NpSCI2A, a Potato Inhibitor I Protein from a Wild Sugarcane Relative, Negatively Regulates Drought Tolerance by Targeting Subtilases

Background Drought severely limits sugarcane productivity. The wild relative of sugarcane, Narenga porphyrocoma , is highly drought-tolerant and represents a valuable source of stress-resistance genes. We identified NpSCI2A , encoding a s potato inhibitor I that is significantly downregulated under prolonged drought. Results Overexpression of NpSCI2A in rice markedly increased drought sensitivity, as evidenced by more severe leaf wilting, reduced antioxidant enzyme activities, elevated H₂O₂ and MDA levels, and lower survival rates. Transcriptome profiling revealed that NpSCI2A overexpression together with drought additively suppressed chloroplast- and photosynthesis-associated genes. Through co-expression analysis and protein–protein interaction assays, we identified two subtilases, SBT1.2 and SBT1.8, as direct physical interactors of NpSCI2A. Interestingly, SBT1.2 and SBT1.8 transcript levels were paradoxically upregulated in NpSCI2A-overexpressing plants under drought, suggesting a compensatory feedback response to impaired protease activity. Conclusions These findings establish NpSCI2A as a negative regulator of drought tolerance that acts by inhibiting SBT proteases, thereby disrupting chloroplast protein homeostasis and exacerbating oxidative damage. Our study uncovers a functional link between protease inhibitor-mediated proteolytic regulation and chloroplast function, offering new insights into plant drought adaptation and a potential target for the molecular breeding of stress-tolerant sugarcane.