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Subversion of insect resistance: A novel role for a plant viral silencing suppressor

Reference	BB/F014376/1
Principal Investigator / Supervisor	Professor John Carr
Co-Investigators / Co-Supervisors	Professor Alison Smith
Institution	University of Cambridge
Department	Plant Sciences
Funding type	Research
Value (£)	390,163
Status	Completed
Type	Research Grant
Start date	01/05/2008
End date	31/10/2011
Duration	42 months

Abstract

We have discovered that a virus can manipulate plant defences to make the host less able to resist the insects that transmit the virus. The cucumber mosaic virus (CMV) 2b protein is a suppressor of virus resistance mediated by RNAi and salicylic acid (SA). We analyzed Affymetrix arrays of gene expression profiles in 2b-transgenic Arabidopsis plants treated with resistance-inducing chemicals. Surprisingly, the most comprehensively inhibited defensive signalling pathway in 2b-transgenic plants was not the SA pathway, but the pathway regulated by jasmonic acid (JA). JA is a key signal in resistance to insects. This is highly significant because CMV, along with 70% of all plant viruses, is transmitted by insect vectors- specifically aphids. In the course of other experiments using CMVdelta2b (a CMV mutant lacking the gene for the 2b protein) it was noted that on plants infected with CMVdelta2b, aphid reproduction was inhibited and aphid mortality was increased. We hypothesize that inhibition of resistance to insects by the 2b protein favours aphid colonisation of CMV-infected plants, which in turn leads to increased numbers of viruliferous aphid vectors, and consequently a higher rate of transmission. To explore this concept further we will determine: 1. Which viral gene product(s) of CMVdelta2b elicit anti-insect responses or if this elicitation requires the presence of a replicating virus; 2. Which anti-insect responses are induced (induction of protease inhibitors or other directly JA-induced gene products or increased biosynthesis of low molecular weight chemicals), and which of these factors are enhancing aphid morbidity and mortality, and 3. How the 2b protein interferes with the induction of these anti-insect defences, in particular by assessing the role of RNAi in this subversion.

Summary

Cucumber mosaic virus (CMV) is one of the two most important viruses of field-grown and glasshouse horticultural crops in the UK and worldwide, causing considerable economic damage annually. Moreover it attacks more crop and wild-plant species than any other virus. An essential contributor to the success of CMV is a recently discovered factor made by the virus, which is called the 2b protein. Until recently the most important known role for the 2b protein was that it counter-attacks the resistance mechanisms that plants use to defend themselves against virus infection. We have now discovered another remarkable and novel property of the 2b protein: it inhibits plant defences against insects. This is very important because under natural conditions, CMV (like most plant viruses) is transmitted from plant to plant by aphids. Aphids are a group of insects that feed on plants and in doing so cause damage and promote the spread of disease caused by viruses and other disease-causing organisms. We can imagine that it will be in the interests of the virus to inhibit defences against insects so that sufficient aphids will be born on CMV-infected plants to pass on the infection to new host plants. We found that the 2b protein blocks the action of a natural chemical signal, jasmonate, which stimulates anti-insect defences in plants. We also discovered that in plants infected with a mutant strain of CMV that is unable to make the 2b protein, these defences are activated. We will determine whether it is a substance made by the virus itself, or the stress of viral infection, which triggers the anti-insect responses. In addition we will identify the anti-insect materials produced by plants in response to infection with the mutant strain of the virus, and how the 2b protein inhibits this response in plants infected with the naturally-occurring virus.

Committee	Closed Committee - Plant & Microbial Sciences (PMS)
Research Topics	Crop Science, Microbiology, Plant Science
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

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