食细菌线虫与溶磷菌相互作用促进冬小麦生长的土壤磷素活化机制

The interaction between bacterial-feeding nematodes and bacteria and their ecological functions in terrestrial ecosystems have been one of the hot issues of soil ecology researches. Our previous studies have shown that the interaction between bacterial-feeding nematodes and bacteria lead to the increase in available phosphate content of soil, thus promoted the plant growth. However, the mechanism still remains unknown. Thus two questions that how bacterial-feeding nematodes increase the phosphate content of soil through the interaction with phosphate solubilizing bacteria and whether the plants responses to the increased phosphate content are needed to be solved. To clearly understand the mechanism of the plant growth promotion led by phosphate increase, which is resulted from the interactions between bacterial-feeding nematodes and bacteria, gnotobiotic microcosm experiment and natural soil microcosm experiment were carried out. The phosphate solubilizing bacteria strains used in the experiments were isolated from natural soil. By molecular biology methods, such as FISH, the influence of bacterial-feeding nematodes on the amount and the colonization of the phosphate solubilizing bacteria in the roots of winter wheat is detected; by soil phosphorus fractionation scheme, the influence of soil phosphate activation degree through the interaction between soil bacterial-feeding nematodes and phosphate solubilizing bacteria is detected; by qRT-PCR method, the change of the phosphate transporter expressions is detected. The results lay a foundation for the excavation and utilization of the beneficial soil organisms, which can do help in the regulation of plant growth.

土壤食细菌线虫与细菌相互作用及其生态功能是土壤生态学的热点问题之一。前期研究得到土壤食细菌线虫与溶磷菌相互作用能够提高土壤有效磷含量，促进植物生长的效果，而土壤食细菌线虫对土壤磷素活化和植物生长的促进机制尚不清楚：食细菌线虫如何提高土壤中有效磷含量？植物对食细菌线虫作用下有效磷增加的响应机制如何？本项目拟筛选高效溶磷菌，在悉生和自然土壤微缩培养体系下，通过荧光原位杂交技术阐明食细菌线虫对溶磷菌数量、在冬小麦根际定殖情况的影响，采用土壤磷素分级手段揭示不同土著优势食细菌线虫与溶磷菌相互作用对土壤磷素活化程度的影响，同时采用实时荧光定量PCR技术检测冬小麦中响应磷素基因表达量的变化，全面阐明食细菌线虫与溶磷菌相互作用促进植物生长的土壤磷素活化机制，为发掘和合理利用调控植物生长的有益土壤生物奠定基础。

土壤食细菌线虫是数量较大、代谢活性较高的一类土壤食细菌动物，它们可以通过取食土壤中的细菌，对微生物的数量、活性以及周转产生影响。食细菌线虫参与了土壤系统中的生态过程，在土壤系统中具有非常重要的地位。土壤食细菌线虫促进植物生长的“氮素作用机制”已得到确认，而“磷素作用机制”尚不清楚。本研究通过悉生和自然土壤微缩培养体系相结合的手段，揭示食细菌线虫对溶磷菌数量以及在冬小麦根际定殖情况，二者相互作用对土壤磷素活化程度的影响，冬小麦中响应磷素基因表达量的变化，全面阐明土壤食细菌线虫促进植物生长的磷素活化机制。结果表明：从供试土壤中分离得到2株土著高效溶磷菌，分别命名为X3和X8。通过传统的细菌形态和生理生化特征以及16S rDNA基因序列分析，判断菌株X3为沙福芽孢杆菌（Bacillus safensis），属革兰氏阳性菌；菌株X8为绿针假单胞菌（Pseudomonas chlororaphis），属革兰氏阴性菌，并使用绿色荧光蛋白（GFP）标记了这两株菌。同时从供试土壤中分离得到2种土著食细菌线虫，中杆属Mesorhabditis sp. ( cp 1)和拟丽突属Acrobeloides sp. (cp 2)。在两种土壤微缩培养体系中，食细菌线虫通过取食溶磷菌促进了溶磷菌的数量、活性及其在冬小麦根际的定殖，刺激了溶磷菌对磷素活化的能力，显著提高了土壤中有效磷、微生物量磷、草酸以及磷酸酶的含量，从而显著增加了冬小麦地上部的含磷量并且显著影响了冬小麦根系形态的变化，主要表现为根变长变粗、根尖数变多上。这些差异都因食细菌线虫的种类和培养时间的不同而不同，接种中杆属线虫比拟丽突属线虫效果明显。同时，接种食细菌线虫使得冬小麦根系内部磷素响应基因PHT1;1、PHT1;10、PHT2;1的表达都显著增加，说明土壤中磷素的活化提高了磷素响应基因在冬小麦根系内部的表达。这些结果不仅从分子生物学角度证实了食细菌线虫对冬小麦生长的促进作用确实存在着磷素活化效应，也加深了土壤食细菌线虫与溶磷菌在有益土壤生物合理利用方面的认知。