Characteristics of Organic Acid Secretion Associated with the Interaction between Burkholderia multivorans WS-FJ9 and Poplar Root System.

The objective of this study was to investigate whether plant-bacteria interaction affects the secretion of organic acids by both organisms and to assess whether the production of IAA by the bacterium increases the secretion of organic acids by root exudates, and if the stress produced by low available phosphorus (P) affects the production of organic acids by bacteria, by roots, or by root exudates in presence of bacterial cultures. With this purpose, we used as a biological model poplar plants and one strain of Burkholderia multivorans able to solubilize P. High performance liquid chromatography was utilized to measure organic acids. The tests, the inductive effects of exogenous indole-3-acetic acid (IAA) on secretion of organic acids, the 2 × 4 × 2 factorial design experiment, and the ability of organic acids to solubilize tricalcium phosphate were performed to investigate the interactive effects. The results showed that, after B. multivorans WS-FJ9 interacted with the poplar root system, the key phosphate-solubilizing driving force was gluconic acid (GA) which was produced in three ways: (1) secreted by the root system in the presence of IAA produced by B. multivorans WS-FJ9; (2) secreted by B. multivorans WS-FJ9; and (3) secreted by the poplar root system in the presence of phosphorus stress. When phosphorus stress was absent, the GA was produced as outlined in (1) and (2) above. These results demonstrated that inoculating B. multivorans WS-FJ9 into the poplar root system could increase the amount of GA secretion and implied that the interaction between B. multivorans WS-FJ9 and the poplar root system could contribute to the increase of P available fraction for poplar plants.

Biosafety and colonization of Burkholderia multivorans WS-FJ9 and its growth-promoting effects on poplars.

Burkholderia cepacia complex (Bcc) is a group of bacteria with conflicting biological characteristics, which make them simultaneously beneficial and harmful to humans. They have been exploited for biocontrol, bioremediation, and plant growth promotion. However, their capacity as opportunistic bacteria that infect humans restricts their biotechnological applications. Therefore, the risks of using these bacteria should be assessed. In this study, Burkholderia multivorans WS-FJ9 originally isolated from pine rhizosphere, which was shown to be efficient in solubilizing phosphate, was evaluated with respect to its biosafety, colonization in poplar rhizosphere, and growth-promoting effects on poplar seedlings. Pathogenicity of B. multivorans WS-FJ9 on plants was determined experimentally using onion and tobacco as model plants. Onion bulb inoculated with B. multivorans WS-FJ9 showed slight hypersensitive responses around the inoculation points, but effects were not detectable based on the inner color and odor of the onion. Tobacco leaves inoculated with B. multivorans WS-FJ9 exhibited slightly water-soaked spots around the inoculation points, which did not expand or develop into lesions even with repeated incubation. Pathogenicity of the strain in alfalfa, which has been suggested as an alternative Bcc model for mice, was not detectable. Results from gene-specific polymerase chain reactions showed that the tested B. multivorans WS-FJ9 strain did not possess the BCESM and cblA virulence genes. Scanning electron microscopy revealed that the colonization of the WS-FJ9 strain reached 1.4 × 10(4) colony forming units (cfu) g(-1) rhizosphere soil on day 77 post-inoculation. The B. multivorans WS-FJ9 strain could colonize the rhizosphere as well as the root tissues and cells of poplars. Greenhouse evaluations in both sterilized and non-sterilized soils indicated that B. multivorans WS-FJ9 significantly promoted growth in height, root collar diameter, and plant biomass of inoculated poplar seedlings compared with controls. Phosphorus contents of roots and stems of treated seedlings were 0.57 and 0.55 mg g(-1) higher than those of the controls, respectively. Phosphorus content was lower in the rhizosphere soils by an average of 1.03 mg g(-1) compared with controls. The results demonstrated that B. multivorans WS-FJ9 is a nonpathogenic strain that could colonize the roots and significantly promote the growth of poplar seedlings.