In plants, the first line of defence against microbes involves the recognition of molecules that are produced by a wide variety of microbes. For example, many plants can detect the protein EF-Tu — which is essential for DNA replication in bacteria — and a molecule called chitin, which is found in the cell walls of fungi. Detection of these molecules activates defence responses that are thought to be able to repel most microbes.
However, some microbes are able to avoid these defences and so plants need to be able to employ other defence strategies that are targeted at those particular microbes. These defences involve genes known as resistance (or R) genes that detect specific molecules produced by the microbe. These genes have been widely used in the breeding of wheat and other crops to improve disease resistance. However, this form of resistance can quickly become ineffective as the microbes mutate or lose the gene(s) that make the molecules detected by the R genes. So, the search is on for more durable forms of resistance.
Molecules like EF-Tu and chitin play important roles in the survival of microbes, so they cannot easily be changed, or lost to allow the microbe to avoid being detected. Therefore, enhancing the ability of plants to detect these molecules may provide resistance to a broader range of microbes that could be effective for much longer periods of time.
Members of the Brassicaceae family of plants (e.g. cabbage, oil seed rape and the model plant Arabidopsis thaliana) have a receptor protein called EFR. This receptor protein can recognise a section of the EF-Tu protein called elf18. When the EFR gene from A. thaliana was expressed in tobacco and tomato plants, these plants showed increased resistance to infection with bacteria (1).
Three different groups of researchers have been studying the effect of inserting the EFR gene into cereal crops. The cereals belong to the monocot group of flowering plants and so they are less closely related to A. thaliana than tobacco and tomato, which are all eudicots (for more information read this post).
Two of the research groups independently inserted the EFR gene into rice (2,3). They found that this enabled the rice plants to detect and respond to the presence of elf18. Also, the plants had increased resistance to infection with the bacteria species that cause the diseases rice blight and rice bacterial brown stripe. The other group of researchers inserted the EFR gene into wheat (4). Similar to the rice plants, EFR enabled the wheat plants to respond to elf18, and the plants had increased resistance to infection with the bacteria Pseudomonas syringae pv. oryzae, which causes bacterial leaf blight.
These findings demonstrate that the EFR gene can be transferred successfully from the Brassicaceae to cereals, and shows that immune signalling pathways are fairly similar across these distant groups of plants. Therefore, transferring the genes involved in the first line of plant defence against microbes between different plant families may be a useful strategy to boost resistance to a variety of diseases.
- Lacombe, S et al. (2010). Interfamily transfer of a plant pattern-recognition receptor confers broad-spectrum bacterial resistance. Nature Biotechnology. 28, 365–9.
- Lu F, et al. (2014) Enhancement of innate immune system in monocot rice by transferring the dicotyledonous elongation factor Tu receptor EFR. Journal of Integrative Plant Biology
- Schwessinger B et al. (2014). Transgenic expression of the dicotyledonous pattern recognition receptor EFR in rice leads to ligand-dependent activation of defense responses. BioRxiv. doi:10.1101/006155.
- Schoonbeek, H.-J.J et al. (2015). Arabidopsis EF-Tu receptor enhances bacterial disease resistance in transgenic wheat. New Phytologist.