Direct beneficial effects of cytokinin-producing rhizobacteria on plant growth
Garcia de Salamone, Ines Eugenia
A study was conducted to determine the role of cytokinin production by certain plant growth promoting rhizobacteria (PGPR) strains in the promotion of plant growth and development. PGPR were screened for cytokinin production in pure culture and in association with Raphanus sativus (radish) cv. Cherry Belle plants and tobacco callus tissues of Nicotiana tabacum, var. Xanthi. The cytokinins isopentenyl adenosine (IPA), zeatin riboside (ZR) and dihydrozeatin riboside (DHZR) were measured by immunoassay. The ability of selected PGPR strains to promote plant growth and development was studied in Triticum aestivum (wheat) cv. Katepwa and radish grown in growth pouches and in pots in controlled environment chambers. After 336 h in MM+Gl medium, Pseudomonas fluorescens strain G20-18WT, a rifampicin resistant mutant and two Tn5 insertion transconjugants produced 8.60, 6.80, 0.90 and 0.80 pmol mL-1 of [IPA+ZR+DHZR] respectively. Addition of 10-5 M adenine increased cytokinin production of strain G20-18WT by approximately 67%. Strain G20-18WT and several mutants colonized Katepwa wheat plants similarly but only G20-18WT significantly increased emergence, shoot and root biomass. At anthesis, leaf area, root biomass, number of tillers and visible ears were significantly increased by this strain, however grain yield was not affected. Radish roots were colonized by all tested PGPR strains, however, only strain G20-18WT increased emergence, root and shoot length and biomass of plants grown in both growth pouches and greenhouse. Similar effects were observed with exogenous applications of zeatin ranging between 0.5 and 10 n' M' and cytokinin combinations lower than 0.5 n'M'. Inoculation with G20-18WT increased the concentration of cytokinins in the rhizosphere fivefold. The concentrations of IPA and DHZR in radish rhizospheres and plant tissues inoculated with G20-18WT were twofold higher than in those inoculated with the mutants. Fresh weight increases of tobacco callus cultured in the presence of strains G20-18WT, were 4.8 times higher than those of callus grown on control plates with a cytokinin-free medium. These increases were correlated with direct and indirect estimates of [IPA+ZR+DHZR] in the medium. These results are the first evidence showing a direct link between plant growth promotion and cytokinin production by a Pseudomonas PGPR strain.