Proteobacterial dominance in endophytic bacterial diversity in switchgrass growing under nitrogen range and effect on plant growth

Switchgrass (<i>Panicum virgatum</i> L.) is native to North America and cultivated as a forage and bioenergy crop. Inorganic fertilizers enhance biomass production, increase production costs, and pollute the environment. Switchgrass cultivation using an eco-friendly approach might be achieved by inoculation with beneficial microbes. Therefore, the diversity of cultivable endophytic bacteria from roots and shoots of switchgrass growing under a nitrogen regime was studied. The potential of bacteria for plant growth promotion (PGP) was tested under <i>in vitro</i> conditions. A total of 216 bacterial isolates obtained belonged to four phyla and 33 genera, and most isolates were obtained from plants growing under no (0 kg/ha) or low nitrogen (90 kg/ha) input, rather than higher N (180 kg/ha). Higher numbers of isolates belonged to the phylum Proteobacteria, and genus-wise representation showed the dominance of <i>Pseudomonas</i>, <i>Enterobacter</i>, and rhizobia. Bacterial isolates were tested for PGP properties, e.g. phosphate solubilization, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, Indole Acetic Acid (IAA) production, and nitrogen fixation. Many isolates were positive for one or more PGP properties. In <i>in vitro</i> analysis, selected bacterial isolates were inoculated in two commercial switchgrass cultivars and a showed cultivar-specific response. PGP isolates can be used for pot or field trials and eventually for the sustainable cultivation of switchgrass.

柳枝稷（Panicum virgatum L.）原产于北美洲，常作为饲草与生物能源作物被栽培。无机肥料虽可提升其生物量产量，但同时会增加生产成本并造成环境污染。通过接种有益微生物，有望实现柳枝稷的生态友好型栽培，因此本研究针对不同氮素施用量下生长的柳枝稷根、茎组织中可培养内生细菌的多样性展开了探究。
本研究在体外（in vitro）条件下检测了候选细菌的植物生长促生（Plant Growth Promotion, PGP）潜力，共获得216株细菌分离物，隶属于4个门与33个属；绝大多数分离物来自施氮量为0 kg/ha（无氮）或90 kg/ha（低氮）的植株，而非高氮（180 kg/ha）处理组。分类学分析显示，变形菌门（Proteobacteria）为优势菌门，属水平上以假单胞菌属（Pseudomonas）、肠杆菌属（Enterobacter）及根瘤菌类群占主导地位。
研究团队对所有细菌分离物的PGP性状进行了检测，涵盖磷酸盐溶解能力、1-氨基环丙烷-1-羧酸（ACC）脱氨酶活性、吲哚乙酸（IAA）合成能力与固氮能力。结果表明，多数分离物可表现出一项或多项PGP相关性状。在体外验证实验中，选取的部分细菌分离物被接种至两个商用柳枝稷品种中，结果显示菌株的促生效果呈现品种特异性。
具备PGP活性的细菌分离物可进一步开展盆栽或田间试验，最终为柳枝稷的可持续栽培提供支撑。