Analysis of proline accumulation, antioxidant capacity and HSP expression in mutant rice lines with different heat tolerance

Three mutant rice (Oryza sativa L.) lines (AG1, AG2 and AG3) were selected as heat tolerant mutants from a gamma-ray-irradiated population of a heat-susceptible line (AG), based on their floret fertility grown under high temperatures. They were subjected to heat stress treatment (45°C, 22 hrs) at the 5-leaf stage, together with a heat-tolerant cultivar N22 and AG. Analysis of seedling root growth by WinRHIZO scanning revealed that N22 and AG3 were more heat-tolerant than the other lines (AG being the most heat susceptible). Following heat stress, a significantly higher level of oxidative damage, as indicated by TBARS, was observed in AG than in N22 and AG3. The proline accumulation was significantly higher in N22 and AG3（12- to 13.5- fold）than AG (2.5- fold). Similarly, significantly greater increases of total antioxidant capacity (T-AOC) and superoxide dismutase (SOD) activity were observed in N22 and AG3 than in AG. The expression of four heat shock proteins was also investigated using qPCR: OsHSP16.9, OsHSP80.3 and OsHSP100.9were induced by heat stress to various levels while OsHSP72.6was down-regulated in all tested lines. The heat-induced expression of OsHSP16.9 and OsHSP100.9 in N22 and AG3 was about twice that of AG. Higher proline accumulation and expression of the three HSP genes, as well as elevation ofT-AOC and SOD activity were observed in the heat-tolerant N22 and the mutant line AG3 under heat stress