Chloroplast Protein Degradation Pathways During Leaf Senescence: Coordination and Roles in Nitrogen Remobilization

Leaf senescence is a highly programmed developmental process that enables nutrient remobilization from source to sink organs in plants. Chloroplasts serve as the primary nitrogen reservoir, and the timely degradation of their constituent proteins is essential for sustaining seed fill and crop yield. For efficient nitrogen recycling, the chloroplast proteome must be dismantled in a controlled manner through coordinated proteolytic pathways that function both inside and outside the organelle. Protein abundances are progressively reduced during senescence through the balanced action of degradation and the suppression of new synthesis, depending on the plant’s carbon–nitrogen status. Malfunctioning or damaged chloroplast components are selectively eliminated. To date, three major classes of chloroplast protein degradation systems have been characterized: the cytoplasmic ubiquitin–proteasome system (UPS), vacuolar degradation pathways, and intra-chloroplast proteases. Here we provide an updated, comprehensive overview of the three chloroplast protein degradation machineries and discuss their coordinated roles in nitrogen remobilization. We also consider how these pathways might be manipulated to improve nitrogen use efficiency (NUE) and crop productivity.