Modulation of the Phenylacetic Acid Metabolic Complex by Quinic Acid Alters the Disease-causing Activity of Rhizoctonia solani on Tomato

A tomato foliar blight disease of unknown etiology was observed in North Carolina (NC) during 2005--2006. The morphology of isolates from symptomatic leaves was consistent with that of the soil fungus Rhizoctonia solani Kuhn. Because the pattern of symptom expression suggested that basidiospores were the primary inoculum source, Koch's postulates were fulfilled using a method to generate basidiospores in planta. Isolates were characterized by morphology, DNA sequence analysis, hyphal anastomosis, and somatic hyphal interactions, all of which supported placement of the isolates in R. solani anastomosis group 3 (AG-3). To our knowledge, this is the first detailed report of tomato foliar blight caused by R. solani AG-3 in North America.;R. solani produces phenylacetic acid (PAA), hydroxy (OH-), and methoxy (MeO-) derivatives of PAA with demonstrated phytotoxic activities. However, limited information is available on the specific role that these compounds play in the development of Rhizoctonia disease symptoms and concentration(s) required to induce a host response. Recent reports that PAA inhibits the growth of R. solani conflict with the established ability of the fungus to produce and metabolize PAA. Experiments were conducted to clarify the role of the PAA metabolic complex in Rhizoctonia disease by determining the concentrations of PAA, and derivatives of PAA required to induce development of root necrosis and stem cankers on tomato in the absence of the fungus, and the concentration at which PAA inhibits the growth of R. solani. The effects of exogenous PAA and derivatives of PAA on tomato seedling growth were also investigated. PAA and derivatives of PAA induced root necrosis and stem cankers, and reduced root growth. R. solani biomass was reduced when PAA was added to the growth medium. Gas chromatography-mass spectrometry (GC-MS) was used to quantify PAA and derivatives of PAA from the culture medium of 14 genetically diverse isolates of R. solani belonging to three anastomosis groups. Correlation analyses revealed relationships between the production of PAA and derivatives of PAA and the disease causing activities of R. solani isolates on tomato seedlings.;The metabolic control of plant growth regulator production by R. solani and consequences associated with the parasitic and saprobic activity of the fungus were investigated by growing isolates of R. solani on Vogel's minimal media with and without the addition of a 25 mM quinic acid (QA) source of carbon. The effect of QA on fungal biomass was determined by measuring the dry weight of mycelia produced under each growth condition. QA stimulated growth of 13 of 14 isolates of R. solani examined. The production of PAA and derivatives of PAA on the two different media was compared by GC-MS. The presence of QA in the growth medium of R. solani altered the PAA production profile, limiting the conversion of PAA to derivative forms. The effect of QA on the ability of R. solani to cause disease was examined by inoculating tomato plants with 11 isolates of R. solani AG-3 grown on media with and without the addition of 25 mM QA. Mean percent survival of tomato plants inoculated with R. solani was significantly higher when the fungal inoculum was generated on growth medium containing QA. The results of this study support the hypotheses that utilization of QA by R. solani leads to reduced production of the plant growth regulators belonging to the PAA metabolic complex which can suppress plant disease development.