Rhizobacterial mediation of plant hormone status

Plant growth-promoting rhizobacteria are commonly found in the rhizosphere (adjacent to the root surface) and may promote plant growth via several diverse mechanisms, including the production or degradation of the major groups of plant hormones that regulate plant growth and development. Although rhizobacterial production of plant hormones seems relatively widespread (as judged from physico-chemical measurements of hormones in bacterial culture media), evidence continues to accumulate, particularly from seedlings grown under gnotobiotic conditions, that rhizobacteria can modify plant hormone status. Since many rhizobacteria can impact on more than one hormone group, bacterial mutants in hormone production/degradation and plant mutants in hormone sensitivity have been useful to establish the importance of particular signalling pathways. Although plant roots exude many potential substrates for rhizobacterial growth, including plant hormones or their precursors, limited progress has been made in determining whether root hormone efflux can select for particular rhizobacterial traits. Rhizobacterial mediation of plant hormone status not only has local effects on root elongation and architecture, thus mediating water and nutrient capture, but can also affect plant root-to-shoot hormonal signalling that regulates leaf growth and gas exchange. Renewed emphasis on providing sufficient food for a growing world population, while minimising environmental impacts of agriculture because of overuse of fertilisers and irrigation water, will stimulate the commercialisation of rhizobacterial inoculants (including those that alter plant hormone status) to sustain crop growth and yield. Combining rhizobacterial traits (or species) that impact on plant hormone status thereby modifying root architecture (to capture existing soil resources) with traits that make additional resources available (e.g. nitrogen fixation, phosphate solubilisation) may enhance the sustainability of agriculture.