The short-root mutation srt5 defines a sugar-mediated root growth in rice (Oryza sativa L.)

We previously reported a mutant, srt5, in which the short-root phenotype at the seedling stage could be partially rescued by exogenous abscisic acid (ABA) [Plant Sci. 163 (2002) 217]. In this paper, we describe that exogenously applied sucrose (Suc), glucose (Glu) or fructose (Fru), but not mannitol (Mtl) or glucose analogs, can also rescue root growth of the mutant to extents greater than ABA. Combined treatment reveals that root growth of the mutant under 1 μM ABA treatment can be further promoted by the addition of sugars. In contrast, application of ABA shows no promotive effects on 100 mM sugar-treated roots of srt5 seedlings. We also observed that endogenous sucrose contents in seeds and seedlings of srt5 are significantly lower, and starch breakdown in endosperm of srt5 is normal compared to wild-type. Furthermore, when treated with ABA, sucrose content in roots increases greatly in srt5, but decreases significantly in the wild-type. Collectively, these results suggest that energy deficiency is the cause of the srt5 phenotype, and ABA regulates root elongation of the mutant in a sugar-mediated way. The possible mechanism for ABA to promote root growth by enhancing the level of sugar in roots of srt5 is discussed.