Role of the Salt Tolerance-Related Protein (STRP) in the responses of Arabidopsis thaliana to infection by Pseudomonas syringae pv. tomato DC3000

The Salt Tolerance-Related Protein (STRP) is a small (16 kDa), intrinsically disordered protein of Arabidopsis thaliana, which has been shown to play a protective role against cold and salt stress. However, its role in biotic stress responses remained unexplored. Hence, here we investigated its role in the response to infection by the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), a model organism for studying plant-pathogen interactions. Upon Pst DC3000 infection, STRP rapidly accumulates in the cytosol and nucleus. RT-qPCR and treatment with the proteasome inhibitor MG132 revealed that protein accumulation was not due to increased transcription, but rather to reduced degradation via the 26S proteasome. Reverse genetics experiments using the strp knockout mutant and STRP-overexpressing (STRP OE) lines showed that STRP limits disease symptoms, since necrotic lesions were larger in strp mutants and reduced in STRP OE plants with respect to wild-type plants. STRP also modulates typical Pattern-Triggered Immunity (PTI) responses, such as stomatal closure and callose deposition, which were impaired in strp mutants but enhanced in STRP OE plants. Consequently, strp mutants exhibited higher endophytic colonization. Overall, the findings demonstrate a positive correlation between STRP levels and the activation of plant defense responses to Pst DC3000, suggesting a novel role for STRP in biotic stress tolerance mechanisms in Arabidopsis thaliana.