Nakabayashi Izumi, Karahara Ichirou, Tamaoki Daisuke, Masuda Kyojiro, Wakasugi Tatsuya, Yamada Kyoji, Soga Kouichi, Hoson Takayuki, Kamisaka Seiichiro
Department of Biology, Faculty of Science, Toyama University, Toyama, 930-8555, Japan.
Ann Bot. 2006 Jun;97(6):1083-90. doi: 10.1093/aob/mcl055. Epub 2006 Mar 14.
The xylem plays an important role in strengthening plant bodies. Past studies on xylem formation in tension woods in poplar and also in clinorotated Prunus tree stems lead to the suggestion that changes in the gravitational conditions affect morphology and mechanical properties of xylem vessels. The aim of this study was to examine effects of hypergravity stimulus on morphology and development of primary xylem vessels and on mechanical properties of isolated secondary wall preparations in inflorescence stems of arabidopsis.
Morphology of primary xylem was examined under a light microscope on cross-sections of inflorescence stems of arabidopsis plants, which had been grown for 3-5 d after exposure to hypergravity at 300 g for 24 h. Extensibility of secondary cell wall preparation, isolated from inflorescence stems by enzyme digestion of primary cell wall components (mainly composed of metaxylem elements), was examined. Plants were treated with gadolinium chloride, a blocker of mechanoreceptors, to test the involvement of mechanoreceptors in the responses to hypergravity.
Number of metaxylem elements per xylem, apparent thickness of the secondary thickenings, and cross-section area of metaxylem elements in inflorescence stems increased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on the increase both in the thickness of secondary thickenings and in the cross-section area of metaxylem elements, while it did not suppress the effect of hypergravity on the increase in the number of metaxylem elements. Extensibility of secondary cell wall preparation decreased in response to hypergravity. Gadolinium chloride suppressed the effect of hypergravity on cell wall extensibility.
Hypergravity stimulus promotes metaxylem development and decreases extensibility of secondary cell walls, and mechanoreceptors were suggested to be involved in these processes.
木质部在增强植物体强度方面发挥着重要作用。过去对杨树张力木以及回转的李属树木茎干中木质部形成的研究表明，重力条件的变化会影响木质部导管的形态和机械性能。本研究的目的是检测超重力刺激对拟南芥花序茎中初生木质部导管的形态和发育以及对分离的次生壁制剂机械性能的影响。
在光学显微镜下观察拟南芥植株花序茎横切面的初生木质部形态，这些植株在300 g超重力条件下暴露24小时后再生长3 - 5天。检测通过酶解初生细胞壁成分（主要由后生木质部细胞组成）从花序茎中分离得到的次生细胞壁制剂的伸展性。用机械感受器阻滞剂氯化钆处理植株，以测试机械感受器是否参与对超重力的响应。
花序茎中每个木质部的后生木质部细胞数量、次生增厚的表观厚度以及后生木质部细胞的横截面积因超重力刺激而增加。氯化钆抑制了超重力对次生增厚厚度和后生木质部细胞横截面积增加的影响，但未抑制超重力对后生木质部细胞数量增加的影响。次生细胞壁制剂的伸展性因超重力刺激而降低。氯化钆抑制了超重力对细胞壁伸展性的影响。
超重力刺激促进后生木质部发育并降低次生细胞壁的伸展性，且提示机械感受器参与了这些过程。
