Disease detectives explore nutrient pathways

 

 

 

Recently published research ‘highly accessed’ The collaborative research of Naidu Rayapati, James Harbertson and colleagues recently has been published in the journal BMC Plant Biology, where it is marked “highly accessed,” indicating broad interest due to the number of times the article has been downloaded.   “We are the first group to address this kind of research,” Rayapati said. “We are approaching the disease at a molecular level to dissect the cascade of events occurring at different levels in the grapevine.”   Rayapati and Harbertson’s collaborators include Linga Gutha, a post-doctoral research associate in Rayapati’s lab who conducted studies on gene expression and real-time PCR (polymerase chain reaction), and Luis Casassa, a graduate student in Harbertson’s lab who conducted HPLC (high-performance liquid chromatography) analysis of anthocyanins and other compounds identified in the study.

Colorful leaves are among the delights of fall – unless you’re growing wine grapes. In grapes, it’s the fruit, not the leaves, that is supposed to change color.

 

In some varieties, leaf color change is a symptom of grapevine leafroll disease. WSU plant pathologist Naidu Rayapati and his colleagues are researching the intricate biochemistry and molecular biology of the disease.

 

Leafroll is a complex viral disease that can cause a marked decline in grapevine vigor, grape quality and fruit productivity. It can reduce yields 50 percent or more, depending on the severity of infection, and it accounts for about 60 percent of grape production losses worldwide, Rayapati said.

 

A few years ago, it was estimated that nearly 10 percent of Washington’s vineyards had grapevine leafroll disease. Anecdotal evidence suggests it is more widespread, raising alarm among industry stakeholders.

 

Colorful conundrum
One of the most perplexing questions Rayapati is addressing is one of symptomology: Why do some grape varieties show the “fall colors” symptoms in their leaves while others do not?

 

The reds and oranges in leaves we see in fall are due to a fascinating group of natural chemical compounds called anthocyanins. Anthocyanins are valuable to plants, giving fruit its red, blue and purple colors. These provide protection against UV radiation while attracting insects that pollinate and animals that disperse seeds.

 

Grape berries change color during véraison, or the onset of ripening, with red varieties deepening to their characteristic purple. Other complex changes, not visible to the eye, result in an accumulation of sugars and other beneficial compounds prized by winemakers and table-grape eaters.

 

When vines are infected with leafroll disease, however, fruit ripens unevenly while, at least in the red grape varieties, the leaves turn color after véraison. This is puzzling because, in fact, the viruses associated with grapevine leafroll can be detected in diseased vines throughout the season.

 

Nutrient transport affected
“There must be something going on in the bioregulation of anthocyanin pathways,” Rayapati said.

 

In living organisms, things rarely occur in straightforward ways. Instead, growth, ripening and other functions occur through complex networks of interactions called bioregulatory pathways. If there is a blockage or perturbation anywhere along that chain, as with a disease, there are often unexpected and undesirable results.

 

“What we suspect,” Rayapati hypothesized, “is that the virus is present in the phloem so that the translocation of nutrients may be affected.” The phloem is akin to the circulatory system in an animal in as much as it transports or translocates nutrients. The leaves of a grape plant should, under disease-free conditions, metabolize sugars and carbohydrates and transfer these nutrients through the phloem to the berries.

 

But that’s not happening. Instead, infected plants produce berries with less color and fewer sugars. It appears that anthocyanins are not being synthesized in berry skins. And that’s not only a mystery to scientists, it’s a problem for growers.

 

James Harbertson, a wine chemist in the WSU School of Food Sciences and colleague of Rayapati, also is working to shed light on the mystery of anthocyanin bioregulation in various grape cultivars.

 

“We have the tools enabling us to very sensitively measure which compounds are being accumulated where in the plant,” Harbertson said. “And what we see is that, indeed, specific anthocyanin compounds are accumulating in the symptomatic leaves of Merlot grapevines.”

 

Practical application
The value of their collaborative research, Rayapati said, “is that we are now able to use this information to establish a baseline in order to measure symptom expression in other cultivars. Using these tools, we should be able to better understand the differences in impact the leafroll viruses have on various cultivars.”

 

Understanding the symptomology of grapevine leafroll disease at the molecular level is critical if growers want to correctly identify the cause of changes in leaf color.

 

“From a practical point of view, this kind of research helps us to differentiate disease symptoms from nutrient disorders, which also can cause color change and reddening, and make appropriate recommendations to growers,” Rayapati said.

 

The work also has great value because, in hunting down the pathways of pathogens, researchers also expose the molecular regulation of beneficial compounds prized by winemakers. It’s a bit like taking apart a watch to remove a speck of dust and learning how a complex set of gears interact to tell time.

 

Work continues
Exactly how grapevine leafroll disease affects the anthocyanin pathway remains a mystery and is the subject of ongoing research.

 

As Rayapati pointed out, the disease is caused by a complex of viruses. If identifying the pathology of a single virus is a bit like hunting down a needle in a haystack, then Rayapati and his colleagues are searching for multiple needles – hidden in piles of fall leaves.