Submitted to: Current Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2005
Publication Date: 6/3/2006
Citation: Lakshman, D.K., Liu, C., Mishra, P.K., and Tavantzis, S.M. 2006. Characterization of the arom gene in Rhizoctonia solani, and transcription patterns under stable and induced hypovirulence conditions. Current Genetics. 49:166-177. Interpretive Summary: Certain species of fungi typically cause disease in plants. Among these is Rhizoctonia solani, which causes black scurf disease, or stem and stolon canker, on potatoes, and other diseases on many other agronomic and ornamental plants. However, certain isolates of the fungus lose the ability to cause disease, a phenomenon called hypovirulence. These hypovirulent isolates can protect the crop from the effects of virulent (disease-causing) isolates of the fungus. In Rhizoctonia solani, it has been determined that hypovirulence, or reduced ability to cause disease, is associated with the presence of virus-like double-stranded RNA (dsRNA) molecules in the fungus, and that presence of the dsRNA causes the quinic acid biosynthetic pathway to be active at all times. In pathogenic isolates of the fungus, the quinic acid biosynthetic pathway is normally inactive, but can be induced by quinic acid in the soil from decaying organic materials such as crop residues. Increased activity in the quinic acid pathway results in decreased activity in the shikimic acid pathway, which is important in disease induction by the fungus. In this work regulation of the shikimic acid biosynthetic pathway was compared in virulent and hypovirulent isolates of the fungus Rhizoctonia solani in order to understand the disease process. Increased understanding of the disease interaction will aid in protection of plants against disease caused by Rhizoctonia solani.
Technical Abstract: We have previously shown that in the hypovirulent, M2 dsRNA-containing Rhizoctonia solani isolate Rhs 1A1, the quinic acid pathway is constitutive, in the absence of environmental quinate, but in the wild-type virulent isolate Rhs 1AP this pathway is inducible by quinic acid. Constitutive expression of the quinate pathway results in down-regulation of the shikimic acid pathway, which includes the pentafunctional arom gene in Rhs 1A1. The arom gene has 5,323 base pairs (bp) including five introns as opposed to a single intron found in the arom of ascomycetes. A 199-bp upstream sequence has a GC box, no TATAA box, but two GTATTAGA repeats. The largest arom transcript is 5,108 nucleotides long, excluding the poly(A) tail. It contains an open reading frame of 4857 bases, coding for a putative 1618 residues pentafunctional AROM protein. A Kozak sequence (GCGCCATGG) is present in the region between +127 and +135. The 5’-end of the arom mRNA includes two nucleotides (UA) that are not found in the genomic sequence, and are probably added post-transcriptionally. A certain degree of size and sequence heterogeneity was observed at both 5’- and 3’-end of the mRNA. Both northern blot hybridization and suppression subtractive hybridization analyses showed that presence of a low amount of quinate, inducer of the quinate pathway, resulted in increased levels of arom mRNA, consistent with the compensation effect observed in ascomycetes.