Isolation and characterization of an abscisic acid-insensitive mutation that affects specifically primary root elongation in rice (Oryza sativa L.)

The isolation and detailed characterization of a monogenic recessive root growth mutant, srt6, which was derived from an M₂ population of rice (Oryza sativa L.) cv. Oochikara treated with NaN₃, is described. Besides greatly reduced primary root length and diameter, the mutant at the seedling stage also shows inhibited lateral root elongation and altered root hair formation. Hydroponic cultivation reveals that root length of the mutant recovers to the level of the wild type at the booting stage, but both lateral root elongation and root hair formation in the mutant remain inhibited at all growth stages. These observations suggest that the expression of the srt6 gene is phase-specific, and that the effects of SRT6 on root growth are restricted specifically to the development of primary roots. Physiological characterization reveals that the mutant has pleiotropic defects in abscisic acid (ABA) responses, which are similar to those reported for Arabidopsis ABA-insensitive mutants abi1 and abi2, i.e. reduced sensitivity of root growth and seed germination, and excessive water loss. Combined with its normal response of root growth to fluridone, the ABA biosynthesis inhibitor, suggesting that the action of the srt6 gene might be related to ABA perception or signal transduction but not ABA biosynthesis in the controlling of early primary root elongation in rice.