For hundreds of years, the fungus Botrytis cinerea has been key to making the world’s finest dessert wines. New research shows how the fungus changes plant metabolism to produce new flavors and aromas in white-skinned grapes.
Under moderately moist conditions, the fungus produces the benign “noble rot,” ideal for making particular high-value dessert wines, known as “botrytized wines” or Sauternes. But when moist conditions prevail in the vineyard, Botrytis spawns noble rot’s evil twin, the so-called “bunch rot.” Bunch rot is a destructive disease that inflicts severe crop losses in all grape-growing regions around the world.
The researchers, working with Dolce Winery in Napa Valley, report that during noble rot infection, Botrytis induces metabolic processes in white grapes normally seen only during the ripening of red-skinned grapes. This was a novel observation, because white grapes are developmental mutants that cannot activate several ripening pathways such as the synthesis of anthocyanins, the molecules that impart the red color in the skin of red grapes.
The research also confirms that the reprogramming of grape metabolism by Botrytis results in the accumulation of key aroma and flavor compounds that make sweet wines made from botrytized grapes so special.
The work also may lead to new approaches to improving quality traits in grapes and other fruit, says plant biologist Dario Cantu of the University of California, Davis department of viticulture and enology. He led the study along with postdoctoral researcher Barbara Blanco-Ulate.
Winemakers have made use of the beneficial impacts of Botrytis cinerea—in the form of noble rot—since the 1500s. For example, white-skinned grapes, naturally infected with noble rot, have been used to produce some of the world’s highest quality dessert wines, referred to as botrytized wines. These wines were first produced in specific regions of Hungary, Germany, and France, but now are also made in Italy, Australia, New Zealand, South Africa, and California.
Botrytized wines are valued for their natural sweetness and distinctive flavor and aroma profiles, which include raisin, pineapple, apricot, pear, and honey features.
In the newly published study, the researchers collected a white-skinned grape variety called Sémillon over three years at the same time the grapes were being harvested for winemaking. They then used state-of-the-art RNA sequencing and metabolomics approaches, as well as enzymatic assays, on those grapes to demonstrate that noble rot:
- induces grape stress responses involved in generating aroma and flavor metabolites; and
- triggers regulators of metabolic pathways that are typically associated with red-skinned grape-ripening.
The researchers also profiled the metabolites of commercial botrytized wines produced from the same vineyard where the grapes were collected and verified that key compounds that result from noble rot carry over to the wines.
Findings from the new study appear in the journal Plant Physiology. Additional coauthors are from UC Davis; Washington State University; Dolce Winery; and CEBAS-CSIC research institute in Spain; and CONICET, the National Scientific and Technical Research Council of Argentina.
The UC Davis College of Agricultural and Environmental Sciences funded the work, and instrumentation support came from Agilent Technologies.
Source: UC Davis