Tomato aspartic proteinases harbouring PSI domains reveal stress responsiveness, organ specificity, and conserved features

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.