Swarbrick Philip J, Schulze-Lefert Paul, Scholes Julie D
Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield, S10 2TN, UK.
Plant Cell Environ. 2006 Jun;29(6):1061-76. doi: 10.1111/j.1365-3040.2005.01472.x.
In a compatible interaction biotrophic fungi often lower the yield of their hosts by reducing photosynthesis and altering the fluxes of carbon within the infected leaf. In contrast, comparatively little is known about the metabolic consequences of activating resistance responses. In this study we investigated the hypothesis that the activation of both race-specific (Mla12) and broad-spectrum (mlo) resistance pathways in barley leaves infected with Blumeria graminis represents a cost to the plant in terms of carbon production and utilization. We have shown, using quantitative imaging of chlorophyll fluorescence, that during a susceptible interaction, photosynthesis was progressively reduced both in cells directly below fungal colonies and in adjacent cells when compared with uninoculated leaves. The lower rate of photosynthesis was associated with an increase in invertase activity, an accumulation of hexoses and a down-regulation of photosynthetic gene expression. During both Mla12- and mlo-mediated resistance, photosynthesis was also reduced, most severely inhibited in cells directly associated with attempted penetration of the fungus but also in surrounding cells. These cells displayed intense autofluorescence under ultraviolet illumination indicative of the accumulation of phenolic compounds and/or callose deposition. The depression in photosynthesis was not due only to cell death but also to an alteration in source-sink relations and carbon utilization. Apoplastic (cell wall-bound) invertase activity increased more rapidly and to a much greater extent than in infected susceptible leaves and was accompanied by an accumulation of hexoses that was localized to areas of the leaf actively exhibiting resistance responses. The accumulation of hexoses was accompanied by a down-regulation in the expression of Rubisco (rbcS) and chlorophyll a/b binding protein (cab) genes (although to a lesser extent than in a compatible interaction) and with an up-regulation in the expression of the pathogenesis-related protein 1 (PR-1). These results are consistent with a role for invertase in the generation of hexoses, which may supply energy for defence reactions and/or act as signals inducing defence gene expression.
在亲和性互作中，活体营养型真菌常常通过降低光合作用以及改变被感染叶片内的碳通量来降低宿主的产量。相比之下，关于激活抗性反应的代谢后果却知之甚少。在本研究中，我们探讨了这样一个假说：在被禾本科布氏白粉菌感染的大麦叶片中，激活小种特异性（Mla12）和广谱（mlo）抗性途径对植物的碳生产和利用而言是有代价的。我们利用叶绿素荧光定量成像表明，在感病互作过程中，与未接种叶片相比，直接位于真菌菌落下方的细胞以及相邻细胞中的光合作用均逐渐降低。较低的光合速率与转化酶活性增加、己糖积累以及光合基因表达下调有关。在Mla12介导和mlo介导的抗性过程中，光合作用也会降低，在与真菌试图穿透直接相关的细胞中抑制最为严重，但在周围细胞中也会受到抑制。这些细胞在紫外光照射下呈现强烈的自发荧光，表明酚类化合物积累和/或胼胝质沉积。光合作用的降低不仅是由于细胞死亡，还归因于源 - 库关系和碳利用的改变。质外体（细胞壁结合）转化酶活性比感染感病叶片时增加得更快且程度更大，同时伴随着己糖积累，这些己糖定位于叶片中积极表现出抗性反应的区域。己糖积累伴随着核酮糖 - 1,5 - 二磷酸羧化酶/加氧酶（rbcS）和叶绿素a/b结合蛋白（cab）基因表达下调（尽管程度小于亲和性互作），以及病程相关蛋白1（PR - 1）表达上调。这些结果与转化酶在己糖生成中的作用一致，己糖可能为防御反应提供能量和/或作为诱导防御基因表达的信号。
