Genetic Analysis And Gene Mapping Of Cold-induced Seedling Chlorosis In Rice

Leaf color mutants (chlorophyll mutant) are often seen in rice. Based on phenotypes, these mutants can be divided into 8 types including albino, etiolation, jade green, pale green, olivine, stripe etc. In addition, according to their lethality, they can be divided into lethal and nonlethal types. The nonlethal type includes variegated, leaky and conditional mutants. Indica rice variety Dular is a conditional mutant depending on temperature and development. Under the condition of natural low temperature in early spring, the seedlings (before three-leaf stage) of Dular showed chlorosis, while the seedlings of japonica rice cultivar Lement remained green.In this study, with Lement as control, the chlorophyll contents in the albino seedlings (≤20°C) and recovered seedlings (>22°C) of Dular were measured. Results showed that the chlorophyll content in the albino seedlings of Dular was only 0.05 SPAD on average; as the temperature increased, the seedlings gradually recovered to normal and the chlorophyll content increased to 20.29 SPAD, similar to that of Lement (20.40). This suggests that the mutant phenotype of Dular seedling had little effects on later growth.The chloroplast structures in Dular seedlings with different phenotypes were observed with transmission electron microscope. Results showed that the chloroplasts in albino seedlings could not develop normally but formed many little vesicles instead. When the seedlings recovered green, the chloroplast structure also became normal, same as that of Lement. This suggests that the chloroplast structure of Dular changed when it became albino.An F₂ population of Lement×Dular was constructed. Under the low temperature of early spring, segregation of normal green and chlorosis occurred in the population, x² test indicated that the segregation followed the expected ratio of 3 : 1, suggesting that the difference of cold tolerance at seedling stage between Dular and Lement is controlled by a single major gene, with the chlorosis allele being recessive. We named the gene cisc (t). With the method of bulk segregant analysis (BSA), the gene was located on chromosome 9. Further analysis on 2800 F₂ albino seedlings, the gene was mapped in a 136-kb interval between SSR markers RM24545 and RM24554.