Auxin Efflux Carrier OsPIN5c Positively Regulates Cold Tolerance by Modulating Reactive Oxygen Species and Water Homeostasis in Rice (Oryza sativa L.)

Low temperature severely constrains plant growth, development, and geographic distribution. Although auxin is well known for its role in plant growth and development, its involvement in plant cold tolerance remains largely unclear. Here, we explored the role of OsPIN5c, an auxin efflux carrier of PIN-FORMED (PIN) family that modulates auxin homeostasis through IAA metabolism, in regulating cold tolerance in rice ( Oryza sativa L.). OsPIN5c is predominantly expressed in rice root stele cells and coleoptiles at the seedling stage, and low temperature rapidly and continuously reduces its transcript levels. Loss-of-function mutants ( C5c ) exhibited significantly increased conjugated auxin content and altered agronomic traits. Under cold stress, C5c mutants showed impaired cold tolerance compared with wild-type (WT) plants, as evidenced by lower survival rate, higher electrolyte leakage, elevated malondialdehyde (MDA) content, and reduced soluble sugar levels. The mutants also accumulated more reactive oxygen species (ROS), including hydrogen peroxide (H 2 O 2 ) and superoxide anion radicals (O₂̇⁻), likely due to the reduced activities of antioxidant enzymes catalase (CAT) and peroxidase (POD) under cold stress. Furthermore, cold-stressed C5c mutants showed a marked decrease in relative leaf water content and a concurrent decline in root activity. Collectively, these findings demonstrate that OsPIN5c plays a critical role in rice cold tolerance and represents a promising candidate for the genetic improvement of cold tolerance in rice.