Clone libraries of bacterial 16S rRNA genes (a total of 1,980 clones) were constructed from the leaf blades, petioles, taproots, and lateral roots of sugar beet (Beta vulgaris L.) grown under different fertilization conditions. A principal coordinate analysis revealed that the structures of bacterial communities in above- and underground tissues were largely separated by PC1 (44.5%). The bacterial communities of above-ground tissues (leaf blades and petioles) were more tightly clustered regardless of differences in the tissue types and fertilization conditions than those of below-ground tissues (taproots and lateral roots). The bacterial communities of below-ground tissues were largely separated by PC2 (26.0%). To survey plant growth-promoting bacteria (PGPBs), isolate collections (a total of 665 isolates) were constructed from the lateral roots. As candidate PGPBs, 44 isolates were selected via clustering analyses with the combined 16S rRNA gene sequence data of clone libraries and isolate collections. The results of inoculation tests using sugar beet seedlings showed that eight isolates exhibited growth-promoting effects on the seedlings. Among them, seven isolates belonging to seven genera (Asticcacaulis, Mesorhizobium, Nocardioides, Sphingobium, Sphingomonas, Sphingopyxis, and Polaromonas) were newly identified as PGPBs for sugar beet at the genus level, and two isolates belonging to two genera (Asticcacaulis and Polaromonas) were revealed to exert growth-promoting effects on the plant at the genus level for the first time. These results suggest that a community analysis-based selection strategy will facilitate the isolation of novel PGPBs and extend the potential for the development of novel biofertilizers.

中文翻译：

---

基于群落分析的甜菜植物促生长菌筛选。

从在不同施肥条件下生长的甜菜 (Beta vulgaris L.) 的叶片、叶柄、主根和侧根构建细菌 16S rRNA 基因的克隆文库（共 1,980 个克隆）。主要坐标分析显示，地上和地下组织中的细菌群落结构在很大程度上被 PC1 (44.5%) 分开。无论组织类型和受精条件如何，地上组织（叶片和叶柄）的细菌群落都比地下组织（主根和侧根）的细菌群落更紧密。地下组织的细菌群落很大程度上被 PC2 (26.0%) 分开。为了调查植物生长促进细菌 (PGPB)，从侧根构建了分离物集合（总共 665 个分离物）。作为候选 PGPB，通过聚类分析选择了 44 个分离株，并结合了克隆文库和分离株集合的 16S rRNA 基因序列数据。甜菜幼苗接种试验结果表明，8 个分离株对幼苗具有促生长作用。其中，7个属（Asticcacaulis、Mesorhizobium、Nocardioides、Sphingobium、Sphingomonas、Sphingopyxis和Polaromonas）的7株被新鉴定为甜菜属水平的PGPB，2个属（Asticcacaulis和Polaromonas）首次在属水平上对植物发挥促生长作用。