Role of Riboflavin and Thiamine in Induced Resistance Against Charcoal Rot Disease of Soybean

Charcoal rot (Macrophomina phaseolina (Tassi) Goidanich) of soybean (Glycine max (L.) Merr.) is a disease of economic significance throughout the world. Pathogenicity of 14 isolates of Macrophomina phaseolina was tested on soybean cv. Giza 21 under greenhouse conditions. They capable to infect soybean plants caused charcoal rot on the basil stem with various degrees of diseases severity. Isolate S13 caused the highest charcoal rot severity (60%) followed by isolates S11 and S8 (57.9 and 56.3 %, respectively).
The positive effect of two inducer chemicals, i.e riboflavin (vitamin B2) and. thiamine (vitamin 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 recorded.
Under greenhouse conditions, 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 grow higher than plants treated with sterilized distilled water (SDW) and inoculated with pathogen, while increased fresh and dry weight of soybean plants. At 10 mM concentration of riboflavin and thiamin recorded the highest dry and fresh weights. In time course observation, it was observed that riboflavin and thiamine treated soybean plants were inducing resistance one day after treatment and reached its maximum level from 5 to 7 days in case of riboflavin and 6-8 days in case of thiamine and then decreased.
Under field conditions, the percentages of damping-off, root rot and/or charcoal rot severity were significantly reduced due to soaking the seeds in any of riboflavin and thiamine before sowing in both trial seasons (2008-2009 and 2009-2010). Also, these treatments significantly increased nodule numbers plant ^-1, fresh and dry weight of nodules plant ^-1 in both experimental 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, polyphenol oxidase, phenylalanine ammonia lyase, pathogenesis related (PR) protein (chitinase), were increased in inoculated and non-inoculated plants treated with the thiamine and riboflavin individually, during the experimental period. In general, activity of these enzymes begins to accumulate after two days of treatment and reached maximum levels at 8, 6, 8 and 8 days for PO, PPO, PAL and chitanase, respectively, then the activities of these enzymes were 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 6th days from application, while lignin recorded the highest level at 10th days from application. These results suggested that these chemicals may be play an important role in controlling the soybean charcoal rot disease, though they have induction of systemic resistance in soybean plants.