2015 Sigalas Assyrtiko

I'm waiting on a reference paper so I can finish up my odyssey regarding how the world came to get white wines. So, in the meantime, I thought I would blog about this 2015 Sigalas Assyrtiko that we drank a few nights ago.
Drinking this wine brings back some fond memories from 2004 of going to Sigalas located on Santorini. We went there in August and the weather was hot, hot, hot and dry. We came upon Sigalas Winery and stopped to relax, have a glass of wine and a bite of food. We had heard that the grape vines in Santorini were fashioned like a basket on the ground to protect the grapes from the heat and winds that are prevalent on Santorini. Sadly, Sigalas seemed to have given up that type of grape growing, in favor of the more conventional trellising.

The Assyrtiko grape is indigenous to Santorini and is an ancient variety that grows on the volcanic soils of the island. This wine was fruity, but it also had acidity and minerality to balance and round out the flavors. What's more, it brought back some wonderful memories spent in the company of good friends.

Genes Involved in Anthocyanin Synthesis

Have you ever wondered why we have red and white wines? I found a really good research paper that explained this phenomenon almost a year ago and then I fell into a rabbit hole. I needed to explain first, why we have red grapes, and it took me a few blogposts to get there.
In my previous blogpost Why Do We Have White Grape Varieties?, I wrote that white grapes appear to lack anthocyanins because they did not express UFGT (UDP glucose-flavonoid 3-o-glucosyl transferase). Then, I finished the post with a cliff hanger. In 1996, when Boss et al.,¹ did their seminal experiments they didn't really know the underlying genetic reasons why white grapes did not contain anthocyanins in their skins.
Ten years later in 2006, Lijavetzky and fellow collaborators answered the question in their research paper titled Molecular genetics of berry colour variation in table grape.²
The Role of Transcription Factors
To get a handle on what they did, requires some knowledge of transcription factors. The Wikipedia defines a transcription factor as a protein that controls the conversion of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence.(For a really good explanation on transcription factors check out Khan Academy on Transcription Factors.)
Two groups of transcription factors called MYB and bHLH are the main regulatory genes associated with the control of anthocyanin biosynthesis in plants. In 2014, Huang et al. mapped out that phylogenetic tree for the MYB transcription factors involved in the phenylpropanoid pathway.³

In the diagram shown above, the main character implicated in anthocyanin production in the grape berry skin is the gene called VvmybA1 located on chromosome 2.⁴ The details of the investigation into how the gene VvmybA1 is responsible for color will sadly, need to be postponed for another blogpost. Please stay tuned, it's pretty cool.
References:
1. Boss P, Davies C, Robinson S (1996b), "Expression of anthocyanin biosynthesis pathway genes in red and white grapes", Plant Mol Biol, 32:565 - 569.
2. Lijavetzky et al., Molecular genetics of berry colour variation in table grape, 2006, Mol. Genet. Genomics, 2006: 427-435.
3. Yung-Fen Huang, Sandrine Vialet, Jean-Luc Guiraud, Laurent Torregrosa, Yves Bertrand,Veronique Cheynier, Patrice This and Nancy Terrier, A negative MYB regulator of proanthocyanidin accumulation, identified through expression quantitative locus mapping in the grape berry, New Phytologist, (2014) 201: 795–809 doi: 10.1111/nph.12557.
4. A Fournier-Level, T Lacombe, L Le Cunff1, J-M Boursiquot1, and P This, Evolution of the VvMybA gene family, the major determinant of berry colour in cultivated grapevine (Vitis vinifera L.), Heredity (2010) 104, 351–362; 18 November 2009, doi:10.1038/hdy.2009.148.

Why Do We Have White Grape Varieties?

