Tobacco rattle virus and spraing disease in potato

Tobacco rattle virus (TRV) has a worldwide distribution and is known to infect more than 400 different plant species, including food crops, ornamentals, woody plants and many weeds. The virus is spread by root-feeding nematodes, from the genera Trichodorus and Paratrichodorus (referred to as trichodorid nematodes), and sometimes through infected seed.

In the UK, northern Europe and North America the most important disease caused by TRV occurs in potato. TRV infection primarily occurs where the plants are grown in sandy soils that favour the activity of the vector nematodes, however, in some potato cultivars the virus can be retained in the tuber and emerge as a secondary infection if that tuber is replanted.

It is currently thought that there are three types of response to TRV by different potato cultivars. Some cultivars, such as Bintje, are immune to TRV infection, although isolates of TRV have arisen that can overcome this resistance. A second group of cultivars, such as Nadine, are completely susceptible to TRV, so that the virus spreads throughout the infected plant causing little or no observable symptoms.

However, previous work at the James Hutton Institute revealed that in these susceptible cultivars, yield, tuber size and uniformity were all reduced by TRV infection. The third group of cultivars, such as Pentland Dell, react to TRV infection by producing arcs of discolouration (known as spraing) inside the tubers, as well as surface lesions and malformations. Potato crops with spraing symptoms can be rejected for sale.

TRV is an RNA virus that has two genome segments (RNA1 and RNA2) that normally are packaged separately into rod-shaped virus particles. Long particles contain the larger, RNA1 whereas short particles contain the smaller, RNA2. In a susceptible potato cultivar both RNAs (and both particles) spread within the plant. However, in spraing-reacting cultivars the RNA2 is often lost and only the RNA1 multiplies and spreads in the plant.

As RNA2 codes for the virus coat protein, loss of this RNA means that the remaining RNA1 is not assembled into a virus particle but must infect the plant in a “naked” form. Loss of RNA2 from a TRV infection has only been observed naturally in potato and is actually very difficult to achieve experimentally. It might be that spraing-reacting potatoes possess a defensive response that actively targets multiplication of RNA2 or its co-ordination with RNA1.

We are studying the responses of susceptible and spraing-reacting potatoes to different strains to TRV, carrying different virus genes, which we have isolated and assembled in the laboratory. We are analysing potato gene expression in response to TRV infection, looking at the biochemical basis of spraing production and investigating whether particular genes or proteins of TRV are elicitors of or targets of the spraing reaction.