Increased production of IAA by Rhizoctonia solani is induced by culture filtrate from rice suspension cultures

To investigate the role of the plant hormones produced by fungi, we tried to construct a system to examine the interaction between Rhizoctonia solani Kuhn MAFF305219 and rice cells in suspension culture (Oc). R. solani was previously found to produce IAA, with the main biosynthetic pathway via the indole-3-pyruvate pathway. The amount of IAA in the medium produced by R. solani was increased by cocultivation with rice cells (Oc) and by culture filtrate (CF) of Oc, Further analysis revealed that the factor(s) that induced the enhanced accumulation of IAA was sensitive to heat, to freezing and thawing and lyophilization, and the molecular weight was estimated to more than 10,000. These results suggest that the active agent(s) in the medium was (a) protein or a proteinous substance. Among suspension cultures of various plants, Oc and another line of rice cells (Ok) had the ability to induce the accumulation of IAA in the fungal medium 4 h after inoculation but other cultures of plant cells were ineffective. The promotive effect of rice CF on the accumulation of IAA was also observed with some strains of R. solani that belong to a different anastmosis group from MAFF305219. Thus, the accumulation of IAA was not related to the host specificity.