Regulation of de-etiolation and stomatal opening responses by HYPOSENSITIVE TO LIGHT and HYPERSENSITIVE TO RED AND BLUE 1.

Abstract

Light is an important environmental cue and regulates plant development processes including de-etiolation, phototropism, shade avoidance, photoperiodic flowering, stomatal movement and so on. In Chapter II, we reported cloning of an Arabidopsis light-signaling component, HYPOSENSITIVE TO LIGHT or HTL. htl mutants displayed a long hypocotyl phenotype under red, far-red, and blue light, as well as other photomorphogenic defects. HTL belongs to an alpha/beta fold protein family and is localized predominantly in the nucleus. ELONGATED HYPOCOTYL5 (HY5), a BZip transcription factor, directly binds to both C/G-box and G-box in HTL promoter and regulates HTL expression. Evidences suggest that HTL represents a new signaling step downstream of HY5 in de-etiolation responses. In Chapter III, we discovered the role of HYPERSENSITIVE TO RED AND BLUE1 (HRB1) and PHOSPHATE7 (PP7) in lightinduced stomatal movement response. Besides its short hypocotyl phenotype upon isolation, hrb1 mutant also showed reduced water loss and stomatal aperture phenotypes under blue light. We identified PP7 as an HRB1 interacting protein. PP7 dephosphorylated HRB1 in vivo and HRB1 required a functional PP7 for both its stomatal opening and hypocotyl elongation responses. HRB1 was found in a protein complex of 193 kDa in the dark and blue light induced a size shift to 285 kDa. However, the size shift of HRB1 protein complex was largely impaired and HRB1 was predominately phosphorylated in pp7 mutant. We propose that a modification of HRB1 by PP7 under blue light is essential to acquire a proper conformation of the HRB1 protein complex or to bring in a new component for the assembly of a functional HRB1 protein complex.