LIGHT PROMOTION OF HYPOCOTYL GRAVITROPISM OF A STARCH-DEFICIENT TOBACCO MUTANT CORRELATES WITH PLASTID ENLARGEMENT AND SEDIMENTATION

Dark-grown hypocotyls of a starch-deficient mutant (NS458) of tobacco (Nicotiana sylvestris) lack amyloplasts and plastid sedimentation, and have severely reduced gravitropism. However, gravitropism improved dr amatically when NS458 seedlings were grown in the light. To determine the extent of this improvement and whether mutant hypocotyls contain s edimented amyloplasts, gravitropic sensitivity (induction time and int ermittent stimulation) and plastid size and position in the endodermis were measured in seedlings grown for 8 d in the light. Light-grown NS 458 hypocotyls were gravitropic but were less sensitive than the wild type (WT). Starch occupied 10% of the volume of NS458 plastids grown i n both the light and the dark, whereas WT plastids were essentially fi lled with starch in both treatments. Light increased plastid size twic e as much in the mutant as in the WT. Plastids in light-grown NS458 we re sedimented, presumably because of their larger size and greater tot al starch content. The induction by light of plastid sedimentation in NS458 provides new evidence for the role of plastid mass and sedimenta tion in stem gravitropic sensing. Because the mutant is not as sensiti ve as the WT, NS458 plastids may not have sufficient mass to provide f ull gravitropic sensitivity.