大豆疫霉对寄主、非寄主根分泌物中信号分子的差异性应答机制

Phytophthora sojae (Ps) is an important soil-borne oomycete pathogen, whose host only restricts to soybean. Isoflavones in root exudates have been reported as host-specific signaling molecules during Ps infection in its host. We found that amino acids and sugars in root exudates also have chemoattractivity to Ps zoospores. However, it is not clear why Ps zoospores do not response to the root exudate from common bean, in which isoflavones and amino acids as well as sugars exist, and why Ps do not infect rotating graminaceous crops. Although, the mask or reduction in chemoattraction between two signaling molecules was reported, no comprehensive research on associative chemoattraction among several signaling molecules has been reported. Furthermore, the mechanism of Ps response to mixed signaling molecules is not clear. In this project, the mechanism of Ps response to mixed signaling molecules will be studied on the basis of understanding of main signaling molecules in root exudates and their secretion levels. 1) Mining and identifying signaling molecules in host and non-host root exudates, measuring their chemoattraction or suppression on Ps; 2) Measuring the changes in intensity of mixed signaling molecules, defining mode and intensity of associative chemoattraction as well as ecological relevant concentrations of chemoattractants corresponding to the specific developmental changes of Ps; 3) Revealing the mechanism of Ps response to mixed signaling molecules through the developmental changes and molecular responses of Ps to mixed signaling molecules. The research of the relationship of Ps and signaling molecules in host and non-host root exudates will elucidate the mechanism of Ps response to mixed signaling molecules in root exudates, reveal the contribution of the difference in responses of Ps to different combination of signaling molecules for Ps to establish its host specificity, and provide a basis for designing highly effective fungicides and realizing the prediction and regulation of the rhizosphere behavior of Ps.

大豆疫霉(Ps)为重要土传病原卵菌，仅侵染大豆。普遍认为异黄酮是Ps侵染寄主的专化性信号分子，我们发现氨基酸和糖对Ps也有信号作用，但却不能解释为什么Ps对同样含有这些物质的菜豆根分泌物完全不应答，也不侵染轮作禾本作物。虽然有信号分子间阻滞报道，但缺乏对多种信号分子联合作用综合研究，更不清楚Ps应答机制。本项目拟在明确根分泌物中信号分子种类及水平基础上，研究Ps对多种信号分子联合作用应答机制。1) 充分挖掘、鉴定寄主、非寄主根分泌物中信号分子，明确对Ps诱导/抑制作用；2) 测定信号分子混合后对Ps信号强度变化，明确联合作用方式、强度及与Ps特异性发育变化关联浓度；3) 从发育变化和转录翻译层面分析Ps应答机制。通过对Ps与寄主、非寄主根分泌物中信号分子间关系研究，阐明Ps应答不同信号分子组合的差异，揭示其对Ps建立寄主专化性的贡献，为设计高效卵菌药剂，实现Ps根际行为预测和调节提供依据。

众所周知，大豆疫霉根腐病的病原菌是大豆疫霉，该菌为土传性卵菌，主要靠游动孢子进行传播，游动孢子一旦接触到适宜的寄主种子或根系便会在种子或根表面成囊、萌发产生芽管实施侵染。大豆疫霉在自然界只侵染栽培大豆，故大豆疫霉与寄主和非寄主种子和根系的互作机制是研究大豆疫霉寄主选择性的关键。通过本项研究，查明主导大豆疫霉建立寄主专化性的化学信号分子为种子/根系分泌物中的异黄酮、糖、氨基酸等，同时挖掘出酚酸类抑菌活性物质和防御蛋白类物质；明确寄主种子/根系分泌物中的异黄酮、氨基酸和糖通过吸引大豆疫霉游动孢子、促进其成囊和萌发来促进其侵染；发现种子/根系分泌物中的信号分子不是单独起作用，而是存在复杂的相互作用，共同参与决定大豆疫霉的寄主选择性；阐明大豆疫霉应答种子分泌物及其关键信号分子的分子机制：①在应答寄主种子分泌物时，调控游动孢子趋化性的蛋白上调，同时正向调控游动孢子成囊的蛋白显著上调，而负向调控游动孢子细胞壁形成的蛋白下调，这些蛋白在应答感病品种种子分泌物时表达变化是应答抗病品种的几倍，说明在感病品种的种子际或根际，游动孢子趋化性和成囊基因被高效表达，利于游动孢子寻找寄主并实施侵染；②在应答非寄主玉米种子分泌物时，花生四烯酸途径中的关键蛋白下调，游动孢子产生趋避反应，无法靠近玉米因而不能侵染；而应答非寄主菜豆种子分泌物时，保护细胞膜结构的蛋白质显著下调，导致游动孢子溶解，也无法侵染；③大豆疫霉对寄主非专化性信号分子天冬氨酸的应答通路部分与寄主专化性信号分子异黄酮重叠，但对寄主非专化性信号分子葡萄糖的应答完全不同于对异黄酮和天冬氨酸的应答，说明不同信号分子间既有相同的作用机制，也有完全不同的作用机制，但殊途同归，最后对大豆疫霉生长发育的影响是相同的。