Vitha S, Yang M, Kiss J Z, Sack F D
The Ohio State University, Department of Plant Biology, Columbus 43210, USA.
Plant Physiol. 1998 Feb;116(2):495-502. doi: 10.1104/pp.116.2.495.
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 dramatically when NS458 seedlings were grown in the light. To determine the extent of this improvement and whether mutant hypocotyls contain sedimented amyloplasts, gravitropic sensitivity (induction time and intermittent stimulation) and plastid size and position in the endodermis were measured in seedlings grown for 8 d in the light. Light-grown NS458 hypocotyls were gravitropic but were less sensitive than the wild type (WT). Starch occupied 10% of the volume of NS458 plastids grown in both the light and the dark, whereas WT plastids were essentially filled with starch in both treatments. Light increased plastid size twice as much in the mutant as in the WT. Plastids in light-grown NS458 were sedimented, presumably because of their larger size and greater total starch content. The induction by light of plastid sedimentation in NS458 provides new evidence for the role of plastid mass and sedimentation in stem gravitropic sensing. Because the mutant is not as sensitive as the WT, NS458 plastids may not have sufficient mass to provide full gravitropic sensitivity.
烟草（林烟草）淀粉缺陷型突变体（NS458）暗中生长的下胚轴缺乏造粉体和质体沉降，并且其向重力性严重降低。然而，当NS458幼苗在光照下生长时，其向重力性显著改善。为了确定这种改善的程度以及突变体下胚轴是否含有沉降的造粉体，对在光照下生长8天的幼苗的向重力性敏感性（诱导时间和间歇刺激）以及内皮层中质体的大小和位置进行了测量。光照下生长的NS458下胚轴具有向重力性，但比野生型（WT）更不敏感。在光照和黑暗条件下生长的NS458质体中，淀粉占据其体积的10%，而在两种处理中WT质体基本上都充满了淀粉。光照使突变体质体大小增加的幅度是野生型的两倍。光照下生长的NS458中的质体沉降，可能是因为它们更大的尺寸和更高的总淀粉含量。光照诱导NS458质体沉降为质体质量和沉降在茎向重力性感知中的作用提供了新证据。由于突变体不如野生型敏感，NS458质体可能没有足够的质量来提供完全的向重力性敏感性。
