根瘤菌可以在其宿主上结瘤并固定大气中的分子态氮，在复杂的共生过程中包含着一系列不同信号物质的交流包括识别，转换和转移等。根瘤菌诱导基因的表达在其与宿主豆科植物成功建立早期共生关系的过程中，对根瘤菌的不同生理功能如结瘤效率、生长速度、胞外多糖的产生以及固氮效率等方面都起着重要作用。 本研究以华癸中生根瘤菌Mesorhizobium huakuii As9为研究对象，用转座子插入的方法构建突变子文库，该转座子中含有无启动子的km抗性报告基因，最终从8000个随机突变子中筛选得到8株可被紫云英种子分泌诱导的突变株，通过 Arbitrary PCR的方法和亚克隆的方法得到转座子插入侧翼基因的序列，经GeneBank序列同源性比对发现：这8个突变株中转座子均插入到了katG基因上，该基因所编码的蛋白为过氧化氢酶，过氧化氢酶在微生物抗氧化和抵御环境中多酚类的氧化物质方面起着重要作用。本文选择了其中一株突变株WYT37继续深入进行研究。 通过构建katG-lacZ融合菌株，检测在有紫云英种子浸提物的情况下，报告基因lacZ的表达情况,结果显示：katG基因能被紫云英种子浸提物诱导表达，说明紫云英浸提物中有某种物质能够诱导根瘤菌中katG基因的表达，为了种子浸提物确切的诱导成分，本文试验了三种多酚类物质，即槲皮素，芦丁和单宁，通过对融合菌株的诱导作用，结果发现诱导能力均很弱，明显低于紫云英种子浸提物的诱导能力，这说明可能是其他物质在诱导katG基因的表达或者是多种物质在共同起作用。 用不同浓度的种子浸提物对突变株进行杀菌试验，结果发现：作用一段时间后突变株的数量大大减少，而野生型却变化不大，说明该基因表达产物在根瘤菌抵御环境中种子分泌的多酚类物质的杀菌中起着重要作用。体外根毛吸附实验证明katG基因突变株在紫云英根毛上的吸附量要比野生型低5倍左右，可能是由于KatG蛋白的功能缺失，不能抵御种子浸提物而被杀死所致。用突变株做结瘤实验，结果显示，30天时katG突变株结瘤的数量明显低于野生型的结瘤数量，但是到了50天后两者之间几乎没有差别。 通过对华癸中生根瘤菌katG基因的研究发现，过氧化氢酶的表达在共生过程的早期，在防御其宿主分泌的毒素物质时起着重要作用；同时宿主植物为了和根瘤菌更有利的共生，通过分泌根瘤菌能抵御的毒素物质来杀死植株周围潜在的有害细菌，对根际菌群进行一次初步筛选，进而提高了共生环境中根瘤菌的竞争力和数量，为共生结瘤打下基础。

The rhizobia establish nitrogen-fixing symbiosis with its leguminous host plant, following a series of continuous signal exchanges. The complexity of the host exudates and the amount of rhizobia induced genes may be required for early stages of the symbiosis. In early stages such as nodulation efficiency, growth rate, exopolysaccharide production, and nitrogen fixation, all of which are important for the establishment of a successful symbiosis. In this study, we used transposon mutagenesis to screen for Mesorhizobium huakuii As9 strain which mutated with mariner transposon. Eight induced mutants were obtained by screening approximately 8,000 Mesorhizobium huakuii As9 random transposon insertion mutants. By arbitrary PCR and subcloning, sequencing of the region flanking the transposon insertion showed that all the mutants were inserted in the same gene katG just not same sites. The expressive protein of katG was the catalase. Catalase was important in that allowd the rhizobia to overcome toxic concentrations of plant polyphenols and oxidative stress conditions.WYT37 mutant was chosen for further experiments, because it had the better inducing phenotype by seed exudates. lacZ fusion experiments confirmed that katG was in deed induced by seed exudates. We want to know which substance in the presence of seed exudates was inducing action.We used the polyphenols of quercetin、rutin and tannin inducing lacZ fusion mutant, found that the each inducing ability of these substances was obviously lower than the inducing ability of seed exudates. The resean of this result may be that the other substance was inducing or many substances were inducing at the same time. The wild type of As9 was adapted to live in polyphenol-rich environments.When the mutant lived in different concentratoin of seed exudates for 16 hours,we found that the survival probability was obviously low .The result maybe showed that the katG was very important in resisted the polyphenol of seed exudates. In vivo experiments proved that katG mutant’s ability of adherence on root-hair was lower than wild-type’s. This because that in katG mutant the katG was no function, the polyphenol of seed exudates killed mutants. Nodulation experiments suggest that at thirty days after inoculation, the average number of the nodules per plant in inoculated with the katG mutant were less than the wild-type strain. But fifty days after inoculation, there is no significantly difference between plants nodulated by the wild-type and the mutant strain. Based on these datas, we speculate that katG-encode catalase plays a key role in the early symbiotic process. The polyphenol of plant exudates can kill the potential pathogens, but not kill the wild type stains. This can make Mesorhizobium huakuii As9 an advantage strain in rhizosphere.This is very profitable for the symbiotic nodulation.