Chatelet David S, Matthews Mark A, Rost Thomas L
Section of Plant Biology, University of California, Davis, CA 95616, USA.
Ann Bot. 2006 Sep;98(3):483-94. doi: 10.1093/aob/mcl124. Epub 2006 Jun 21.
Bacterial leaf scorch occurring in a number of economically important plants is caused by the xylem-limited bacterium Xylella fastidiosa (Xf). In grapevine, Xf systemic infection causes Pierce's disease and is lethal. Traditional dogma is that Xf movement between vessels requires the digestion of inter-vessel pit membranes. However, Yersinia enterocolitica (Ye) (a bacterium found in animals) and fluorescent beads moved rapidly within grapevine xylem from stem into leaf lamina, suggesting open conduits consisting of long, branched xylem vessels for passive movement. This study builds on and expands previous observations on the nature of these conduits and how they affect Xf movement.
Air, latex paint and green fluorescence protein (GFP)-Xf were loaded into leaves and followed to confirm and identify these conduits. Leaf xylem anatomy was studied to determine the basis for the free and sometimes restricted movement of Ye, beads, air, paint and GFP-Xf into the lamina.
Reverse loading experiments demonstrated that long, branched xylem vessels occurred exclusively in primary xylem. They were observed in the stem for three internodes before diverging into mature leaves. However, this stem-leaf connection was an age-dependent character and was absent for the first 10-12 leaves basal to the apical meristem. Free movement in leaf blade xylem was cell-type specific with vessels facilitating movement in the body of the blade and tracheids near the leaf margin. Air, latex paint and GFP-Xf all moved about 50-60% of the leaf length. GFP-Xf was never observed close to the leaf margin.
The open vessels of the primary xylem offered unimpeded long distance pathways bridging stem to leaves, possibly facilitating the spread of bacterial pathogens in planta. GFP-Xf never reached the leaf margins where scorching appeared, suggesting a signal targeting specific cells or a toxic build-up at hydathodes.
多种具有经济重要性的植物中发生的细菌性叶焦枯病是由木质部受限细菌——木质部难养菌（Xylella fastidiosa，简称Xf）引起的。在葡萄中，Xf的系统性感染会导致皮尔氏病，且具有致死性。传统观点认为，Xf在导管间移动需要消化导管间纹孔膜。然而，小肠结肠炎耶尔森菌（Yersinia enterocolitica，简称Ye，一种在动物体内发现的细菌）和荧光珠能在葡萄木质部内迅速从茎移动到叶片，这表明存在由长而分支的木质部导管组成的开放通道，便于被动移动。本研究基于并扩展了先前关于这些通道的性质及其如何影响Xf移动的观察结果。
将空气、乳胶漆和绿色荧光蛋白（GFP）标记的Xf加载到叶片中，并追踪以确认和识别这些通道。对叶片木质部解剖结构进行研究，以确定Ye、珠子、空气、漆和GFP-Xf能够自由且有时受限地进入叶片的基础。
反向加载实验表明，长而分支的木质部导管仅存在于初生木质部中。在茎中观察到它们在三个节间，然后分支进入成熟叶片。然而，这种茎与叶的连接具有年龄依赖性，在顶端分生组织基部的最初10 - 12片叶中不存在。叶片木质部中的自由移动具有细胞类型特异性，导管有利于在叶片主体中移动，而叶边缘附近的管胞则有利于移动。空气、乳胶漆和GFP-Xf都移动了约叶片长度的50 - 60%。从未在靠近叶边缘处观察到GFP-Xf。
初生木质部的开放导管提供了从茎到叶的畅通无阻的长距离通道，可能有利于植物体内细菌病原体的传播。GFP-Xf从未到达出现焦枯的叶边缘，这表明存在靶向特定细胞的信号或水孔处有毒物质的积累。
