Phosphate-solubilizing bacteria in the rhizosphere of Al-tolerant and Al-sensitive wheat varieties growing in Australian acid soils.

H. Fisher ^1,2, P. Mele¹ and R. Ford².

¹Department of Natural Resources and Environment, Rutherglen Research Institute, RMB 1145, Rutherglen, Victoria, AUSTRALIA, 3685

²Joint Centre for Crop Innovation, The University of Melbourne, Victoria, AUSTRALIA, 3010 ²

E-mail: Helen.Fisher@nre.vic.gov.au

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Phytotoxic aluminium (Al) levels are often associated with acid soils, forming complex compounds with essential elements like phosphorus (P)¹. This complexation limits P availability for wheat roots. The application of P-containing fertilisers can be reduced by the development of management practices using P-solubilizing rhizosphere bacteria. Such bacteria may condition the rhizosphere, resulting in beneficial growth of wheat in Al-phytotoxic soils.

Root architecture and phenotypic differences were assessed in two near-isogenic wheat lines (Al-tolerant and Al-sensitive) growing under Al-toxic field conditions. This was to confirm expression of Al tolerance gene, and to locate site of removal of potential P-solubilizing bacteria. Increased numbers of nodal roots and plant tillers, and a different response in plant tissue colour to Al-toxicity was observed for the Al-tolerant line compared to the Al-sensitive line.

Two approaches are being employed; a culture dependent technique and a direct extraction molecular technique. P-solubilizing microorganisms were isolated from rhizosphere samples using selective media containing dicalcium phosphate and Al-phosphate. Of the total culturable microbial population, 60% were able to solubilize P from media containing dicalcium phosphate (pH 7.2). Further screening, on media containing Al-phosphate (pH 6.0), resulted in 53% and 41% P-solubilization, by microbes collected from the Al-tolerant and Al-sensitive rhizosphere, respectively. The mechanism of P-solubilization was assessed using a phosphatase enzyme assay. Five isolates (from Al-tolerant wheat) showed strong phosphatase activity. One of these strains has been selected for field trials to observe its persistence in the rhizosphere of Al-tolerant wheat growing in Al-toxic soil. Al-tolerant and Al-sensitive rhizosphere community diversity is being assessed and compared to one another, using T-RFLP analyses with eubacterial 16S rDNA specific primers. Preliminary data will be presented.

Whitelaw, M.A., Harden, T.J., and Heylar, K.R. (1999) Soil Biology and Biochemistry. 31, 655-665