Quinic Acid-Mediated Induction of Hypovirulence and a Hypovirulence-Associated Double-Stranded RNA in Rhizoctonia Solani

This study is a part of a project focused on the relationship between dsRNA and hypovirulence in R. solani. Hypovirulence refers to a condition in which a pathogen has a drastically reduced ability to cause disease. Rhizoctonia solani is a soil-borne pathogen causing diseases in numerous plants. Phenyl acetic acid (PAA), a metabolite of phenylalanine, causes Rhizoctonia disease symptoms on potato in the absence of the pathogen itself. The amount of PAA produced by a hypovirulent isolate is 10% of that produced by virulent isolates. A 3.6 kb dsRNA (M2) has been shown to be associated with hypovirulence in R. solani. Polypeptide A (PA) encoded by the M2 dsRNA is phylogenetically related to the pentafunctional polypeptide AROM of the shikimate pathway and QUTR, repressor of the quinate pathway in fungi. A hypothesis has been proposed to explain the mechanism of the M2 dsRNA-mediated hypovirulence. Polypeptide A may interfere with both the shikimate and quinate pathways, leading to a decreased production of aromatic amino acids and PAA, thus leading to a decreased virulence. Results of this study 1) supported this hypothesis, and 2) verified the relationship between the M2 dsRNA and hypovirulence in R. solani. A protein band of the expected size (83 kDa) was detected only in M2-containing isolates. The hypovirulent isolate Rhs 1A1 has a constitutive quinate pathway whereas the ' virulent isolate Rhs IAP has an inducible quinate pathway. Moreover, Rhs IAP has a higher level of expression for the shikimate pathway than Rhs 1Al. Data also sowed that phenylalanine levels were positively correlated with virulence in Rhs IAP. The addition of quinate converted virulent Rhs IAP to hypovirulent, and induced the synthesis of 1) a polypeptide of the same size as pA and reacting with anti-pA antibodies, and 2) the respective M2-specific transcript. For the first time, the arom gene has been cloned from Basidiomycetes. The R. solani arom gene has five introns as compared to one intron found in arom genes from other fungi. The deduced R. solani AROM polypeptide contains all of the highly conserved motifs and enzyme domains found in AROM polypeptides from other fungal species.