Plant Growth-Promoting Rhizobacteria Associated with Winter Wheat in Saskatchewan, Canada
Freitas, José Renato de
The association of root colonizing bacteria (rhizobacteria) with winter wheat (Triticum aestivum L. cv. Norstar) was investigated in a series of laboratory, growth chamber and field studies. In an initial screening, 111 bacteria were tested for their effects on the growth and development of winter wheat. Nine pseudomonad isolates significantly (P<O.OI) increased plant height, root and shoot biomass, and number of tillers of plants grown in a Gray Luvisol (Choiceland) in a growth chamber. In an Orthic Black Chernozem (Laird), some of these pseudomonads significantly increased seedling emergence rates. Identification of the nine rhizobacteria was made using standard biochemical tests and API 20E and API rapid strips. Three of the isolates were tentatively classified as Pseudomonas aeruginosa, and two each as Pseudomonas cepacia, Pseudomonasfluorescens and Pseudomonas putida. In vitro interaction studies on potato dextrose agar and King's B agar plates revealed an antagonistic ability of these rhizobacteria against the plant pathogenic fungi Rhizoctonia solani and Leptosphaeria maculans. Three possible mechanisms of plant growth promotion were assessed: (i) antibiosis towards phytopathogenic fungi (i.e., biocontrol), (ii) production of hormones such as IAA, and (iii) stimulation of nutrient uptake by winter wheat plants inoculated with wild-type parents and their antibiotic-resistant (RifT Nall) counterparts. Seed inoculation of winter wheat with P. cepacia R55, P. cepacia R85, or P. putida R104, antagonists towards plant pathogenic fungi in vitro, significantly increased plant growth in soil infested with Rhizoctonia solani isolates AG-1, AG 2-1, and AG- 3. Compared to nontreated seeds grown in the infested soil, inoculation with P. cepacia R55 and P. putida RI04 significantly (P<O.05) increased plant biomass (shoot, root, and plant iii total weight). Inoculation with P. cepacia R85, significantly (P<0.05) increased shoot biomass only. Although these increases ranged from 17 to 215%, total dry matter yields were only 63% of that obtained in the absence of the pathogens. Each of these three pseudomonads produced fluorescent pigments when cultured in 10w-Fe3+ medium. This suggests that one possible explanation for the antibiosis towards fungi was the production of a siderophore-like substance. In fact, all nine pseudomonad isolates produced watersoluble fluorescent pigments with absorbance peaks between 320 and 480 nm when cultured in low-Fe3+ medium. P. cepacia R55 and R85, P. aeruginosa R61 and R75, and P. fluorescens Rill produced compounds with similar absorbance peaks near 320 nm, whereas P. aeruginosa R80, Pi fluorescens R92 and P. putida R104 produced compounds with absorbance peaks in the range of 380 to 420 nm.