Friendly bacteria show key role in “priming” plant defences

New research into plant disease resistance shows that bacteria attracted to maize roots induce a state of 'early warning' in the cereal, resulting in a faster and stronger response to attack by pathogens or herbivores. The findings, published in Plant Signaling and Behavior, point to a possible mechanism to reduce the amount of pesticides used in the future if this natural defence mechanism can be exploited through breeding.

It has been known for some time that certain plants exude chemicals from their roots which attract organisms to the surrounding soil.

Dr Andrew Neal of Rothamsted Research and Dr Jurriaan Ton of the University of Sheffield have now shown that the plants respond to the attracted bacteria with an 'induced systemic resistance', where the plant's natural resistance mechanisms to pathogens and herbivores are placed in an 'enhanced state of awareness'.

In a previous paper, Dr Neal and Dr Ton recently found that maize plants exude chemicals known as benzoxazinoids which attract soil bacteria.

Once at the root surface, these bacteria increase the availability of important nutrients like iron and phosphorous, and so help maximise plant growth. They have now identified that the bacteria also act in a similar way as vaccinations in humans, enabling the maize to be more ready to defend itself from threats.

Dr Neal explained: "What we have now found is not only does maize recruit microbes by exuding benzoxazinoids, but maize plants respond to the presence of these beneficial microbes by acquiring an enhanced state of preparedness. We have identified that rhizobacteria prime transcription of maize defence genes and chemicals, which are important in systemic resistance against plant pests and diseases. Plants respond to attack in a much more powerful way and more quickly than plants whose roots have not been colonized by rhizobacteria. This response helps protect leaves from further damage."

Dr Ton said: "Now we have a better understanding of how and why maize attracts certain soil microbes we can explore the possibility of breeding maize, and potentially other cereals, with increased levels of root exudation of naturally occurring benzoxazinoids to 'woo' more beneficial soil bacteria, which in turn prime maize natural defences against pests of diseases. If successful, this could reduce the use of fungicides and insecticides which are applied to crops."

The research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and the EU PURE project, supported by the European Commission through the Seventh Framework Programme.