Is your favorite wine a Chardonnay, Riesling, or one of our favorites, Chenin Blanc? I did not know this until I took the U.C. Davis online course, that if it was not for a certain genetic phenomenon, the world would be full of only red grape varieties. I looked into this a little further and learned some interesting factoids.
First of all, the red color in grapes is attributable to the anthocyanins located in the grape berry skin.¹
Just as I did when blogging about red wine color, I'm going to break up this blog about white grapes into little pieces otherwise the blog will become too long and difficult to follow.
So, to recap: The production of anthocyanin, which is responsible for red color in wine is after the enzyme leucocyanidin dioxygenase or LDOX. In the figure below, the expression of mRNAs homologous to each of the anthocyanin biosynthesis genes was determined in the white varieties Riesling, Muscat Gordo, Semillon, Chardonnay, and Sultana as well as the red varieties Cabernet Sauvignon, Pinot Noir and Shiraz using northern blot analysis.¹

The northern blot experiment clearly showed that none of the white varieties expressed anthocyanins as shown by the lack of expression of UFGT (UDP glucose-flavonoid 3-o-glucosyl transferase. Furthermore, there was a difference in expression of genes in the white varieties between Riesling, Semillon and Chardonnay versus Muscat Gordo and Sultana.
When this paper was written in 1996, the authors speculated that:

• It is quite likely that the white grape has arisen independently a number of times from different red varieties
• The mutations could be different in each case so it is not surprising that patterns of gene expression were different in the various white varieties
• Loss of anthocyanin synthesis can arise by mutations in individual structural genes in the flavonoid pathway
• Loss of anthocyanin synthesis can arise by alterations of regulatory genes controlling expression of a number of the structural genes

In the world of scientific investigation, this is ending the experimental work in a mystery. Stay tuned for updated experiments that give more insight into why white grape varieties exist in our world today.
References:
1. Boss P, Davies C, Robinson S (1996b), "Expression of anthocyanin biosynthesis pathway genes in red and white grapes", Plant Mol Biol, 32:565 - 569.
2. Lijavetzky et al., 2006, Mol. Genet. Genomics, 2006: 427-435.
3. José Tomás Matus, Felipe Aquea and Patricio Arce-Johnson, BMC Plant Biology, 2008, 8:83.
4. Yung-Fen Huang, Sandrine Vialet, Jean-Luc Guiraud, Laurent Torregrosa, Yves Bertrand,Veronique Cheynier, Patrice This and Nancy Terrier, A negative MYB regulator of proanthocyanidin accumulation, identified through expression quantitative locus mapping in the grape berry, New Phytologist, (2014) 201: 795–809 doi: 10.1111/nph.12557.

Recap: Why Do We Have Red Wines?

About a year ago, I learned the answer to why we have white wines. (One answer: they come from white grape juice or from red grapes where there was no skin contact in the making of the wine.)
I found a very well written research paper and started to create a blog about it and the blog became a mile long. I decided to approach what seems to be an easy answer in a stepwise fashion so I took the opposite approach, writing first about how red wines get their color:
       Anthocyanins Give Red Wine Their Color
I couldn't stop at there because I wanted to explain how anthocyanins were produced and so I wrote the following blogposts about anthocyanin production beginning with the amino acid phenylalanine and proceeding through the Flavonoid Pathway:
       Why is Red Wine Colored Red
I then found another reference that looked at both the General Phenylpropanoid Pathway and the Flavonoid Pathway in more detail and felt compelled to share this:
       General Phenylpropanoid/flavonoid Pathway
Finally, I was able to blog about how Boss et al. determined that anthocyanin was produced only in the grape berry skin, based on some elegant Northern blot experiments:
       Anthocyanin Production in Grape Berry Skins
Stay tuned for the next blogpost which will use this information on red wine color to explain how the world came to have white grapes and white wine.
References:
1. Boss P, Davies C, Robinson S (1996b), "Expression of anthocyanin biosynthesis pathway genes in red and white grapes", Plant Mol Biol, 32:565 - 569.
2. Lijavetzky et al., 2006, Mol. Genet. Genomics, 2006: 427-435.
3. José Tomás Matus, Felipe Aquea and Patricio Arce-Johnson, BMC Plant Biology, 2008, 8:83.
4. Yung-Fen Huang, Sandrine Vialet, Jean-Luc Guiraud, Laurent Torregrosa, Yves Bertrand,Veronique Cheynier, Patrice This and Nancy Terrier, A negative MYB regulator of proanthocyanidin accumulation, identified through expression quantitative locus mapping in the grape berry, New Phytologist, (2014) 201: 795–809 doi: 10.1111/nph.12557.