Caspar T, Pickard B G
MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing 48824, USA.
Planta. 1989;177:185-97.
The starch-statolith theory of gravity reception has been tested with a mutant of Arabidopsis thaliana (L.) Heynh. which, lacking plastid phosphoglucomutase (EC 2.7.5.1) activity, does not synthesize starch. The hypocotyls and seedling roots of the mutant were examined by light and electron microscopy to confirm that they did not contain starch. In upright wild-type (WT) seedlings, starch-filled plastids in the starch sheath of the hypocotyl and in three of the five columellar layers of the root cap were piled on the cell floors, and sedimented to the ceilings when the plants were inverted. However, starchless plastids of the mutant were not significantly sedimented in these cells in either upright or inverted seedlings. Gravitropism of light-grown seedling roots was vigorous: e.g., 10 degrees curvature developed in mutants rotated on a clinostat following a 5 min induction at 1 g, compared with 14 degrees in the WT. Curvatures induced during intervals from 2.5 to 30 min were 70% as great in the mutant as the WT. Thus under these conditions the presence of starch and the sedimentation of plastids are unnecessary for reception of gravity by Arabidopsis roots. Gravitropism by hypocotyls of light-grown seedlings was less vigorous than that by roots, but the mutant hypocotyls exhibited an average of 70-80% as much curvature as the WT. Roots and hypocotyls of etiolated seedlings and flower stalks of mature plants were also gravitropic, although in these cases the mutant was generally less closely comparable to the WT. Thus, starch is also unnecessary for gravity reception in these tissues.
利用拟南芥（Arabidopsis thaliana (L.) Heynh.）的一个突变体对淀粉 - 平衡石重力感受理论进行了验证。该突变体缺乏质体磷酸葡萄糖变位酶（EC 2.7.5.1）活性，不能合成淀粉。通过光学显微镜和电子显微镜检查该突变体的下胚轴和幼苗根，以确认它们不含淀粉。在直立生长的野生型（WT）幼苗中，下胚轴淀粉鞘以及根冠五个柱状层中的三层中充满淀粉的质体堆积在细胞底部，当植株倒置时会沉降到细胞顶部。然而，该突变体的无淀粉质体在直立或倒置的幼苗的这些细胞中均未发生明显沉降。光照培养的幼苗根的向重力性很强：例如，在1 g条件下诱导5分钟后，在回转器上旋转的突变体根产生了10度的弯曲，而野生型为(14)度。在2.5至30分钟的时间段内诱导产生的弯曲，突变体是野生型的(70%)。因此，在这些条件下，淀粉的存在和质体的沉降对于拟南芥根感受重力并非必需。光照培养的幼苗下胚轴的向重力性不如根强，但突变体下胚轴的弯曲度平均为野生型的(70 - 80%)。黄化幼苗的根和下胚轴以及成熟植株的花茎也具有向重力性，不过在这些情况下，突变体与野生型的可比性通常较差。因此，在这些组织中，淀粉对于重力感受也不是必需的。
