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THE EFFECT OF PLASTID MUTATIONS ON GRAVITROPISM OF ROOTS, HYPOCOTYLS, AND INFLORESCENCE STEMS OF ARABIDOPSIS.   K. Yamamoto, J. Z. Kiss. Department of Botany, Miami University, Oxford OH 45056.

Gravity is one of the stimuli that is important in plant growth and development. The sites of gravity perception are the columella cells in roots and endodermal cells in hypocotyls and inflorescence stems. Since plastids likely play a role in these cells for graviperception, we investigated gravitropism in plastid mutants of Arabidopsis. The arc (accumulation and replication of chloroplasts) mutants (arc6 and arc12) are known to have variations in plastid number, size, and morphology in leaf mesophyll cells. However, in this study, we observed plastid mutations not only in mesophyll cells but also throughout the entire plant body. Seedlings and inflorescence stems of both wild-type and arc6 were fixed and sectioned for anatomical studies. Plastid numbers per cell were examined in these sections with light microscopy. In the wild-type, an average of 5 plastids was observed per cell in columella and endodermal cells. However, in arc6, an average of only 1 - 2 plastids was found, and these were larger in size compared to those of the wild-type. Time course of curvature studies were conducted with seedlings and inflorescence stems of arc12, arc6, and their wild-types. Results demonstrated that gravisensitivity differed depending on the plant organ even though mutations were expressed through the entire plant body. The organs that demonstrated inhibition of gravisensitivity were as follows: hypocotyls of light-grown seedlings and inflorescence stems of arc6, roots of light-grown seedlings and inflorescence stems of arc12. Studies are currently in progress to clarify the effects of variation in number and size of plastids on gravitropism in Arabidopsis.

 (Special thanks to Dr. Kevin Pyke for providing the arc mutants and to NASA grant NAG 2 - 1017 for financial support.)