N⁷-SSPP Confers Drought Tolerance in Arabidopsis

Drought tolerance is critical for plant survival and productivity and is tightly linked to redox homeostasis and senescence regulation. SENESCENCE-SUPPRESSED PROTEIN PHOSPHATASE (SSPP), a negative regulator of leaf senescence, has previously been implicated in salt stress tolerance. However, whether SSPP functions in drought stress responses remains unknown. Here, we demonstrate that SSPP enhances drought tolerance in Arabidopsis thaliana. Although drought represses SSPP transcription, drought treatment attenuated N⁷-mediated SSPP degradation, in which N⁷, the N-terminal 1-14 amino acids of AtACS7, functions as a conditional degradation signal, resulting in stress-responsive accumulation of SSPP protein in N⁷-SSPP-overexpressing plants. Both SSPP- and N⁷-SSPP-overexpressing plants exhibited enhanced drought tolerance, with survival rates after rewatering reaching approximately 95% and 70%, respectively, whereas the sspp-1 mutant displayed pronounced drought sensitivity. Mechanistically, SSPP overexpression upregulated reactive oxygen species (ROS)-scavenging genes, enhanced antioxidant enzyme activities, and reduced drought-induced ROS accumulation, thereby mitigating oxidative damage. Notably, the N⁷ element enables conditional accumulation of SSPP under adverse conditions while preventing growth inhibition under normal conditions. Together, our findings reveal SSPP as a regulator connecting senescence-associated processes with drought stress adaptation and highlight the N⁷-SSPP fusion as a versatile strategy for improving stress resilience without compromising plant growth.