Induction of resistance in rice against Pyricularia oryzae by two plant growth promoting rhizobacteria

Rice blast disease caused by Pyricularia oryzae is one of the most important rice diseases worldwide. Given the growing demand for non-toxic and chemical-free products, identification of plant growth-promoting microorganisms that can assist in mitigating challenges to plant growth is very important in sustainable rice cultivation. This study aimed to investigate the effect of two plant growth-promoting rhizobacteria (PGPR) isolates, Alcaligenes faecalis strain O1R4 and Bacillus idriensis strain MR2, on the induction of resistance-related enzymes such as peroxidase and catalase and on the severity of rice blast in Hashemi and Hassan Sarai cultivars.

The roots of the seedlings (cv. Hashemi and Hassan Sarai) were inoculated with a suspension of bacterial isolates (106-107 CFU/ml) for one hour and planted into pots. The plants were then sprayed in the four-leaf stage with an aqueous spore suspension (2×105 spore/ml) until run-off. The inoculated plants were transferred to the greenhouse at 28 °C and 90% relative humidity. Catalase and peroxidase activity in rice leaf extracts from different treatments were assayed one, three, five, and seven days after inoculation with blast fungus using a spectrophotometer. The activity was expressed in U µg–1 protein. The experiment was done in a completely randomized design with eight treatments and three replications for each cultivar. Also, the blast severity was assessed in the leaves of inoculated plants with six replications. The data were analyzed by SAS software (version 9.1) and Tukey's test at 0.05 and 0.01 probability levels. 

According to the analyses of variance, there was a significant influence of bacterial inoculation on the activity of peroxidase enzyme on the third, fifth, and seventh days. On the other hand, the activity of catalase enzyme increased on the first day to the seventh day at the 1% probability level. Mean comparison revealed in the treatments consisting of A. faecalis + P. oryzae, the levels of peroxidase and catalase enzymes were respectively elevated by 97% and 43% in Hashemi cultivar and by 174% and 125% in Hassan-Sarai cultivar compared to the positive control. Also, in the presence of B. idriensis + P. oryzae, the amount of peroxidase and catalase enzymes were respectively heightened in cv. Hashemi by 69% and 33% and in cv. Hassan-Sarai by 195% and 121% compared to the positive control. In the simultaneous use of two bacterial isolates and P. oryzae, the amount of catalase enzyme increased in Hashemi and Hassan-Sarai cultivars by 73% and 166% and the amount of peroxidase enzyme reached 174% and 319% in Hashemi and Hassan-Sarai cultivars, respectively compared to the positive control. Finally, the results showed that the blast severity was reduced using A. faecalis strain O1R4 and B. idriensis strain MR2 by 37% and 20% in the cultivars respectively.

In the present study, peroxidase activity in all treatments consisting of blast fungus was approximately at the same level within 24 hours post-inoculation. Then, the peroxidase level increased and maintained until the seventh day. Further, the catalase activity increased in the presence of the blast fungus and PGPRs. The level of peroxidase activity increased one to three days after inoculation. In comparison, the level of catalase increased within 24 hours after inoculation. The simultaneous use of two bacterial isolates along with pathogenic fungus was more effective than usage of each isolate separately. In general, the results of this research were consistent with previous studies that indicate the bacterial isolates can reduce the blast severity by inducing systemic resistance in rice.