Involvement of Nitric Oxide Signal in Alternaria Alternata Toxin Induced Defense Response in Rauvolfia Serpentina Benth. Ex Kurz Calli

This study elucidates Alternaria alternata toxin induced defense enzyme activity and phenolics accumulation and its correlation with nitric oxide (NO) generation. Role of NO in plant defense signaling to biotic stress has been strongly established by earlier investigators. Here, in vitro treatment of Rauvolfia serpentina Benth. ex Kurz callus with A. alternata toxin induced a rapid production of NO at 50 mg L-1 toxin concentration. This increase in toxin induced NO production was further confirmed by real time visualization of NO burst using a fluorescent probe, 4, 5-diaminofluorescein diacetate (DAF-2DA) in cells of R. serpentina. Parallely, the same concentration of toxin also induced the production of peroxidase (PO), polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL), beta-1, 3 glucanase and phenolics accumulation in the callus tissue of R. serpentina. Further, inhibition of NO production by co-treatment with NO synthase inhibitor, NG -nitro-L-arginine-methyl ester (L-NAME) at 10 muM and NO scavenger, 2-(4- carboxyphenyl)- 4,4,5,5- tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) at 100 μM reduced the accumulation of all the defense enzymes and total phenols in the callus signifying the regulatory role of NO in the induction of defense enzyme and phenol accumulation. The effect of NO in host defense response was crosschecked further by sole application of NO donor sodium nitroprusside (SNP). Treatment of R. serpentina callus with 100 muM SNP showed the similar pattern of defense responses. So, the overall results demonstrated that increased production of NO in R. serpentina callus treated with fungal toxin, might act as an essential signaling molecule for triggering the activation of PO, PPO, PAL, beta-1, 3 glucanase and phenolics accumulation.