Gravitropism of inflorescence stems in starch-deficient mutants of Arabidopsis

Previous studies have assayed the gravitropic response of roots and hypocot yls of wild type Arabidopsis thaliana, two reduced-starch strains, and a st archless strain. Because there have been few reports on inflorescence gravi tropism, in this article, we use microscopic analyses and time-course studi es of these mutants and their wild type to study gravitropism in these stem s. Sedimentation of plastids was observed in endodermal cells of the wild t ype and reduced-starch mutants but not in the starchless mutant. In all of these strains, the short inflorescence stems (1.0-2.9 cm) were less respons ive to the gravistimulus compared with the long stems (3.0-6.0 cm). In both long and short inflorescence stems, the wild type initially had the greate st response; the starchless mutant had the least response; and the reduced starch mutants exhibited an intermediate response. Furthermore, growth rate s among all four strains were approximately equal. At about 6 h after reori entation, inflorescences of all strains returned to a position parallel to the gravity vector. Thus, in inflorescence stems, sedimentation of plastids may act as an accelerator but is not required to elicit a gravitropic resp onse. Furthermore, the site of perception appears to be diffuse throughout the inflorescence stem. These results are consistent with both a plastid-ba sed statolith model and the protoplast pressure hypothesis, and it is possi ble that multiple systems for gravity perception occur in plant cells.