Julius-von-Sachs-Institut fuer Biowissenschaften, Lehrstuhl für Pharmazeutische Biologie, Universitaet Wuerzburg, Julius-von-Sachs-Platz 2, 97082 Wuerzburg, Germany.
Mol Plant. 2010 Nov;3(6):1037-48. doi: 10.1093/mp/ssq053. Epub 2010 Sep 10.
There is increasing evidence that pathogens do not only elicit direct defense responses, but also cause pronounced changes in primary carbohydrate metabolism. Cell-wall-bound invertases belong to the key regulators of carbohydrate partitioning and source-sink relations. Whereas studies have focused so far only on the transcriptional induction of invertase genes in response to pathogen infection, the role of post-translational regulation of invertase activity has been neglected and was the focus of the present study. Expression analyses revealed that the high mRNA level of one out of three proteinaceous invertase inhibitors in source leaves of Arabidopsis thaliana is strongly repressed upon infection by a virulent strain of Pseudomonas syringae pv. tomato DC3000. This repression is paralleled by a decrease in invertase inhibitor activity. The physiological role of this regulatory mechanism is revealed by the finding that in situ invertase activity was detectable only upon infection by P. syringae. In contrast, a high invertase activity could be measured in vitro in crude and cell wall extracts prepared from both infected and non-infected leaves. The discrepancy between the in situ and in vitro invertase activity of control leaves and the high in situ invertase activity in infected leaves can be explained by the pathogen-dependent repression of invertase inhibitor expression and a concomitant reduction in invertase inhibitor activity. The functional importance of the release of invertase from post-translational inhibition for the defense response was substantiated by the application of the competitive chemical invertase inhibitor acarbose. Post-translational inhibition of extracellular invertase activity by infiltration of acarbose in leaves was shown to increase the susceptibility to P. syringae. The impact of invertase inhibition on spatial and temporal dynamics of the repression of photosynthesis and promotion of bacterial growth during pathogen infection supports a role for extracellular invertase in plant defense. The acarbose-mediated increase in susceptibility was also detectable in sid2 and cpr6 mutants and resulted in slightly elevated levels of salicylic acid, demonstrating that the effect is independent of the salicylic acid-regulated defense pathway. These findings provide an explanation for high extractable invertase activity found in source leaves that is kept inhibited in situ by post-translational interaction between invertase and the invertase inhibitor proteins. Upon pathogen infection, the invertase activity is released by repression of invertase inhibitor expression, thus linking the local induction of sink strength to the plant defense response.
越来越多的证据表明，病原体不仅会引发直接的防御反应，还会导致碳水化合物代谢的显著变化。细胞壁结合的转化酶属于碳水化合物分配和源库关系的关键调节剂。虽然到目前为止，研究主要集中在病原菌感染后转录诱导转化酶基因上，但对转化酶活性的翻译后调控作用却被忽视了，这也是本研究的重点。表达分析表明，拟南芥源叶中三种蛋白转化酶抑制剂之一的高 mRNA 水平在受到强毒力丁香假单胞菌 pv.番茄 DC3000 感染后受到强烈抑制。这种抑制作用与转化酶抑制剂活性的降低平行。这一调控机制的生理作用是通过发现只有在感染丁香假单胞菌时才能检测到原位转化酶活性而揭示出来的。相比之下，在从感染和未感染叶片中制备的粗提物和细胞壁提取物中，可以在体外测量到高的转化酶活性。对照叶片的原位和体外转化酶活性之间的差异以及感染叶片中高的原位转化酶活性可以通过病原菌依赖的转化酶抑制剂表达抑制和转化酶抑制剂活性的同时降低来解释。化学转化酶抑制剂阿卡波糖的应用证实了将转化酶从翻译后抑制中释放出来对防御反应的重要性。在叶片中渗透阿卡波糖对细胞外转化酶活性的翻译后抑制作用被证明会增加对丁香假单胞菌的敏感性。在病原菌感染过程中，抑制光合作用和促进细菌生长的时空动力学中，细胞外转化酶的抑制作用支持了其在植物防御中的作用。在 sid2 和 cpr6 突变体中也可以检测到阿卡波糖介导的敏感性增加，并且导致水杨酸水平略有升高，这表明这种效应独立于水杨酸调节的防御途径。这些发现为源叶中发现的高可提取转化酶活性提供了一种解释，这种活性通过转化酶与转化酶抑制剂蛋白之间的翻译后相互作用在原位被抑制。在病原菌感染后，通过抑制转化酶抑制剂的表达来释放转化酶活性，从而将局部诱导的库强度与植物防御反应联系起来。
