m-Hydroxyphenylacetic and m-Methoxyphenylacetic Acids of Rhizoctonia solani: Their Effect on Specific Root-Nodule Activity and Histopathology in Soybean

m-Methoxyphenylacetic acid (m-OMePAA), a derivative of m-hydroxyphenylacetic acid (m-OHPAA), having the same chemical composition as that phytotoxic compound produced in culture by Rhizoctonia solani, a fungal pathogen of soybean, reduced growth and symbiotic N₂-fixation activity of ‘Tracy’ soybeans in soil: perlite at 1.5 × 10^-4 M, the lowest concentration used. At twice this concentration m-OMePAA was strongly teratogenic and caused root hypertrophy and root fasciation. At 1.2 × 10^-3 M, m-OMePAA nearly suppressed seed germination. m-OMePAA at the minimum concentration used and equivalent concentrations of the culture filtrate fraction (m-OHPAA and m-OMePAA) caused cytopathological and histopathological disturbances in the nodule central tissue, extrusion of the nucleoli, and abnormal nuclci. The data indicate that these phytotoxic compounds of R. solani are involved in nodule impairment and reduced N₂-fixation in soybean.