Straighthead Resistance Is Controlled by Two Dominant and Additive Genes in Rice.

Straighthead is a physiological disorder and causes grain sterility, which could result in complete yield loss when it is severe in rice. Most U.S. cultivars are susceptible to the disorder and water management that increases production cost is used for its prevention. Genetic understanding of straighthead resistance is the primary step for breeding new cultivars resistant to this disorder and improving production efficiency. Previous study has identified forty-two resistant cultivars that have no yield reduction under severe straighthead infestation. Two resistant cultivars, Zhe 733 (PI 629016) and Jing185-7 (PI 615205) from China were crossed with susceptible R312 (PI 614959) and Cocodrie (PI 606331), respectively for genetic study. R312 is from China and Cocodrie is popularly grown in the southern states. In 2007, 10 plants of each F1 and 300 individuals of each F2 for the populations were evaluated for straighthead disorder induced using monosodium methanearsonate (MSMA). The inferred genotypes were verified by their corresponding F3 families to each of the F2 individuals in 2008. Segregation ratio of 9 resistant : 6 intermediate : 1 susceptible individuals was true in both populations with χ²=2.41, P = 0.343 in Zhe733/R312, and χ²= 2.993, P = 0.224 in Cocodrie/ Jing185-7. The result demonstrates that straighthead resistance is controlled by two genes in additive action where the resistance is dominant over the susceptibility. An effort has been made to develop recombined inbred lines (RIL) from the populations for genomic characterization of straighthead resistance in order to assist breeding with molecular means.