Pathogenic Mechanism Of Rhizoctonia Solani Toxin And Its Effects On Activity Of Defensive Enzymes In Rice Plants

The crude toxin of Rhizoctonia solani produced in improved Richard's medium was extracted when the cultured filtrate was absorbed by active carbon with methanol as eluant and then the methanol was evaporated by rotavapor. When the rice seeds were treated by the crude toxin, the radicle growth was obviously inhibited and the higher the toxin concentration was, the more virulent it was.The crude toxin of R. solani have obvious damage to the cell membrane of rice sheath and leaves. When rice sheath and leaves were treated by the crude toxin, the permeability of cell membranes was changed and many substrates in the cells outflowed. When rice plants were treated by the crude toxin diluted 5 to 10 times, the damage rate of cell membranes was more than 80% and the longer the treatment time was, the more serious the damage was. In addition, the crude toxin could significantly interfere with the chlorophyll synthesis. Treated by the crude toxin diluted 10 times, the chlorophyll cotent in the rice plants was reduced by over 85%. The higher the toxin concentration was and the longer the treatment time was, the lower the chlorophyll cotent was.The observation by TEM showed that the crude toxin could seriously destroy the cells and cell ultramicroscopic structures of rice sheath. Treated by the crude toxin for 12 hours, chloroplast membrane was damaged, and chloroplast lamella was relaxed and swollen, with some cava in the chloroplast. Treated by the crude toxin for 36 hours,the chloroplast was disassembled, and other organelles such as mitochondria became unclear and disappeared, and some parts of the cell wall were thin and loose.After the treatment of R. solani crude toxin, the activity of defensive enzymes such as peroxidase (POD), polyphenol oxidase (POD), phenylalanie ammonialyase (PAL) and superoxide dismutase (SOD) in rice sheath and leaves was stimulated to some intension. When rice plants were treated with the crude toxin diluted 20 to 100 times, the activity of these enzymes were significantly increased, while treated with the crude toxin diluted 1000 times, the enzyme activity was little changed. Also, the results showed that the higher the toxin concentration was and the longer the treatment time was, the higher the enzyme activity in rice plants was. When treated with the crude toxin for 48 to 60 hours, the enzyme activity reached the maximum, and then decreased as the treatment time was prolonged.