Functions of a novel chitinase-like effector family unique to aphids

Aphids are intrinsically fascinating insects, but they are also damaging pests in global agriculture, causing losses through feeding on plant sap and through spreading many viruses that cause plant diseases. Aphid populations can increase extremely fast due to their clonal reproduction and their unusual "Russian Doll" telescoped generations.

Although chemical pesticides are widely used, problems often arise due to the aphids evolving resistance and no longer being able to be controlled. In addition, there may be environmental or health issues associated with certain pesticides that have led to some being withdrawn or restricted. There are a few crops such as tomato, melon, soybean and rice, with known genetic resistance to some aphids and related species.

The genes involved are called R-genes (for resistance) and commonly code for receptor proteins that recognise invasion by the pest insect. However, insects in turn can evolve new means to overcome the plant's R-gene defences. Overall there is a pressing need to find new robust and sustainable ways to manage pests in agriculture.

In this context, we compared molecular differences between aphids that thrived or died depending on which plant type they were attacking, and recently discovered a new class of proteins in aphid saliva that distantly resemble chitinase enzymes. Chitinases are present in all insects, being essential for maintaining the chitin polymer in their exoskeleton and mouthparts.

However, these salivary proteins appear unable to act as chitinase enzymes, so we need to find another function to explain how they benefit the aphids. Intriguingly, this protein group is unique to aphids, being absent even from other closely related species of sap-sucking insects. Because aphids evolved as a distinct group of species around 300 million years ago, we think that the chitinase-like proteins have ancient origins that may underpin the enormous success of these organisms.

Our starting idea is that chitinase-like proteins may still have the ability to stick to chitin, in effect mopping up fragments of these molecules. This would prevent the host plant from detecting the presence of chitin, allowing the aphids to go under the radar of the plant's immune system. Other work on fungi shows that plants carry chitin-detecting receptor proteins on their cell surface, and there is no reason to think that these receptors couldn't also detect aphid chitin.

Our second idea comes from finding one particular aphid chitinase-like protein that is strongly associated with exactly the opposite process: triggering plant immunity through an R-gene. Here we will directly test both these concepts.

To uncover how chitinase-like proteins work, we will take a wide range of research approaches. We want to work out the common rules for the whole family, and also to discover the specific properties of the one member that leads to plant immunity and aphid death. First, we will make purified forms of the proteins to enable laboratory scale experiments.

In particular, we will test whether the proteins really bind to chitin, or whether they have some other unexpected functions. We will also hunt for proteins or other components of plant cells that bind to the incoming chitinase-like salivary proteins. Both immune-suppressing and immune-activating modes are commonly known to result from protein-to-protein interactions in responses to fungal and bacterial diseases, so we will look for parallels in our aphid system.

Working with colleagues in France, we will apply a genome editing technology called CRISPR to specifically knock-out the gene that codes for the immune-activating salivary protein. This will give us new aphids to see if they do better or worse than aphids that still have this gene. Finally, we discovered that plant immunity is suppressed under stressful environments, including drought that are increasingly prevalent due to climate, so we will explore mechanisms of immune robustness.