Enhancement of sheath blight tolerance in transgenic rice by combined expression of tobacco osmotin (ap24) and rice chitinase (chi11) genes

Agrobacterium-mediated transformation was used to introduce pathogenesis-related protein genes into scutellum-derived callus of rice (Oryza sativa L. subsp. indica 'White Ponni') as a means of increasing resistance to sheath blight disease caused by Rhizoctonia solani Kuhn. Transformation with a tobacco osmotin (ap24) gene driven by the Cauliflower mosaic virus 35S promoter (P35S) yielded six single-copy transgenic lines. Homozygous T-2 plants of all lines accumulated high levels of the ap24 transcript and the osmotin protein and reduced the percent disease index (PDI) of sheath blight disease from 100% down to 49-77%. Transformation with the binary plasmid pNSP2, which harbored the P35S-ap24 gene and a maize ubiquitin promoter (PUbi1)-driven rice chitinase (chi11) gene in the same TDNA, yielded three single-copy transgenic lines: CO1, CO2, and CO3. Homozygous T-2 plants of all three lines accumulated high levels of the chi11 transcript and the chitinase protein. Accumulation of ap24 transcript and osmotin protein was high in homozygous CO1 and CO2, but very low in CO3. At 7 d post infection with R. solani, the transgenic rice lines C18a (with chi11 alone), O8 (with ap24 alone), and CO1, CO2, and CO3 (with chi11 + ap24) reduced the sheath blight PDI from 100% to 64.2, 57.2, 43.3, 50.2, and 58.6%, respectively. At 15 d post infection, the sheath blight disease PDI was significantly lower in the CO1 and CO2 transgenic rice lines, which expressed both chi11 and ap24, than in those which expressed either chi11 or ap24 individually.