Scientists root out the secret life of a plant disease.

Scientists from the Sainsbury Laboratory (SL), Norwich report in the journal Nature that important plant diseases previously thought only to infect plants through their leaves may also enter through the plant’s roots. They report that the rice leaf blast fungus is able to use very different routes and means of attacking the rice plant by switching between two completely different programmes of developmental events; one programme is characteristic of leaf-infecting fungi and the other characteristic of root-infecting fungi. If this previously unsuspected ability is widespread amongst diseases of important crops it will have implications for our current strategies for controlling diseases by using chemical sprays and plant breeding, and for our understanding of how changing agriculture practices may alter disease prevalence.

“This is a fascinating discovery” says Dr Anne Osbourn (leader of the research team at the SL). “Plant diseases are usually highly-specialised to be able to infect a particular plant tissue. We have demonstrated that a fungus that we normally associate with the rice leaf and that has a sophisticated system for entering and infecting the leaf tissue, can switch on a completely different infection system to enable it to penetrate the rice plant’s root. When it comes to invading its host plant, the rice leaf blast fungus is keeping its options open”.

Rice is the staple food for half of the world’s population. Rice blast is one of the most damaging diseases of cultivated rice and so is a constant threat the world’s food supply. Strategies to control the disease depend on the use of varieties that are resistant to disease attack and the application of fungicides, but neither of these methods is particularly effective. The development of durable, environmentally friendly strategies for the control of rice blast disease will depend on a better understanding of how the disease organism infects its host.

In laboratory experiments where rice seedlings were infected with the blast fungus through their roots, typical blast disease symptoms appeared on the aerial parts of the plant, indicating that the fungus had spread systemically throughout the plant. The appearance of normal disease symptoms in a significant number of root infected plants suggests that this might be an important infection route in the field. Rice blast has several close relatives that enter their cereal host plants by root-infection and cause major diseases, such as take-all, and so the possibility that root-infection is a significant aspect of the rice blast life cycle should be taken seriously. Although the importance, in the field, of rice blast’s ability to switch between infection routes is unknown, the existence of these alternative strategies should be considered when changing agronomic practices.

Recently, other important pathogens that were thought to infect the aerial parts of their hosts have been reported to be able to infect through the roots as well. If further research shows that it is common for pathogens to have “secret lives” that enable them to switch infection routes, this will be an important but previously unsuspected factor to consider when changing agricultural practices and breeding for plant disease resistance. Any change that makes it more difficult for a pathogen to infect a crop through its normal infection route could select for a change in the pathogen’s behaviour that exploits alternative infection routes. Such a shift could alter the prevalence and ease of controlling current diseases and highlights the need to fully understand plant disease in order to develop new and effective strategies for disease control.