ASGSB 1998 Annual Meeting Abstracts

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ROOT PHOTOTROPISM AND GRAVITROPISM IN WILD-TYPE AND STARCHLESS MUTANTS OF ARABIDOPSIS.   S. Vitha^{1, 2} and F.D. Sack¹. ¹Department of Plant Biology, Ohio State University, Columbus, and ²Dept Biology, Univ. Nevada, Reno (current address).

Arabidopsis roots are negatively phototropic (grow away from the light), and gravitropism-impaired mutant roots (aux1) displayed stronger phototropism than the wild-type (WT; Okada & Shimura 1992 Aust J Pl Physiol 19: 439).

In many gravitropism experiments, the light came from above, a configuration that could exaggerate the effect of gravitropism if phototropism were not taken into account.

Many data indicate that starch-filled amyloplasts trigger gravitropic sensing. However, roots of the starchless mutant, pgm-1 (TC75), have been reported to be gravitropic, especially when grown in the light. But studies at threshold g-doses have determined that pgm-1 roots are about twelve times less sensitive than WT roots (Kiss et al. 1989 Planta 177: 198-206). These results demonstrate that starch is not necessary for some gravitropism but is required for full sensitivity. However, these data were obtained with light from above and thus it is possible that the contribution of root phototropism led to an overestimate of the gravitropic sensitivity of either or both genotypes.

To determine the contribution of phototropism to the measurement of apparent root gravitropism, various measures of gravitropism were performed so that the responses with light from above or below were compared in the WT and in the starchless mutants pgm-1 and adg1-1 (TL255) of Arabidopsis. The position of the light significantly influenced the measurement of virtually every gravitropic parameter tested, and in all cases the root angles of the starchless mutants were affected more by phototropism than the WT. By accounting for the contribution of root phototropism, the pgm-1 mutant was found to be significantly less sensitive to gravity than the WT compared to previous estimates. These results provide additional support for the importance of amyloplasts in gravitropic sensing and also point to the need for consideration of light position in the design of gravitropism experiments. (Supported by NASA grants NAGW-4472 and NAG5-3774)

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