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As almost everyone in the wine business knows, Botrytis cinerea is a fungal pathogen that attacks many plants, including grapes, which can cause significant crop and quality losses and ultimately economic losses for wineries and vineyards. Physiologically, B. cinerea excretes enzymes onto the plant leaves and fruit which results in the degradation of the plant cell walls and ultimately infections and necrotic lesions. Under most circumstances, grapes infected with B. cinerea are not fit to produce wine, as they would possess significant off aromas and flavors that are undesirable for the majority of wine consumers.
When faced with continuous damp or humid environmental conditions, Botrytis cinerea infections can wreak havoc on vineyards, resulting in what’s commonly referred to as “grey rot”. However, under drier conditions, B. cinerea is sometimes desirable, as Botrytis infections of dried or raisinated grapes results in “noble rot”, or in other words a high quality concentrated sweet wine. Under most circumstances around the world, Botrytis cinerea is highly undesirable and steps to avoid widespread contamination are frequently practiced.
Studies on other plants have shown that examining the metabolic changes of theplant under stressful conditions such as a fungal infection may provide important information on how plants protect themselves and recover from disease, as well as how this stress affects the entire plant and not just the sites of infection. These types of studies are not very common in the grapevine, and grapevine metabolic changes induced by Botrytis cinerea infections in particular have not been studied much at all. Thus, the goal of the study presented today was to examine and determine how Botrytis cinerea infections influence metabolic changes in the grapevine, which could have important implications for Botrytis cinerea management as well as early detection in the field.
Five bunches of healthy Chardonnay grapes as well as five bunches of botrytised Chardonnay grapes were randomly selected and harvested from a vineyard in Champagne, France in 2010. Skins were manually separated from the pulp for all treatments. Treatments included: 1) the skins and pulp from healthy grapes on healthy bunches; 2) skins and pulp from healthy grapes on botrytised bunches; and 3) skins and pulp from botrytised grapes on botrytised bunches.
Metabolites were then extracted and measured from each of the samples using Nuclear Magnetic Resonance (NMR) methods.
• Using NMR, skin and pulp extracts were found to be significantly different in regards to their metabolite content between healthy grapes, healthy grapes from botrytised bunches, and botrytised grapes.
• D-Gluconic acid was only found in botrytised grape samples and was 2x higher in the skin than in the pulp.
• Valine, isoleucine, leucine, threonine, arginine, and proline were significantly increased and sucrose, caffeic acid, trans-coumaric acid, and quercetin/kaempferol-3-O-glucoside were decreased in the skins of healthy grapes from botrytised bunches compared with the skins of healthy grapes from healthy bunches.
• Alanine, glutamate, succinate, fructose, and glucose were significantly increased and sucrose was decreased in the skins of botrytised grapes compared with the skins of healthy grapes from healthy bunches.
• Caffeic acid, trans-coumaric acid, and quercetin/kaempferol-3-O-glucoside were not found in botrytised grape skins.
• Glycerol was significantly increased in the skin of botrytised grapes.
• Valine, isoleucine, threonine, proline, glutamine, and glutamate were significantly increased in the pulp of healthy grapes from botrytised bunches compared with healthy grapes from healthy bunches.
• Succinate, arginine, and γ-aminobutyrate were significantly increased in the pulp of botrytised grapes compared with the pulp of healthy grapes from healthy bunches.
• Glycerol was significantly increased in the pulp of botrytised grapes.
Discussion and Conclusions
Overall, the results of this study indicate that Botrytis cinerea infections affect the metabolic profile of not only botrytised grapes, but also the healthy grapes that are located on a bunch with botrytised berries. They are not affected in the same manner, as then compared with health grapes from healthy bunches, the three populations are significantly different from one another in regards to their metabolite content, however, it is important to note that even healthy grapes on botrytised bunches have significant changes in their metabolite content that are not seen in healthy grapes from healthy bunches.
Many of the metabolites that were found to be increased in botrytised grapes as well as the healthy grapes from botrytised bunches are known to be involved in many plant defense mechanisms. Specifically, these metabolites respond to the infection by synthesizing into more complex compounds that subsequently react and combine with polyphenols in order to defend and fight against the infection. The metabolites involved with this type of defense mechanisms include (but are not limited to) arginine, glutamate, and alanine.
Other metabolites noted to be increased in the healthy grapes from botrytised bunches are known to be the basis for structural components of plant cell walls. It could be that in response to the Botrytis cinerea infection, the plant sent the still healthy grapes on the bunches increased levels of the amino acids required for cell wall synthesis in order to combat against the increased threat of cell wall degradation as a result of the fungal infection. The metabolites involved with cell wall synthesis that were found increased in healthy grapes from botrytised bunches were: proline, hydroxyproline, valine, threonine, and isoleucine.
Other metabolites were found to be either partially degraded or completely degraded in healthy grapes from botrytised grapes and botrytised grapes, respectively, indicating that Botrytis cinerea infections causes significant metabolic changes and activation of plant defense mechanisms even in berries that look healthy to the human eye.
It was also noted that glycerol and gluconic acid were only found in botrytised grapes. According to the authors, this indicates that Botrytis cinerea infections can trigger the synthesis and increase of certain defense metabolites which are not normally present in healthy grapes.
Overall, this was a very fascinating study that starts to answer some of the questions related to the physiological changes in the metabolite content of grapevines when under attack by Botrytis cinerea. The results indicate that it may be possible to develop a test for monitoring Botrytis cinerea in the vineyard, particularly in very early stages when it is not yet clear by the naked eye that the infection has taken place. Since the metabolite content of the grapes significantly changes in the healthy grapes of infected bunches, this study shows promise that this type of field test is feasible and that this early detection could help vineyard managers combat the infection before it spreads to other plants.
I’d like to see future work implementing this type of Botrytis cinerea early detection test in the field, perhaps by measuring those metabolites that were found to be increased in the healthy grapes from botrytised bunches. If these metabolites are found at a certain threshold (to be determined with more research), then it could indicate that more aggressive Botrytis vineyard defense actions should take place. Would this type of test be applicable to all grape varieties? Or are the results we saw in this study unique only to Chardonnay? Future tests should include multiple varieties of grapes to determine how universal these results are and how the early detection tests may need to be altered depending upon the variety in question.
I’d love to hear what you all think of this study and the implications therein. Please feel free to comment and discuss!
Source: Hong, Y.S., Martinez, A., Liger-Belair, G., Jeandet, P., Nuzillard, J.M., and Cilindre, C. 2012. Metabolomics reveals simultaneous influences of plant defence system and fungal growth in Botrytis cinerea-infected Vitis vinifera cv. Chardonnay berries. Journal of Experimental Botany 63 (16): 5773-5785.