2016_06 Tricks of Chinese Wild Grapes
What was the motivation?
Grapevine is a cash crop, but requires also a lot of effort. Especially protection against diseases such as Downy or Powdery Mildew accounts for some 70% of European fungicide consumption. Society feels progressively uneasy about this ecological price. New grape varieties, where during almost a century of breeding immunity factors from American wild grape had been introduced, represent so far the most efficient tool to reduce fungicide load. German viticulture is worldwide leading in this technology, but nature does not sleep - already new pathogen strains have been discovered that can break the resistance of these new varieties. New strategies to stimulate immunity are therefore urgently needed. For breeding "new strategies" means always: new genes. Powdery Mildew likes warm temperatures and therefore poses increasing problems to viticulture in the Mediterranean, but also overseas (Australia, China, California). But even in Germany we have to prepare, because global warming will make this disease more widespread. All genes that have been used so far to breed for resistance against Powdery Mildew originate from North American wild grapes that have coexisted over a long time with the pathogen and therefore developed efficient resistance factors. As mentioned before, these resistance factors are progressively undermined by novel strains. The idea to search for new resistance factors in other regions of the world may first appear strange - since Powdery Mildew (the same holds true for Downy Mildew) was not found outside North America, before Columbus discovered this continent, it is not to be expected that wild grapes in China or Europe should have been prepared to meet this fungus.
March separately, beat jointly - a SinoGerman success story
However, they do as astonishing this may sound. The immunity used in the resistant grape varieties, is targeted to only one type of disease (Powdery Mildew). However, plants harbour a general basal immunity that protects against many pathogens. This basal immunity has a price, though, since it requires energy that is diverted from growth. Since humans have selected over millenia for fast growth, large berries and high sugar content, this basal immunity has been partially lost in domesticated grapevine. The research team of Prof. Dr. Yuejin Wang from the Northwest Agicultural and Forestry University in Yangling near Xian has been working already for decades on Chinese wild grapes and discovered that specific types of the wild species Vitis pseudoreticulata can cope quite well with infections by Powdery Mildew. Basically same approach, our group has pursued for the European Wld Grape (more...). So, it was straightforward to cooperate on this topic. Funded by the Chinese Scholarship Council a Ph.D. from Prof. Wang's group, Yuntong Jiao, joint the KIT for one year to investigate a specific allele of the gene stilbene synthase that had been discovered in the Chinese wild V. pseudoreticulata.
What was the result?
Stilbene synthase is responsible for the formation of resveratrol and viniferin, efficient defence compounds against dangerous microbes that are also important in the European Wild Grape. The work started with a comparison between the stilbene synthase versions from Vitis pseudoreticulata and domesticated grapevine (Vitis vinifera). The protein itself was almost identical, but the gene promoter controlling the expression of stilbene synthase, was different: here, the wild grape harboured additional regions. While still in China, Yuntong Jiao could demonstrate that Arabidopsis plants where this wild stilbene synthase had been implanted into the genome, were able to successfully defend against a Powdery Mildew species normally able to infect Arabidopsis. The reason for this resistance conferred by the wild stilbene synthase was not clear, however. She therefore joined the KIT to make us of a test system that just had been established in our team. This test allows to measure, how a promoter of interest is switched on. For this purpose, the promoter is placed upstream for the coding sequence of firefly luciferase. Luciferase is the enzyme responsible for the glow of these insects. Each time, when the promoter is active, the cells will start glowing and this glow can be measured., Using this method, it could be shown that the stilbene synthase promoter of the Chinese wild grape is activated very strongly by the above mentioned basal immunity. A similar phenomenon we had seen for the promoter of the gene switch MYB14 from the European Wild Grape (more...). This gene switch binds to the stilbene synthase promoter and thus activates the formation of stilbenes.
The surprise came, when it turned out that salicylic acid (aspirine) was a very efficient trigger as well. This important plant hormone is normally only found in the specialised immunity used in the newly bread resistant varieties, but not linked with basal immunity. When we had worked on the MYB14 promoter in European Wild Grapes, we had also tested salicylic acid, but seen only a very weak reaction of MYB14. The situation in Vitis pseudoreticulata is therefore different - afterwards, we probed by specific inhibitors, how salicylic acid can activate the promoter and found that this runs over the usual steps of basal immunity.
What does this mean?
Obviously, the wild grapes from China and Europe use different strategies to boost their basal immunity - in the European Wild Grape, a very efficient version of the gene switch MYB14 is employed to activate stilbene synthase in the next step, whereas the Chinese wild V. pseudoreticulata recruits the signal salicylic acid (that normally plays a different role in defence) into basal defence using a particular promoter variant of stilbene synthase itself. The result of both strategies is the strong and rapid accumulation of resveratrol, which will stop the fungal attack.
Nature has therefore developed in China and Europe different pathways to reach the same goal. Each of these pathways offers to the breeder a new solution to control this pathogen. The lesson: we just have to observe carefully, try to understand the solutions developed by nature, and then make use of these solutions in a smart way. A second lesson: to exploit the creativity of nature, it is important that scientists from different continents exchange their ideas and materials.
127. Jiao Y, Xu W, Duan D, Wang Y, Nick P (2016) A stilbene synthase allele from a Chinese wild grapevine recruits salicylic acid signalling for efficient defence, J. Exp. Bot. 67, 5841-5856 - pdf