Fungi rooted out as pasture nemesis
DNA tests developed by the Pasture Soil Biology Program with AWI support are helping identify constraints to pasture productivity. Widespread damage to subclover seedling roots has been detected in a survey of pastures across three states encompassing the south-east of South Australia, the south-west of Western Australia and the southern tablelands and south-west slopes of New South Wales.
The damage is most likely associated with root-rotting fungi and was detected at each of 18 sites where CSIRO Plant Industry, the University of Western Australia and South Australian Research and Development Institute (SARDI) researchers established field bioassay experiments during the autumn-winter of 2006.
On average, roots on two-thirds of subclover test plants were damaged. Even in the best situation, damage affected 40 per cent of roots.
"Dry seasons are not conducive to root disease, so this level of damage is alarming and indicates that there is a persistent and costly impact on pasture plant establishment and production that warrants further investigation to develop management strategies," says Dr Bob Hannam, coordinator of the Pasture Soil Biology Program.
Towards that goal, SARDI researchers have developed a series of DNA-based assays for pathogenic and beneficial organisms as well as plant roots. "These assays serve as research tools to better understand soil biology and help minimise biological constraints in Australian pastures," Dr Hannam says.
The assay targets 16 soil-borne pathogens, some mycorrhizal families and three pasture roots. More are planned for development.
Importantly, the SARDI assays can identify and quantify not just the pathogens but also the beneficial microbes and plant roots in the same soil sample.
"These new assays will help us study the interactions between microbes and plant roots under different pasture management systems," Dr Hannam says. "Because a similar suite of DNA assays were previously developed for soil pathogens that affect grain crops, we are now able to study the interplay between soil-borne pasture and crop disease in mixed-farming systems."
An exciting development is the prospect that the pasture DNA assay can also detect functional (or active) plant roots in paddock-scale environments: "Once verified, we expect that the assay's sensitivity means that benchmarks for optimum root performance may be established for different pasture management strategies such as grazing, fertiliser and chemical applications, and also recovery from stress."
Dr Hannam is confident that a better understanding of microbe–root interactions can also serve as a platform for developing techniques to influence the behaviour of roots and microbes.
An Australian National University basic science study is focusing on how soil bacteria near roots emit chemical signals (quorum sensing signals) to influence the bacteria's behaviour, and how plants respond to these chemicals. Pasture species such as subclover, barrel medic and ryegrass have been shown to respond through accelerated germination, altered plant growth and changes in the interaction of the plants with soil bacteria.
"Once these processes are understood, it is conceivable that special signalling compounds could be used to stimulate plants to better fight fungal pathogens and other pathogenic soil-borne organisms," he says. "There is also evidence that plants can detect the bacteria's chemical signals, which may help the plant prepare defensive responses to infection or emit ‘mimic' chemicals to confuse the bacteria behaviour. This opens up the opportunity to breed plants for the ability to secrete compounds that can influence the behaviour of soil bacteria near the root surface."
In another study, CSIRO Plant Industry is looking at the interaction between soil pathogens and the environment surrounding the roots (the rhizosphere).
They are determining how plant roots respond to stress and how this influences microbe colonisation of the rhizosphere.
"The Pasture Soil Biology Program was recently subject to external review by a team of scientists who strongly recommended that the overall research program is sound and should be continued," Dr Hannam says. "We look forward to continuing to develop and validate a comprehensive suite of molecular assays in association with targeted basic science on understanding the interactions between roots and soil biota at the root surface."
The program is jointly supported by AWI, Meat and Livestock Australia (MLA) and the Grains Research and Development Corporation (GRDC). Dr Hannam is inviting other research investors to join the next phase of research to ensure "a strong collaborative funding base" that can foster investment in frontier research of wide interest to farmers and with potential benefit for soil health and productivity.
More information: Pastures information or contact Dr Bob Hannam, rhannam@ozemail.com.au
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