Proteases and the ubiquitin-proteasome system: Understanding protein degradation under heat stress in plants

Enhanced protein damage is a common consequence of heat stress in plants. One approach to removing these damaged proteins is to degrade them into amino acids via proteolytic machinery, such as proteases and the ubiquitin-proteasome system (UPS). Proteases are responsible for the breakdown of proteins inside the organelles, while the UPS conducts proteolysis mainly in the cytoplasm and nucleus by attaching polyubiquitin chains to the target proteins. This process is of particular importance in protecting cells against heat stress, as it prevents the accumulation of toxic aggregates, thereby reducing cellular aging and death while maintaining normal metabolic activities in plants. In this review, we focus on the roles of different protease families in plant responses to heat stress, including serine proteases, aspartic proteases, cysteine proteases, and metalloproteases. Additionally, we summarize and discuss the involvement of the UPS in thermotolerance, with special attention to two key components: E3 ligases and 26S proteasome. Furthermore, recent advances in ubiquitin-omics in the study of abiotic stress are highlighted, suggesting the potential of utilizing ubiquitin-omics as a powerful tool to identify more UPS substrates and to characterize their functions in heat stress response. Understanding of how protein degradation is regulated in response to heat stress provides a deeper insight into thermotolerance mechanisms in plants.