Further enhancement of cold tolerance in rice seedlings by Piriformospora indica collaborating with plant growth-promoting bacteria: Evidence from the antioxidant defense, osmoregulation, photosynthesis, and related genes

• The addition of PGPB could better perform the function of P. indica . • PGPB could help P. indica to further enhance the cold tolerance of rice. • Co-inoculation of PGPB with P. indica enhance the photosynthesis of rice. • Co-inoculation of PGPB with P. indica promoted rice growth through osmoregulation. As the global population continues to grow, so does the demand for food. However, cold stress due to global climate change greatly threatens the safe production of rice. Piriformospora indica plays a pivotal role as a functional microbiota in improving cold tolerance in rice. However, the addition of P. indica alone has a limited effect. In this study, an attempt was made to select plant growth-promoting bacteria ( Agrobacterium rhizogenes and Bacillus subtilis ) as helper bacteria to help P. indica function better and further enhance rice cold tolerance. Under cold stress, the co-addition of the three beneficial microorganisms significantly increased the biomass, photosynthetic performance indicators, osmoregulatory substance contents, and antioxidant enzyme activities of rice. P. indica and helper bacteria significantly reduced the cold stress-induced increases in malondialdehyde content and electrolyte leakage rate in rice leaves and triggered a substantial upregulation of cold-tolerant genes in rice. This suggests that P. indica and helper bacteria can synergistically improve the cold resistance of rice. The findings of the research offer a conceptual foundation and practical assistance for enhancing rice's resilience to cold temperatures.