依赖LjALD1的水杨酸信号途径调控百脉根结瘤的机理探析

Symbiotic nitrogen fixation occurs in plants that harbor nitrogen-fixing bacteria within their tissues and the nitrogen-fixing bacteria are able to convert atmospheric nitrogen to ammonia. The best-studied example is the association between legumes and bacteria in the genus rhizobium. Symbiotic nitrogen fixation plays a significant role in improving the fertility and productivity of low-N soils, and therefore is crucial for the sustainable development of agriculture. In this project, Lotus japonicas mutants Ljald1 (AGD2-like defense response protein 1), transgenic L.japonicus plants overexpressing the LjALD1 (OvLjALD1), and Ljnpr1 (a key gene in salicylic acid (SA) signal pathway) mutants will be used as plant materials. Analyses of genetics, transcriptome sequencing, and plant resistance and nodulation characteristics will be used to reveal the effect and roles of the defense gene LjALD1 in legume-rhizobium interaction. Our research involves primarily the following aspects: 1) the effect of LjALD1 on disease resistance and nodulation in L.japonicus. 2) the mechanism of LjALD1 on nodulation via SA-dependent pathway.3) the effect of reduced edogenous SA on Nod-factor signalling in L.japonicas. The results we will get would help us to illuminate the role of endogenous SA in symbiotic nitrogen fixation and clarify the mechanism of crosstalk mechanism between SA-dependent signalling and Nod-factor signalling; and would also facilitate further investigation of the relationship between plant innate immune system and nitrogen fixation.

共生固氮是固氮微生物与寄主植物形成共生关系，并将大气中的氮气还原成可供植物吸收的氨的过程。充分发挥共生固氮作用，对提高农作物产量和保障可持续农业发展有着重要意义。目前内部免疫体系与豆科植物结瘤的关系还不清楚。本项目拟以百脉根防御响应相关基因LjALD1的突变体和超表达转基因株系；及水杨酸信号途径关键基因NPR1的突变体为材料，采用表型观察、遗传学分析和转录组测序分析的方法，探讨内部免疫系统在豆科植物-根瘤菌互作中的功能。研究内容包括1）LjALD1调控百脉根抗病和结瘤机制的异同。2）SA信号在LjALD1调控结瘤过程中的作用。3）依赖LjALD1的防卫反应途径缺失后对结瘤因子信号的影响。研究结果将揭示依赖LjALD1的水杨酸防卫信号调控百脉根结瘤的分子机理；阐明豆科植物结瘤过程中水杨酸防卫反应信号与结瘤因子信号之间的网络分子互作机制；为深入了解植物内部免疫在根瘤菌侵染过程中的作用奠定基础。

非豆科植物无法识别根瘤菌分泌的几丁质寡糖结瘤因子，不能激活体内结瘤信号途径的开启，从而无法与根瘤菌形成共生关系。近几年研究发现适度抑制体内免疫反应可以使非豆科植物响应结瘤因子信号，并激活结瘤相关基因的表达；并且豆科植物可能通过不同的作用机制抑制结瘤过程中寄主体内的内部免疫体系。本项目用百脉根水杨酸信号途径中关键基因的突变体（Ljald1和Ljnpr1）为材料，研究豆科植物内部依赖LjALD1的SA的抗病防卫反应途径对其结瘤过程的影响。项目主要从三个方面开展研究，第一为LjALD1基因对百脉根抗病性和结瘤过程的影响；第二为LjALD1对结瘤过程的调节与SA信号途径的关系；第三为结瘤过程中依赖LjALD1的防卫反应信号与结瘤因子信号的互作关系。通过国家自然科学基金资助后，我们发现百脉根中ALD1家族有4个同源基因，其中LjALD1只在侵染的根毛及成熟根瘤的非侵染细胞中表达；其他三个同源基因在百脉根的各个组织中均有表达。LjALD1和LjAGD2基因突变后均能加速根瘤菌的侵染，但对白粉病没有表现出明显的敏感性；超表达LjALD1和LjAGD2基因后均能延缓侵染线的形成，表现出比野生型更强的白粉病抗性。另外，与水杨酸合成相关的苯丙氨酸解氨酶PAL也通过影响植株水杨酸的合成以及细胞壁的结构影响根瘤菌的早期侵染。我们的研究表明SA依赖的内部免疫在根瘤菌侵染早期可能被瞬间抑制。研究结果阐明了豆科植物依赖水杨酸的防卫反应信号在寄主-根瘤菌互作中的作用。