Grapes, mind the (cell wall) barricades against grey mould!

There is something rather romantic about vineyards. People have been growing grapes for wine-making since 7,000-6,000 BC in China and Armenia, but today, grapes are also globally important for table grape and raisin production as well as vinegar and grape seed oil. As different cultivars were selected for different purposes, grapes (Vitis vinifera) can be categorised as “wine grapes” and “table grapes”. One of the most striking differences between them is the size of their berries. 

Nowadays, growing grapes is challenged by a myriad of plant diseases and pests. To tackle these, vineyards need to be sprayed with pesticides and fungicides. Post-harvest diseases and handling also spoil many grapes. The most troublesome fungal disease is grey mould, caused by a microscopic fungus called Botrytis cinerea. You might have come across it yourself on ripe, table grapes. Or you might have enjoyed a dessert wine produced by “botrytised grapes” as this fungus can also lead to “noble rot” that dehydrates the grapes and increases the sugar content, giving the wine a sweet flavour. Whilst this fungus causes disease on both table and wine grapes, not much is known if and how the first barrier of plant defence, the cell wall, varies between different cultivars that are more or less susceptible to grey mould. 

Florent Weiller, a PhD student at the South African Grape and Wine Research Institute under the supervision of Dr John Moore and colleagues from Denmark, Germany, the UK and France, investigated cell wall changes in wine and table grapes in response to grey mould infection. There were only visible infection signs on ripe grapes, and overall, table grapes were more susceptible to the fungus. The more diseased the grapes were, the more they differed in their cell wall composition, especially in extensins, glucan and arabinogalactan protein patterns. 

The researchers selected two worldwide-grown wine grape cultivars (white Sauvignon Blanc and black Cabernet Sauvignon) and two table grape varieties (black Barlinka and white Dauphine) that are important in the South African grape industry. These four cultivars allowed a nice comparison between thick/thin/black/white grape skins, and Cabernet Sauvignon tends to have low susceptibility to grey mould. Grapes were harvested at three sugar ripeness stages (two pre-ripe stages and one fully ripe stage) from multiple vineyards. 

All the table grapes were sourced from the Hex River Valley. Grapes have been grown here for over 100 years, and the valley is the biggest producer of table grapes in South Africa. One of the wineries even has the longest harvest season in the world!

Next, grape berries from the four cultivars collected at the three ripeness stages were infected by a droplet of the disease-causing Botrytis cinerea microscopic fungi. The scientists tracked infection signs and cell wall changes every three days until twelve days post-infection. At each sampling time, Weiller and colleagues had a close look at the grape’s surface with microscopes and flash-froze the berries in liquid nitrogen, ground it up and isolated the cell wall components.

Nine monosaccharides (e.g., glucose, arabinose), different classes of polysaccharides (e.g., pectins, xyloglucans) and proteins (e.g., extensions, arabinogalactan proteins) were analysed. The infection was tracked closely with scanning electron microscopy and computerised tomography.  

The infections developed at different rates and in different ways on the four cultivars. The pre-ripe grapes showed very little fungal infection but 6-12 days after infecting ripe grapes developed clear infections. The fungi led to brown lesions on the white grapes whilst a droplet of juice, later overgrown by “white fluff” (fungal mycelium), was notably present on the black grapes. Next time you notice mould on your table grapes – you can make similar observations at home!

“This study is, to our knowledge, the first to profile the berry cell wall of different grape cultivars during B. cinerea infection,” Weiller and colleagues wrote. 

Overall, table grapes were more susceptible to B. cinerea than wine grapes. 

“Plausible reasons for this could be the waxy cuticle and the thicker skins present in the wine grapes, which act as physical barriers to infection.” 

“For Cabernet Sauvignon, the least susceptible cultivar from this study, the skin composition is most likely the key as Botrytis started to develop under the skin rather than at the grape surface as was observed with the other cultivars,” the researchers explained. 

The more diseased the grapes were, the more they differed in their cell wall composition, especially in extensin deposition and degradation, glucan deposition and arabinogalactan protein reorganisation patterns. 

This study neatly compared the disease development in morphologically different grapes at different ripeness stages and detected distinct cell wall changes during the infection. These findings open up many questions about how and where exactly cell wall changes occur in grapes. Working with grapes should come with some perks, and I hope the Weiller and colleagues raised a glass of wine or enjoyed some table grapes when they published their findings! 

Update 26 July 2021. The caption for the Hex River Valley photo was corrected from Australia to South Africa.