细菌效应蛋白AvrB在拟南芥中分子靶标的功能分析

Psyeudomonas Syringae effectors play important roles in pathogenicity. Such as effector AvrB, after deliveried by type III secretion system, can interact and induce host protein RIN4 phosphorylation rapidly. Phosphorylated RIN4 could be recognized by immune receptor RPM1, leading to plant immune responses. In addition to RIN4, multiple additional host AvrB-interacting proteins have been found recently, such as MPK4 and HSP90 chaperons. Those AvrB-interacting proteins are involved in either effector triggered immunity or hormone signaling. Our recent work discovered that AvrB could also interact with a histone modification enzyme (HAE) of Arabidopsis via yeast-two hybrid screening with AvrB as the bait. Further, we found that HAE played a critical role in plant immune response by affecting the accumulation of callose revealed by the inoculation with bacterial pathogens on hae mutant plants. Because the mechanism of AvrB-suppressed plant basal responses remains largely unknown, characterization of HAE should help us gain a comprehensive understanding of AvrB pathogenic mechanisms in basal defense.

丁香假单胞菌的效应蛋白在其致病性中具有重要作用，如效应蛋白AvrB经细菌的III型分泌系统进入植物后，可以迅速引起寄主蛋白RIN4的磷酸化，磷酸化的RIN4蛋白可以被寄主的R蛋白RPM1识别，导致植物的免疫应答反应。除RIN4蛋白外，近期还发现了许多可以和AvrB互作的寄主蛋白，例如MPK4和分子伴侣HSP90等。这些互作蛋白有的参与到效应蛋白激发的免疫应答反应中，有的参与到激素信号通路中。我们近期的酵母双杂实验发现AvrB还可以和拟南芥的一个组蛋白修饰酶（HAE）互作，进一步的研究发现HAE在植物免疫反应过程中起了重要作用；HAE的缺失影响了植物在病菌侵染后的胼胝质（callose）积累。由于AvrB在抑制植物“基础免疫”中的作用还不清楚，因此对HAE的功能研究将有助于全面的揭示AvrB在抑制植物免疫中的作用。

在细菌和植物分子互作关系的模型中,以丁香假单胞菌Pseudomonas syringae pv. tomato DC3000和模式植物拟南芥的研究最为广泛和深入。该菌至少有30多个效应蛋白可以通过它的type III分泌系统进入寄主体内，在不同的信号途径上干扰寄主的免疫应答反应。我们的酵母双杂交实验发现该菌的效应蛋白AvrB可以与拟南芥的一个组蛋白修饰酶HAE互作，AvrB处理会改变HAE的亚细胞定位。进一步的酵母筛库实验发现HAE可以与拟南芥中的importinβ家族蛋白AR2的C terminal互作，在ar2突变体中，HAE的亚细胞定位由细胞核向细胞质转移，表明AR2介导了HAE的入核。进一步实验发现ar2突变体在沾菌处理后，气孔开度比野生型小，表现出一定程度的抗性。因ar2突变体对ABA超级敏感，我们用酵母双杂的方法筛选到AR2可以与ABA信号途径上的负调控因子ABI1互作，但还需要进一步的实验验证。