Studies On The Screening Of Cd-Sensitive/Tolerant Rice Mutant And The Mechanism Of Antioxidant Responses To Cadmium Stress

We chose 700 isolated rice {Oryza sativa L. subsp. Japonica, cv. Zhonghua 11) mutants as material from the mutant system derived from gene transformation mediated via Agrobacterium tumefaciens. By exposure of Cd treatment, we obtained the Cd-sensitive/tolerant mutant rice (ST/TT). Our results indicated that Cd accumulation in leaves and seeds of the Cd-sensitive mutant were higher than that of the wild type, so the capacity of transporting Cd to shoots might be more powerful in the mutant. Furthermore, Cd accumulation in sheaths and seeds of the Cd-tolerant mutant was lower than that of the wild type. Otherwise, we analyzed Cd accumulation in the seeds of different T₂ plants of the Cd-tolerant mutant. We observed a high-accumulation Cd-tolerant mutant (HTT), in which Cd accumulation of the seeds was 937.9 μg·kg^-1 , 63.8% higher than that of the wild type. In addition, we observed a low-accumulation Cd-tolerant mutant (LTT), in which Cd accumulation of the seeds was 166.65 ug·kg^-1, 29% of the wild type. Then the high/low -accumulation Cd-tolerant mutant was added a generation to analyze the amplification patterns of Homomycin gene (Hyg) in T3 plants of Cd-tolerant mutant. The results indicated that there was not the Hyg in T2 and T3 lines, so we concluded that different inheritable character of accumulating Cd in the mutant was not caused by T-DNA inserting. More studies suggested that the capacity of transporting Cd to shoots in the high-accumulation Cd-tolerant mutant might be higher than that of the wild type. However, the Cd accumulation in seeds of the low-accumulation Cd-tolerant mutant was lower, since the Cd transporting from leaves to seeds could be inhibited.We analyzed the different antioxidant responses to cadmium stress in Cd-sensitive mutant rice seedlings. We observed significant phenotypic differences between a Cd-sensitive mutant (ST) and wild type (WT) rice seedlings. Our results indicated that Cd accumulation in the leaves of the mutant was twice that of the wild type after 10 days of 0.5 mmol L^-1 Cd²+ treatment. Furthermore, we observed a rapid Cd-induced H2O2 increase in the mutant leaves, which induced abnormally early activity inantioxidant enzymes such as superoxide dismutase (SOD). However, we found lower catalase (CAT) activities in the mutant leaves. By contrast, guaiacol peroxidase (G-POD) activities were higher in the mutant than in the wild type roots. Together with the Cd toxicity-induced decline of early responsive enzymatic activities in vivo, especially CAT, the inability of mutants to scavenge accumulated H2O2 led to higher lipid peroxide levels. H2O2 might strengthen the expression of G-POD as a signaling molecule. Thus, based on our research presented herein, we conclude that the decline of antioxidant enzymes activities is a physiological explanation for Cd sensitivity in the mutant and G-POD activity is a potential biomarker for reflecting Cd sensitivity in rice seedlings.