| Abstract: | All isolates of Rhizoctonia solani used in this study, when grown in Weinhold medium supplemented with phenylalanine, accumulated phenylacetic acid (PAA) and its ortho-, meta- and para-hydroxy derivatives in culture. Major metabolites which accumulated were PAA and m-hydroxyphenylacetic acid (m-OH-PAA). However, the accumulation in vitro of total phenylacetic acid derivatives by isolates of R. solani was not related to their pathogenicity and/or virulence. On the contrary, the non-pathogenic and weakly virulent isolates of R. solani produced the highest quantities of PAA-derivatives per unit weight of mycelium in comparison to the highly virulent isolates.The phenylacetic acid derivatives used in this investigation, when bioassayed at high concentrations, caused inhibition of growth of tomato and bean seedlings. The most active compounds were PAA and -α-methoxy-phenylacetic acid (MeO-PAA). However, when m-OH-PAA was assayed at low concentrations there was a stimulation of seedling growth. In addition, when m-OH-PAA solutions were injected into bean hypocotyls or into tobacco leaves they never caused necrotic lesions or symptoms resembling those caused by R. solani on beans.These results indicate that PAA-derivatives are not directly involved in pathogenesis of R. solani in beans. |
