Time-course study of the induction of defense enzymes, phenolics and lignin in rice in response to infection by pathogen and non-pathogen

Rice leaf sheaths inoculated with Rhizoctonia solani (pathogen) and Pestalotia palmarum (non-pathogen) were analyzed for the induction of phenylalanine ammonia-lyase (PAL), peroxidase (PO), polyphenol oxidase (PPO), accumulation of phenolics and lignin. PO and PAL activities were significantly increased in rice leaf sheaths after inoculation with R. solani and P. palmarum. However, the increases in these enzyme activities were significantly higher in response to inoculation with non-pathogen compared to inoculation with pathogen. A 3-fold increase in PO and PAL activities was observed 2 days after inoculation with P. palmarum. PO isoenzyme analysis indicated that PO-1, PO-3 and PO-4 were induced 2 days after inoculation with P. palmarum. In contrast, in the compatible interaction these isoenzymes did not accumulate until 3 days after inoculation. PPO isozyme analysis indicated that PPO-2 increased dramatically in rice 1 day after inoculation in the incompatible interaction. The same level of activity corresponding to this isoenzyme was seen 3-4 days after inoculation in compatible interactions. PPO activity also rapidly increased in rice leaf sheaths after inoculation with the non-pathogen. About a 3-fold increase in PPO activity was recorded 2 days after inoculation with P. palmarum compared to control. Non-pathogen inoculated rice leaf sheaths exhibited rapid accumulation of phenolics and the maximum phenolic content was recorded 4 days after inoculation. Inoculation of P. palmarum increased the lignin content of rice by 91% above control levels by 1 day while R. solani caused only 37% increase. The present study provides evidence for the important role of phenolics and lignin in the resistance of rice against non-pathogen.