Evolutionary and Functional Insights into PSI-Containing Aspartic Proteinases in Tomato

Aspartic proteinases (APs) are essential proteolytic enzymes involved in plant growth, development, and defence. This study investigates PSI-containing Aspartic Proteinases (APs) in Solanum lycopersicum (tomato plant), focusing on their molecular characteristics, tissue-specific expression, and response to abiotic stress. While 58 APs were identified in S. lycopersicum , only five (AP V, AP W, AP X, AP Y, and AP Z) contained a C-terminal vacuolar sorting domain and the Plant Specific Insert (PSI) domain, a unique domain implicated in vacuolar trafficking and stress responses. These PSI-containing APs were further analyzed through phylogenetics, revealing their evolutionary relationships with homologous APs from other species. Expression profiling showed that most PSI-containing APs are highly expressed in cotyledons and roots, with AP Y being distinctively predominant in reproductive tissues. Under osmotic and saline stress, expression patterns varied, with AP Z exhibiting a broader stress sensitivity. Biochemical analyses confirmed that severe osmotic stress led to increased oxidative stress markers and antioxidant responses, suggestive of activation of the plant-adaptive mechanisms to abiotic stress. As a whole, it is shown that PSI-containing APs from Solanum lycopersicum are strictly regulated within the plant and exhibit varied responses to environmental stressors.