GRAVITROPISM IN ROOTS OF INTERMEDIATE-STARCH MUTANTS OF ARABIDOPSIS

Gravitropism was studied in roots of wild type (WT) Arabidopsis thalia na (L.) Heynh. (strain Wassilewskija) and three starch-deficient mutan ts that were generated by T-DNA insertional mutagenesis. One of these mutants was starchless while the other two were intermediate mutants, which had 51% and 60%, respectively, of the WT amount of starch as det ermined by light and electron microscopy. The four parameters used to assay gravitropism were: orientation during vertical growth, time cour se of curvature, induction, and intermittent stimulation experiments. WT roots were much more responsive to gravity than were roots of the s tarchless mutant, and the intermediate starch mutants exhibited an int ermediate graviresponse. Our data suggest that lowered starch content in the mutants primarily affects gravitropism rather than differential growth because both phototropic curvature and growth rates were appro ximately equal among all four genotypes. Since responses of intermedia te-starch mutants were closer to the WT response than to that of the s tarchless mutant, it appears that 51-60% of the WT level of starch is near the threshold amount needed for full gravitropic sensitivity. Whi le other interpretations are possible, the data are consistent with th e starch statolith hypothesis for gravity perception in that the degre e of graviresponsiveness is proportional to the total mass of plastids per cell.