The Cysteine Protease Gene MdCP37 Negatively Regulates Drought Tolerance in Apple.

Drought threatens apple (Malus domestica) growth and productivity, often leading to irreversible economic losses. C1-papain family cysteine proteases are key enzymes in plant responses to abiotic stress, yet their specific roles under drought conditions remain largely uncharacterized in apple. Sequence analysis identified MdCP37 as a member of the C1-papain family. Here, we demonstrate that overexpression of MdCP37 (OE) in transgenic apple increases drought sensitivity, while RNAi-mediated silencing of MdCP37 enhances drought tolerance. Under drought stress, MdCP37-OE lines displayed reduced antioxidant enzyme activity and suppressed expression of drought-responsive genes, whereas RNAi lines exhibited opposite trends. Moreover, MdCP37 accelerated chlorophyll and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) degradation and promoted leaf senescence under drought conditions. Collectively, our findings establish MdCP37 as a negative regulator of drought tolerance in apple and offer theoretical support for improving drought resilience in apple breeding programs, particularly in arid regions.