In the same way that mosquitoes transmit the malaria-causing parasite, Plasmodium, between humans, some species of leafhopper transmit phytoplasma bacteria between plants. Phytoplasmas are bacterial pathogens that infect the plant phloem and require sap-sucking bugs to transport them to other plants. Researchers at the John Innes Centre on Norwich Research Park have found that leafhoppers living on plants infected with phytoplasma produced more offspring. This is the first time that a particular protein in the bacteria has been found that reduces the plants’ defensive reactions to pests such as leafhoppers, allowing the bugs to thrive.
The scientists were looking at the witches’ broom strain of aster yellows phytoplasma. This causes plants to grow clusters of stems, giving the appearance of witches’ brooms. This often leads to a reduction in quality and quantity of crop plant yield. There are over 80 species of plants worldwide that are susceptible to these bacteria including onion, lettuce and carrot. Another result of infection is that the usual defence mechanisms that the plants have against bugs that drink their sap are down-regulated, making them more attractive to the bugs.
After sequencing the genome of the phytoplasma, the researchers looked at possible proteins that could be affecting how the insects and plants interact. One such protein, called SAP11, was found to cause the plants to produce less of a defensive chemical, jasmonate, when leafhoppers pierced the stem. On infected plants, leafhoppers laid more eggs and had a greater number of offspring per plant. By attracting leafhoppers to the plants, the phytoplasma are providing themselves with more opportunities for dispersal to other plants.
Currently, the only way to tackle phytoplasma infection is to use insecticides against the insect vectors that transmit the disease, and to remove infected plants from the vicinity of healthy plants. However, this new understanding of which plant pathways the phytoplasma alters give possible opportunities for producing plants resistant to this pathogen in the future.
To find out more about the aster leafhopper, which transmits aster yellows disease, take a look at the Plantwise datasheet: http://www.plantwise.org/?dsid=32158&loadmodule=plantwisedatasheet&page=4270&site=234