Role of riboflavin and thiamine in induced resistance against charcoal rot disease of soybean
Charcoal rots (Macrophomina phaseolina (Tassi) Goidanich) of soybean (Glycine max (L.) Merr.) is a disease of economic significance throughout the world. Pathogenicity of 14 isolates of M. phaseolina was tested on soybean cv. Giza 21 under greenhouse conditions. The obtained data indicated that all the obtained isolates were able to attack soybean plants and caused charcoal rot on the basal stem with various degrees of diseases severity. M. phaseolina isolate S13 caused the highest charcoal rot severity (60%) followed by isolates S11 and S8 (57.9 and 56.3%, respectively). The effect of two inducer chemicals, that is, riboflavin (B2) and thiamine (B1) on the induction of systemic resistance in soybean against charcoal rot disease as well as biochemical changes associated with these treatments in soybean plants was investigated. Under greenhouse condition, the dose effect of 0.1 to 15 mM riboflavin and thiamine showed that 2.5 mM of riboflavin and 5 mM of thiamine was sufficient for maximum induction of resistance; higher concentration did not increase the effect. On the other hand, plants treated with riboflavin and thiamine and inoculated with pathogen grew higher than plants treated with sterilized distilled water (SDW) and inoculated with pathogen and increased fresh and dry weight of soybean plants. 10 mM concentration of riboflavin and thiamin recorded the highest dry and fresh weight compared with the control. In time course observation, it was observed that riboflavin and thiamine treated soybean plants induced resistance one day after treatment and reached its maximum level from 5 to 7 days in the case of riboflavin and 6 to 8 days in the case of thiamine and then decreased. Under field conditions, the percentage of damping-off, root rot and/or charcoal rot severity were significantly reduced due to soaking of the seeds in any of riboflavin and thiamine before sowing compared with the control treatment (seed treated with water) in both seasons (2008 to 2009 and 2009 to 2010). Also, these treatments significantly increased nodule numbers per plant, fresh and dry weight of nodules per plant compared with the control in both seasons. Generally, thiamine gave the best results in most cases under greenhouse and field conditions. In physiological studies, activity of defense-related enzymes, including peroxidase (PO), polyphenol oxidase (PPO), phenylalanine ammonia lyase (PAL), pathogenesis related (PR) protein (chitinase), were increased in the inoculated and non-inoculated plants treated with the thiamine and riboflavin respectively, compared with the control during the experimental period. In general, activity of these enzymes began to accumulate after two days of treatment and reached maximum levels at 8, 6, 8 and 8 days for PO, PPO, PAL and chitinase, respectively then the activities of these enzymes decreased progressively. On the other hand, total phenols and lignin increased in soybean plants inoculated with M. phaseolina and treated with thiamine and riboflavin. The highest accumulation of phenols was recorded 6 days after treatment, while lignin recorded the highest level at the 10th day from application. These results suggested that these chemicals mayplay an important role in controlling the soybean charcoal rot disease, through induction of systemic resistance in soybean plants.
Key words: Soybean, charcoal rot, thiamine, riboflavin, induced resistance.