Focus on proteolysis

Proteolysis is an essential cellular function mediating the processing and turnover of proteins to remove damaged or inactive proteins, alter protein function (binding or enzymatic activities), and ensure appropriate protein stoichiometries in the cell.The post-translational control of protein stability is also a central feature of cellular signaling in eukaryotes, i.e. regulating the turnover of important regulatory proteins, and is crucial for almost all aspects of plant biology, including vegetative growth, development, reproduction, and stress responses.Proteolysis plays a central role in hormone signaling pathways, plant defense against pests and pathogens, abiotic stress responses, and basic cell functions like the cell cycle, metabolism, organellar biogenesis and maintenance, and senescence.Compared with animals, plant genomes encode a highly expanded number of components related to proteolytic pathways including the ubiquitin-proteasome system, autophagy, programmed cell death, endosomal trafficking, and organelle-associated protein degradation.Recent advances in molecular genetics and cell biology, microscopy and high-resolution imaging, in vivo labeling, proteomics, mass spectrometry, and structural biology have led to new insights and understanding in many areas of plant proteolysis, including autophagy of chloroplasts and other organelles, degradation of membrane proteins, the discovery of plant N-degron pathways, and proteolytic processing involved in plant development and environmental responses (immunity and abiotic stress).Knowledge of plant proteolytic systems is also important for agriculture and plant breeding, given the impact on plant growth and development related to yield, as well as plant resistance to pests and diseases.This focus issue on plant proteolysis includes two letters to the editor, one commentary, nine review articles, and eight original research articles.The two letters address the question of whether the polyubiquitin pathway operates inside intact chloroplasts.The question is posed by van Wijk and Adam (2024), with a Reply from Jarvis et al. (2024).Researchers on both sides of the debate make valid points to be considered.We eagerly await more definitive data to arrive at a complete understanding of protein degradation inside chloroplasts.The commentary by Eckardt et al.