have BR-binding activity. Therefore, BAK1 is likely a co-receptor in BR signal transduction. The genetic significance of BAK1 was not clearly demonstrated in previous studies largely due to functional redundancy of BAK1 and its closely related homologues. It was not clear whether BAK1 plays an essential role or only an enhancing role in BR signaling. In this study, we identified all possible BAK1 redundant genes in the Arabidopsis thaliana genome and generated single, double, triple, and quadruple mutants. Detailed analysis indicated that, without BAK1 and its functionally redundant proteins, BR signaling is completely disrupted, largely because BRI1 has lost its ability to activate downstream components. These studies provide the first piece of loss-of-functional genetic evidence that BAK1 is indispensable to the early events of the BR signaling pathway.Yeshttp://www.plosgenetics.org/static/editorial#pee