Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 15, 2005
Publication Date: February 15, 2006
Citation: Brooks, S.A. 2006. Differential response of rice cultivars to RS toxin, a pathogenicity factor in sheath blight disease of rice. Rice Technical Working Group Meeting Proceedings, February 29-March 1, 2006, Houston, Texas. 2006 CDROM. Technical Abstract: Rationale: Sheath blight is widely regarded as the second most important disease of cultivated rice and germplasm improvement is essential for disease management. Genetic sources of tolerance have been identified for the disease, however, phenotypic evaluation is ineffective for rice breeding programs due to environmental influence on disease and replications needed that result in extensive time requirements for reliable phenotypic data. To address this problem we are developing an early generation screen for Sheath Blight tolerance using the host-selective toxin produced by Rhizoctonia solani (Kuhn). RS-toxin assays are non-destructive to rice plants and quickly produce reliable replicated data on large sample sizes. Methods: Using liquid and semi-solid medias optimized for fungal growth and toxin production, cultures of R. solani are grown in constant light for 12 fays at 30 degrees Celsius. Crude toxin is harvested by filtration, purified by organic extraction and precipitation, and concentrated to ten-times the original volume. Bioassays using serial toxin dilutions are performed to determine relative toxin concentration in a preparation. Using a Hagborg device, approximately 30 microliters of toxin is forcibly infiltrated into the leaves of 30-45 day old rice plants. Response is recorded five days post-infiltration as a percentage of necrosis in the infiltrated area. Results: RS-toxin is host selective and when purified from cultures of R. solani it produces a differential response in rice varieties Jasmine 85 (MR, tox-) and Cypress (VS, tox+) that is correlated with disease susceptibility. To confirm this correlation and evaluate RS-toxin as a means to detect disease tolerance, numerous cultivars with known disease ratings are being evaluated for toxin response. Preliminary data shows a correlation exists and the methodology and verification will be reported.