Mitsugu EIGUCHI and Yoshio SANO
National Insititute of Genetics, Mishima, 411 Japan
Plants utilize environmental signals for altering the timing of gene expression so as to respond to changing environments. A drastic change in morphology due to changing environments is known as phenotypic plasticity which is particularly important in plants because of their sessile life style. An example of plastic response is deepwater or floating rice whose internodal elongation starts with increasing water depth to escape submergence, resulting in various changes in morphology. However, no or little internodal elongation occurs in non-deepwater rice as well as air-grown deepwater rice. A number of recent experiments indicated that deepwater tolerance is related to enhanced internodal elongation resulting from the action of the plant hormones ethylene and gibberellin. Although the inheritance of deepwater tolerance seems to be complex, the genetic and developmental regulation gives an excellent oppotunity for understanding the mechanisms of phenotypic plasticity in plants. In the present study we successfully detected a major gene which is responsible for deepwater tolerance in rice. We report here that responding to flood the gene for deepwater tolerance induces elongation of basal internodes whose intercalary meristem gives no elongation without the gene.
The materials used were a perennial type of Oryza sativa (W120 from India) and a near isogenic line of Taichung 65 with wx (T65wx). Generally, a perennial type of O. rufipogon shows tolerance for deepwater since they prefer deepwater and stable habitats while Taichung 65 is a non-deepwater rice cultivar. In order tow introduce an alien factor responsible for deepwater tolerance, the T65wx X W120 F1 was successively backcrossed with T65 used as the recurent parent. In each generation, F2 plants were grown in a deepwater tank and subjected to flood at 6 weeks of age. The water level was rised 10 cm every other day to a maximum depth of 100 cm. Under these conditions, only 1/5 to 1/4 of F2 plants survived and came to flowering which were used for further backcrossings. Segregation data in BC5F1, BC5F2 and BC4F3 consistently supported the assumption that deep-water tolerance is controlled by a single recessive gene which was designated as dw3 (Table 1 and 2).
Table 1. Segregation pattern for deepwater tolerance in BC5F2 of T65wx x W 120 ============================================================================== Segregation ==================================== X2 (3:1) d.f. Non-resistant Resistant Total ============================================================================== 83 18 101 2.78ns 1 (75.75) (25.25) ============================================================================== "ns" shows non-significance. Table 2. Segregation pattern for deepwater tolerance in BC4F3 of T65wxXW120 ============================================================================= No. of F2 lines ============================================= X2 d.f. dw3+/dw3+ dw3+/dw3 dw3/dw3 Total ============================================================================= 6 18 5 29 1.76ns 2 (7.25) (14.5) (7.25) =============================================================================