Plastid Protein Degradation During Leaf Development and Senescence: Role of Proteases and Chaperones

Abstract

During leaf development, plastids undergo dynamic changes in morphology. Chloroplasts develop from proplastids during leaf growth: this process includes synthesis, import, and maturation of numerous chloroplast proteins. During leaf senescence, chloroplasts change gradually into a senescing form termed gerontoplast, with the breakdown of thylakoid membranes and the degradation of photosynthetic proteins. In these developmental processes, it is apparent that the proteolytic activity within chloroplasts is a key to control such remarkable morphological/functional changes. Processing and maturation of chloroplast proteins are very important since chloroplast development requires numerous proteins that are imported from the cytosol. Various efforts to elucidate the functions of chloroplast proteases have revealed the existence of signal peptidases (SPP, PreP, TPP, and PlsP1) that are involved in the processing and the maturation steps. In addition, the quality control of proteins is ­necessary for proper chloroplast development. Recent studies using Arabidopsis mutants have identified several important chloroplastic proteases (Clp, FtsH, Deg, and some intramembrane-proteases), which originated from bacterial homologs, in the quality control of proteins during chloroplast development. In contrast, studies on the degradation of chloroplast proteins during senescence implied that multiple pathways, not limited to chloroplast proteases, control protein degradation in this process. In addition to protein degradation inside the chloroplasts, degradation of engulfed whole-chloroplasts within the vacuole, and small spherical bodies like senescence-associated vacuoles (SAV), and Rubisco-containing bodies (RCB) that include chloroplast stromal proteins are known to occur during leaf senescence. The latter implicates that autophagy plays an important role in delivering chloroplast proteins into the vacuole. This chapter provides an integrated summary on the roles of chloroplast proteases during chloroplast development, and the current view of the chloroplast protein degradation during senescence.