Plant pathogen focus: Pierce's disease and the vineyards of California

Grapevine dying of Pierce's disease. Photo courtesy of PD-GWSS/CDFA.

Xylella fastidiosa  is not your ordinary kind of bug. It made it to the list of the most wanted plant pathogenic bacteria in 2012! (Mansfield, 2012.)

This is well deserved: X. fastidiosa can infect over a hundred species (grapevine, oleander, citrus, almonds,…), and it causes severe symptoms that can kill the infected plant. The Xylella bacteria colonize the xylem vessels, and by doing so they block the transport of water in the plant. The water-deprived leaves dry and scorch, until finally they drop to the ground. 

In 1892, Newton Pierce, California’s first professional plant pathologist, described the disease that now bears his name, although he failed to identify the causative agent of the disease (X. fastidiosa). Around the end of the 19^th century, an epidemic of Pierce’s disease devastated thousands of hectares of vineyards in the Los Angeles Basin. Since then, southern California has considerably reduced its viticulture; this also explains why present-day Californian vineyards are mostly restricted to the north of the state. There is currently no other cure than getting rid of the infected plants!

Xylella fastidiosa rod cells. Photo courtesy of Jeremy Warren.

The dispersion of the disease is associated with the distribution of its vector insects, sharpshooter leafhoppers. Sharpshooters carrying X. fastidiosa in their guts feed on the grapevine’s sap, which allows the bacteria to infect the plant’s xylem in the process. (Sharpshooters are found in North and South America, but not in Europe, hence the absence of the disease in ‘old world’ vineyards.) Along the coast of California, the spread of Pierce’s disease is associated with the distribution of the blue-green sharpshooter. With increasing latitude, as the winter becomes colder, the sharpshooters are less prominent and Pierce’s disease is less of a problem

Pierce’s disease has thus been endemic in California for more than a century. Since the 1990s, however, a new vector insect from southeastern US, the glassy-winged sharpshooter, has colonized California and has considerably worsened the situation. Unlike the native Californian vectors, the glassy-winged sharpshooter has feeding habits that favor chronic infections by X. fastidiosa. The incidence of Pierce’s disease became so alarming that in 1999 the Californian Department of Food and Agriculture funded an ambitious research program, led by the University of California, to fight the spread of the disease. Every year from 2001 to 2011, a research symposium was organized by the Pierce’s Disease Control Program (PDCP) to account for the latest research developments, with UC Davis significantly contributing to the research effort.

What are the strategies to mitigate the disease? There are plenty! First, there is the control of the vector, the glassy-winged sharpshooter. This includes the use of insecticide, but other approaches such as biological control prove to be efficient. In the biocontrol strategy, parasitoid wasps are released in order to destroy the sharpshooters’ eggs before they hatch, and this can drastically reduce the sharpshooters’ population. This work is notably done at the Center for Invasive Species Research at UC Riverside, and they have a nice video explaining the topic:

A different type of biocontrol strategy is to inoculate grapevine with avirulent or weakly virulent strains of X. fastidiosa. These bacteria would cause only mild symptoms in the plants while protecting them from virulent strains that would kill the vine. This is done by the group of Donald Hopkins at the University of Florida.

A lot of efforts are on-going to better understand the biology of Xylella fastidiosa, and thereby finding new ways of controlling the bacteria. The genome of X. fastidiosa was sequenced in 2000, which gave clues about the bacterium’s pathogenicity factors. At present a tremendous amount of research is done on the molecular biology of X. fastidiosa, for instance in the groups of Bruce Kirkpatrick (UC Davis), Caroline Roper (UC Riverside) and Steven Lindow (UC Berkeley). (See for instance Voegel, 2012.)

Ultimately, the best would be to prevent the development of X. fastidiosa in the grapevine. The problem is that none of the grape varieties currently grown in California are resistant to Pierce’s disease. Some resistant varieties exist in southeast US, and plant breeding can be done to render the Californian grapes more resistant. This work is done by the group of Andrew Walker at UC Davis. The pitfall, though, is that for the wine industry it is important to maintain traditional cultivars (Cabernet Sauvignon, Pinot Noir, Chardonnay, etc.), because it ensures fruit quality and good marketing.

Because winemakers prefer their traditional varieties over new breeds, genetic engineering might be a viable alternative to conventional crossings. Currently, many different transgenic grapes are being tested for their ability to withstand Pierce’s disease. For instance, my neighbors-from-the-lab-next-door, in the group of Bruce Kirkpatrick, are developing a transgenic grapevine that produces a protein called hemagglutinin in its sap. This protein, which was isolated from X. fastidiosa itself, can act as a 'molecular glue', triggering the aggregation of the bacteria that invade the xylem and thus limiting their propagation in the plant. The testing of this transgenic grapevine is currently under way.

Another interesting approach is the one pioneered by Abhaya Dandekar, also at UC Davis. The group of Dandekar developed a chimeric protein to target X. fastidiosa in the sap. One part of this chimera is an enzyme that recognizes the bacterium's surface, while the other part is a lytic peptide that creates holes in its cell membrane; the synergistic effect of the two parts kills the Xylella bacteria!  This new type of chimeric proteins is full of promise, not only for Pierce's disease but for all sorts of infections in plants and animals. (Dandekar, 2012.)

Overall, this pluralistic research maximizes the chances to find effective treatments against Pierce's disease. The disease is at present kept at bay, but it is thanks to constant efforts of vineyards management and to the early eradication of infection hotbeds. It will take more to kick Xylella fastidiosa out of the 'ten most-wanted' list, though, but researchers in California are working towards that goal!

More information and videos are available on the website of the PD-GWSS board and on Pierce's disease.org.

References:

• Hopkins D. L, and Purcell A. H. (2002). Xylella fastidiosa: Causes of Pierce's disease of grapevine and other emergent diseases. Plant Disease Vol. 86, pp. 1056-1066.
• Galvez, L.C., Korus, K., Fernandez, J., Behn, J.L., and Banjara, N. (2010). The Threat of Pierce’s Disease to Midwest Wine and Table Grapes. APSnet Features. doi:10.1094/APSnetFeature-2010-1015. 
• Mansfield J. et al. (2012). Top 10 plant pathogenic bacteria in molecular plant pathology. Molecular Plant Pathology Vol. 13, pp. 614-629.
• Voegel T. M. et al. (2012). Identification of a response regulator involved in surface attachment, cell-cell aggregation, exopolysaccharide production and virulence in the plant pathogen Xylella fastidiosa. Molecular Plant Pathology DOI: 10.1111/mpp.12004.
• Dandekar A. M., et al. (2012).  An engineered innate immune defense protects grapevines from Pierce disease. PNAS Vol. 109, pp. 3721-3725.
• Pierce's disease Research Progress Reports (2012). Pierce's Disease Control Program, CDFA.