The role of SHORT HYPOCOTYL UNDER BLUE 1 in Arabidopsis seed and seedling development.

Abstract

SHORT HYPOCOTYL UNDER BLUE 1 or SHB1 was previously identified as a negative regulator of the cryptochrome-mediated blue light response in Arabidopsis early seedling development. shb1, a T-DNA insertional loss-of-function allele, showed a short hypocotyl phenotype only under blue light, whereas shb1-D, a gain-of-function over-expression allele, showed a long hypocotyl phenotype under blue as well as red or far red light. SHB1 is a nuclear protein and contains a N-terminal SPX domain and a C-terminal EXS homologous to yeast SYG1 protein family. Using a biochemical approach, I found that SHB1 is anchored to a large protein complex through both of its SPX and EXS domains. In a genetic screen, I identified five mis-sense mutations within or nearby its N-terminal SPX domain and these mutation impaired its biological function and proper assembly into a protein complex. In contrast, the C-terminal EXS domain, when over-expressed, created a dominant negative phenotype and interfered with the assembly of the endogenous SHB1 into a native protein complex. SHB1 also functions in flowering timing control and this involvement is likely related to its light signaling activity. In addition, SHB1 plays a specific role in a quite different developmental phase, seed development. SHB1 regulates the timing of endosperm cellularization and promotes the subsequent embryo development through its control over both cell division and cell expansion. SHB1 associates with the promoters of MINISEED3 (MINI3), a WRKY transcription factor gene, and HAIKU2 (IKU2), an LRR receptor kinase gene in vivo, and activates the expression of these two genes required for endosperm development. In summary, SHB1 plays a novel signaling role in both Arabidopsis seed and early seedling development, and this conserved function may be implicated for some SHB1-like proteins in many other organisms.