Molecular cloning and functional analysis of two novel polygalacturonase genes in Rhizoctonia solani

Two novel polygalacturonase (PG) genes, RsPG3 and RsPG4, were cloned from Rhizoctonia solani isolate YN-7, which is a member of anastomosis group (AG) subgroup 1A and causes rice sheath blight. The predicted protein product of RsPG3 contained 239 amino acid (aa) residues, with a molecular mass of 25.0 kD, whereas RsPG4 encoded a deduced protein of 345 aa residues, with a mass of 37.5 kD. Sequence alignment, phylogenetic analysis and gene ontology indicated that RsPG3 and RsPG4 had endo-PG and exo-PG activity, respectively. Recombinant RsPG3 and RsPG4 both exhibited PG activity that led to the destruction of rice sheaths and release of reducing sugars. Pathogenicity assays showed that the two PGs induced tissue necrosis in rice sheaths, indicating that both RsPG3 and RsPG4 are important virulence factors in the R. solani-rice interaction. Further studies are underway to more clearly define the role of these enzymes in rice cell wall degradation and their interaction with PG inhibitor proteins.