IDENTIFICATION AND CHARACTERIZATION OF GIBBERELLIN-INSENSITIVE MUTANTS SELECTED FROM AMONG DWARF MUTANTS OF RICE

In rice, many dwarf mutants have been isolated and characterized. We have investigated the relationship between dwarfism and the gibberellin (GA)-mediated control of physiological processes. Twenty-three rice cultivars and mutants (9 normal, 3 semi-dwarf, 11 dwarf) were analyzed in terms of two GA-mediated processes, namely, elongation of shoots and production of alpha-amylase activity in the endosperm. As a result, we identified four different groups (groups N,T,D and E). Two-dimensional plotting of the extent of induction of alpha-amylase in the endosperm versus the extent of enhancement of shoot elongation upon treatment with exogenous gibberellic acid (GA(3)) provided a useful method for the rapid allocation of large numbers of dwarf mutants of rice to the various groups. Members of group N (normal type), which included all normal cultivars and semi-dwarf mutants, showed a slight increase in elongation of shoots and a remarkable increase in production of alpha-amylase with the application of GA(3) during germination. All of the dwarf mutants were classified as being members of the other three groups. Members of group T (Tan-ginbozu type), including three dwarf mutants, were highly responsive to exogenous GA(3) in terms of elongation of shoots and production of alpha-amylase, with associated lower levels of endogenous GA. In contrast, members of the other three groups, including group N, had normal levels of endogenous GAs. Members of group D (Daikoku type) were only slightly responsive to exogenous GA(3), an indication that they are GA-insensitive mutants. Members of group E (Ebisu type) had responses to GA(3) similar to those of group N, not only in terms of elongation of shoots but also in terms of alpha-amylase production, an indication that they are dwarf mutants that can be considered as neither GA-deficient nor GA-insensitive mutants. We also examined a GA-insensitive mutant selected from among 19 near-isogenic dwarflines of 'Shiokari', and we concluded that the d-1 gene is associated with the phenotype of GA-insensitive dwarf mutants.