Dynamics of Seed-Borne Rice Endophytes on Early Plant Growth Stages

Bacterial endophytes are ubiquitous to virtually all terrestrial plants. With the increasing appreciation of studies that unravel the mutualistic interactions between plant and microbes, we increasingly value the beneficial functions of endophytes that improve plant growth and development. However, still little is known on the source of established endophytes as well as on how plants select specific microbial communities to establish associations. Here, we used cultivation-dependent and -independent approaches to assess the endophytic bacterrial community of surface-sterilized rice seeds, encompassing two consecutive rice generations. We isolated members of nine bacterial genera. In particular, organisms affiliated with Stenotrophomonas maltophilia and Ochrobactrum spp. were isolated from both seed generations. PCR-based denaturing gradient gel electrophoresis (PCR-DGGE) of seed-extracted DNA revealed that approximately 45% of the bacterial community from the first seed generation was found in the second generation as well. In addition, we set up a greenhouse experiment to investigate abiotic and biotic factors influencing the endophytic bacterial community structure. PCR-DGGE profiles performed with DNA extracted from different plant parts showed that soil type is a major effector of the bacterial endophytes. Rice plants cultivated in neutral-pH soil favoured the growth of seed-borne Pseudomonas oryzihabitans and Rhizobium radiobacter, whereas Enterobacter-like and Dyella ginsengisoli were dominant in plants cultivated in low-pH soil. The seed-borne Stenotrophomonas maltophilia was the only conspicuous bacterial endophyte found in plants cultivated in both soils. Several members of the endophytic community originating from seeds were observed in the rhizosphere and surrounding soils. Their impact on the soil community is further discussed.

细菌性内生菌几乎普遍存在于所有陆生植物中。随着学界对揭示植物与微生物间互利共生互作的研究日益深入，我们愈发重视能够促进植物生长发育的内生菌的有益功能。然而，目前对于已定殖内生菌的来源，以及植物如何筛选特定微生物群落以形成共生关系，仍知之甚少。本研究采用依赖培养与非依赖培养的两种方法，对经过表面灭菌的水稻种子（涵盖连续两代水稻）的内生细菌群落进行了分析，共分离得到9个细菌属的菌株。尤为关键的是，两代水稻种子中均分离得到了与嗜麦芽窄食单胞菌（Stenotrophomonas maltophilia）和苍白杆菌属（Ochrobactrum spp.）相关的菌株。对种子提取的DNA进行基于聚合酶链式反应的变性梯度凝胶电泳（PCR-DGGE）分析结果显示，第一代种子的细菌群落中约有45%同样存在于第二代种子中。此外，本研究设置了温室实验，以探究影响内生细菌群落结构的非生物与生物因子。对从不同植物部位提取的DNA进行PCR-DGGE分析得到的图谱显示，土壤类型是调控细菌性内生菌群落的主要驱动因子。种植于中性pH土壤中的水稻，其体内的种子源性栖稻假单胞菌（Pseudomonas oryzihabitans）和放射农杆菌（Rhizobium radiobacter）生长更为旺盛；而种植于低pH土壤中的水稻，则以类肠杆菌（Enterobacter-like）和人参迪茨氏菌（Dyella ginsengisoli）为优势类群。种子源性的嗜麦芽窄食单胞菌是唯一一种在两种土壤种植的水稻中均能检测到的优势内生细菌。本研究还在根际及周边土壤中发现了多个源自种子的内生菌群成员，并进一步讨论了它们对土壤群落的影响。