Ridge R W, Sack F D
Department of Plant Biology, Ohio State University, Columbus 43210.
Am J Bot. 1992 Mar;79(3):328-34.
Although the rootcap is required for gravitropic sensing, various classical and contemporary data raise the question of whether additional sensing occurs away from the cap in roots. Roots of Equisetum hyemale L. (horsetail) were examined by light and electron microscopy to determine which cell components were distributed with respect to gravity both in and away from the rootcap. Adventitious roots from stem cuttings were gravitropic in a vertical orientation or if reoriented to the horizontal. Obvious amyloplast sedimentation was found in vertical and in reoriented roots 1) in cells in the center of the rootcap and 2) in young, elongating cortical cells located in two to three layers outside the endodermis. These cortical amyloplasts were smaller than cap amyloplasts and, unlike central cap amyloplasts, were occasionally found in the top of the cell. The nucleus was also sedimented on top of the amyloplasts in both cell types, both in vertical and in reoriented roots. Sedimentation of both organelles ceased as cortical cells elongated further or as cap cells became peripheral in location. In both cell types with sedimentation, endoplasmic reticulum was located in the cell periphery, but showed no obvious enrichment near the lower part of the cell in vertical roots. This is the first modern report of sedimentation away from the cap in roots, and it provides structural evidence that gravitropic sensing may not be confined to the cap in all roots.
虽然根冠是重力感应所必需的，但各种经典和当代数据都提出了一个问题，即根部是否在根冠之外的其他部位也存在重力感应。通过光学显微镜和电子显微镜对木贼（Equisetum hyemale L.）的根进行了检查，以确定哪些细胞成分在根冠内外是相对于重力分布的。茎插条上的不定根在垂直方向上具有向重力性，或者重新定向为水平方向时也具有向重力性。在垂直根和重新定向的根中，在以下部位发现了明显的淀粉体沉降：1）根冠中央的细胞；2）位于内皮层外两到三层的幼嫩伸长皮层细胞。这些皮层淀粉体比根冠淀粉体小，并且与根冠中央淀粉体不同，偶尔会出现在细胞顶部。在垂直根和重新定向的根中，这两种细胞类型的细胞核也都沉降在淀粉体顶部。随着皮层细胞进一步伸长或根冠细胞位置变到外围，这两种细胞器的沉降都停止了。在有沉降现象的两种细胞类型中，内质网都位于细胞周边，但在垂直根中，内质网在细胞下部附近没有明显富集。这是关于根中根冠外淀粉体沉降的第一份现代报告，它提供了结构证据，表明重力感应在所有根中可能并不局限于根冠。
