Cold Stress Mechanisms in Asian Rice Subspecies and Overexpression of OsUBC7 for Enhancing Rice Cold Tolerance

By 2050, the projected 9.7 billion global population will increase food demand, especially for staple crops. Climate change, with extreme temperature fluctuations, heavily impacts cold-sensitive Asian rice (Oryza sativa L.) grown in tropical and subtropical areas. Therefore, understanding the response mechanisms of two Asian rice subspecies, japonica and indica, which have distinctive tolerance to cold stress, becomes important for improving crop cold tolerance. Hence, this study tested our hypotheses to address how japonica can tolerate cold exposure better than indica: (1) japonica selectively adjusts antioxidant activities to counter reactive oxygen species (ROS), while indica rapidly increases antioxidant activities early; (2) japonica increases antioxidants to prevent damage after prolonged exposure, while indica fails to do so; (3) japonica slows water uptake to sustain minimal photosynthesis during cold, while indica’s uptake mechanisms are compromised; (4) Overexpression of OsUBC7, a ubiquitination protein, improves cold tolerance in cold sensitive rice accession. To test these hypotheses, this study investigates the enzymatic antioxidant activities and water uptake strategies employed by both subspecies under different cold treatments. Results reveal distinctive strategies in managing ROS and antioxidant activities, with japonica exhibiting fluctuating antioxidant activities to potentially activate defense pathways, while indica demonstrates rapid but possibly excessive and expensive ROS scavenging responses. Moreover, contrasting water uptake patterns are observed, with indica accessions experiencing significant reductions under cold, in contrast to japonica moderate declines, suggesting relative outcomes. Furthermore, this study explores the role of the cold candidate gene OsUBC7 in cold stress responses and productivity. Overexpression of OsUBC7 in cold sensitive accession enhances plant resilience to cold stress by increasing growth rate, sugar metabolism, and chlorophyll content, ultimately contributing to more efficient recovery and higher survivability. Additionally, OsUBC7 shows potential involvement in flowering and yield, suggesting promising roles in productivity. In conclusion, this work demonstrates the complex response mechanisms of Asian rice subspecies to cold stress, emphasizing the importance of ROS perception and management, water uptake strategies, and genetic factors for improving cold stress tolerance. The findings provide insights into the adaptive strategies of the two subspecies and help develop effective strategies to improve crop cold tolerance in fluctuating environments.