Characterization of Phosphate Solubilizing Bacterial Endophytes and Plant Growth Promotion In Vitro and in Greenhouse

Abstract

Phosphate is one of the most important nutrients for plant growth and development, and only 0.1% of the phosphate in soils is available to plants. Currently, the use of excess phosphate fertilizer has caused surface and ground water pollution and water eutrophication, resulting in algal blooms in lakes and oceans. Therefore, it is imperative to explore alternative ways to solve these problems for sustainable agricultural production and improvement of soil fertility, while protecting the environment. Microorganisms from the rhizosphere and within plants are able to solubilize insoluble soil phosphate, making it available to plants. Five high phosphate solubilizing bacteria from our bacterial endophyte library were chosen for this study and identified as Pantoea vagans IALR611, Pseudomonas psychrotolerans IALR632, Bacillus subtilis IALR1033, Bacillus safensis IALR1035 and Pantoea agglomerans IALR1325. All five bacteria significantly promoted tall fescue growth in vitro. Greenhouse experiments showed that IALR1325 significantly promoted pepper and tomato growth, and IALR632 was the best in promoting tomato growth. In addition, all these bacteria had extracellular acid phosphatase and phytase activities. One of the mechanisms for phosphate solubilization by bacteria is pH reduction caused by gluconic acid production. Our results indicate that P. agglomerans IALR1325 is a promising bacterium for future applications.

A Potential Application of Endophytic Bacteria in Strawberry Production

“…These three endophytic bacteria were also characterized for their plant growth-promoting traits. All three bacteria have the ability to produce the plant hormone auxin and to solubilize insoluble phosphate (Table 2) although the activity is low compared to other bacteria in our library [17]. However, they do not produce siderophore compounds, fix nitrogen from the atmosphere or have ACC deaminase activity (data not shown).…”

“…Once endophytic bacteria were isolated from leaf, stem or root tissues and grown on Luria-Bertani (LB) with 15 g/L agar, glycerol stocks were made for long-term storage at −80 • C in the Plant Endophyte Research Center at the Institute for Advanced Learning and Research in Danville, Virginia, USA. Routine bacterial growth and culture followed the procedures reported previously [17,18].…”

“…The formula for pathogen inhibition is: % Inhibition = [(Radius of mycelial mat with control plug − Radius of mycelial mat with endophyte disc)]/(radius of the control) × 100. The auxin quantification was modified from Patten and Glick's protocol [19] following our previous publication [17] using Salkowski reagent in triplicate for each bacterium. Indole acetic acid (IAA) was used for a standard curve.…”

“…Selected bacteria were tested for phosphate solubilizing ability following our previous protocol [17]. Briefly, the overnight bacterial culture (0.1 mL) was added to 3.9 mL of NBRIP containing 0.5% of tricalcium phosphate (in triplicate for each) and incubated at 28 • C with shaking at 200 rpm for 3 days.…”

“…Selected bacteria were also screened for nitrogen fixation and ACC deaminase activity, and tested for siderophore production following our previous protocol [17].…”