Characterization of a Candidate Gene at qCTS12, a Major QTL for Seedling Cold Tolerance in Rice (Oryza sativa L.).

Cultivated rice (Oryza sativa L.) is sensitive to temperatures below 15 to 20°C. Cold stress at the seedling stage increases mortality and delays development resulting in poor stand establishment and non-uniform crop maturation. Seedling cold tolerance is of importance in the USA where rice is planted using direct seeding methods that greatly increase exposure to low temperature stress. One of our goals is to understand the molecular mechanisms involved in seedling cold tolerance in rice.

M-202 is a popular rice variety grown in California which has good seedling cold tolerance. Previous genetic analysis of M-202, a temperate japonica, resulted in the identification of a major QTL, qCTS12, which confers seedling cold tolerance to IR50, a sensitive indica variety from the Philippines. The effect of qCTS12 is observed under temperature conditions that result in wilting and necrosis in IR50. Fine mapping of the qCTS12 locus identified two members of the zeta class of glutathione S-transferases (GST). One of these genes, OsGSTZ2, has been previously shown to increase germination under low temperature when overexpressed.

Analysis of OsGSTZ2 expression under low temperature conditions indicated 2-3 fold higher expression in M-202 than IR50. Sequencing of the M-202 and IR50 alleles of OsGSTZ2 revealed two SNPs in the coding region. Biochemical assays using recombinant proteins and various GST substrates indicate there are significant differences in enzymatic activity between M-202 and IR50 proteins. Our functional analysis of OsGSTZ2 and will be presented and the possible role it plays in seedling cold tolerance will be discussed.