The plant rhizosphere contains a diverse array of microorganisms, some of which may help plants to adapt to challenges more rapidly than plant genetic adaptation alone. To investigate this, over 170 bacterial isolates from the rhizosphere of five types of plants from three Western Australian soils were tested for their ability to promote plant growth. Two isolates, Burkholderia caledonica and Enterobacter soli, significantly improved the grain yield of wheat in field trials in south-west Western Australia by 23% and 9% respectively. Another isolate, Pseudomonas granadensis, when co-inoculated with rhizobia on peas, significantly improved nodulation by up to 71% and increased the grain yields at harvest by up to 35%. Using strain specific primers developed from the 16S-23S rRNA ITS1 region, the P. granadensis cells were shown to be present in the nodules of field grown peas.
The P. granadensis isolate was also tested in field trials as a co-inoculant with rhizobia on a range of legume species on Christmas Island and significantly increased nutrient uptake, nodulation or plant biomass at low levels of applied fertilizer on soybean, mungbean and lablab compared with the rhizobia-only controls.
Full phosphorus response curve glasshouse trials demonstrated that phosphorus solubilisation by P. granadensis and B. caledonica is not the mechanism of plant growth promotion in wheat. In vitro, these bacteria produced the plant hormone, indole-3-acetic acid (IAA), which is known to alter plant root architecture. In growth pouch assays, root lengths were enhanced by the bacteria and the effect was reduced in the presence of an auxin inhibitor, suggesting that production of IAA by the bacteria may be partly responsible for increasing wheat seedling root lengths.
This study demonstrates that harnessing the ability of soil microbes to improve plant health and productivity may be an important factor in promoting food security in the future.