Molecular mechanism of seed coat discoloration induced by low temperature in yellow soybean.

Seed coat pigmentation is inhibited in yellow soybean. The I gene inhibits pigmentation over the entire seed coat. In yellow soybean, seed coat discoloration occurs when plants are exposed to low temperatures after the onset of fl owering, a phenomenon named ‘cold-induced discoloration (CD)’. Inhibition of seed coat pigmentation results from post-transcriptional gene silencing (PTGS) of the chalcone synthase ( CHS ) genes. PTGS is a sequencespecifi c RNA degradation mechanism in plants and occurs via short interfering RNAs (siRNAs). Similar posttranscriptional suppression is called RNAi (RNA interference) in animals. Recently, we identifi ed a candidate of the I gene designated GmIRCHS . In this study, to elucidate the molecular mechanism of CD, CHS mRNA and siRNA levels in the seed coat were compared between CDsensitive and CD-tolerant cultivars (Toyomusume and Toyoharuka, respectively). In Toyomusume, the CHS siRNA level was reduced markedly by low temperature treatment, and subsequently the CHS mRNA level increased rapidly after treatment. In contrast, low temperature treatment did not result in severe reduction of the CHS siRNA level in Toyoharuka, and the CHS mRNA level did not increase after the treatment. These results suggest that the rapid increase in CHS mRNA level after low temperature treatment may lead to enhanced pigmentation in some of the seed coat cells and fi nally in seed coat discoloration. Interestingly, we found a Toyoharuka-specifi c difference in the GmIRCHS region, which may be involved in CD tolerance.