Hybridization of Cultivated Rice with its Wild Relatives to Incorporate Sheath Blight Resistance

Abstract

Wild relatives of crop plants often have served as additional sources of disease resistance genes. Sheath blight (Rhizoctonia solani Kühn) is one of the two major fungal diseases affecting rice (Oryza sativa L.). The objective of this study was to introduce additional sources of sheath blight resistance from wild relatives of rice (Oryza species) into southern U.S. rice cultivars. O. rufipogon (PI 590420) and 27 IRRI accessions reported as sheath blight resistant and representing eight wild Oryza species, were crossed with the long grain experimental line, RU9401188 and the medium grain cultivar, Bengal. F1 progeny were backcrossed to the cultivated rice parent. Embryo rescue is being used in attempts to obtain hybrids not acquired by emasculation and crossing. Pollen stainability and self seed set were determined for the Oryza species and F1 plants. Ratoon plants of Oryza species and F1 hybrids were 1) inoculated using toothpicks colonized with R. solani, 2) placed in a growth chamber for one week at 100% humidity, 14 h light/10 h dark and 28°C and 3) rated on a 0 to 9 scale for sheath blight resistance.F1 progeny were obtained from crosses with 21 of the 28 accessions and BC1 progeny from crosses with 17 accessions. All but one of the successful crosses was with an A genome species, O. barthii, O. glumaepatula, O. meridionalis, O. nivara or O. rufipogon. Male and female fertility, measured as pollen stainability and self seed set, respectively, were found in some of these F1 and BC1 progeny. One C genome, O. officinalis accession (IRRI no. 101399) hybridized with Bengal and most likely with RU9401188. No BC1 seed was obtained from the Bengal/101399 hybrids. RU9401188/101399 seed germinated and died at the four leaf stage. This appears to be a seedling lethality.Plants were obtained from crosses with three additional accessions using embryo rescue. Two of these accessions, O. australiensis (IRRI no. 103318) and O. latifolia (ERRI no. 100169), are E and CD genome species, respectively. The other accession, O. glumaepatula (IRRI no. 105670), has an A genome. To date, no plants have been obtained from crosses with O. alta (IRRI no. 100967), a CD genome species.Preliminary screening of the Oryza species for sheath blight resistance indicated the species tested were resistant (rating 0-2) or moderately resistant (rating 3–4). The F1 progeny tested were moderately resistant to moderately susceptible (rating 5–6). In field tests, RU9401188 had an overall rating of ‘4’ and Bengal a rating of ‘5–6’. The Oryza species had a rating of 1’ when rated for sheath blight resistance in the field at IRRI. Most likely, the difference in screening methods is part of the reason for the overall ratings being different.KeywordsRice CultivarWild RelativeDisease Resistance GeneRhizoctonia SolaniSheath BlightThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.