Between the Vines


Mary, Mary — how does your vineyard grow?


We’re gaining a better understanding of how genetics determine the way a plant’s various components will develop. By Gary Strachan


I


t's almost impossible to list all the variations that are present in cultivars of the single species


Vitis vinifera. With familiar varieties, many of us can quickly tell the difference between, say, a Pinot blanc and a Chardonnay, or between a Merlot and a Cabernet sauvignon. These are visible, heritable differences and because all commercial vineyards are created from cloned vines, we can expect to observe the same differences in all vineyards of the same varieties. Other traits must be observed during growth, such as mildew resistance. We have found by trial and error that there are horticultural practices which enhance mildew resistance, delay budbreak, or enhance the development of varietal character.


We are now on the threshold of being able to understand how grape vines turn these mechanisms on or off. In future we should be able to revise vineyard management to optimize performance as never before.


All cells of a plant contain DNA, which codes for any cell function the plant can express. A tendril contains the same genes as a leaf, but only the genes to create a tendril are expressed in the tendril. At an appropriate stage in shoot


British Columbia FRUIT GROWER • Fall 2014 19


Waddington’s Epigenetic Landscape, a clever model to illustrate the differentiation of cells, and whether they can revert to a less specialized form. These figures represent the indirect relationship of genotype to phenotype. An organism, represented by the ball, moves through development over a landscape with valleys representing various possible specialized tissues. The shape of this landscape is determined by an individual's overall genotype, which may have dramatic effects on the relative likelihood of different end points. The diagrams represent two different pathways within the same plant, such as cells which become specialized as, for example, leaf cells or tendril cells.


development, a single cell silences all the genes for anything except tendril growth, and all the daughter cells of that modified cell become tendril cells.


Even more interesting is that the same modification of gene expression creates the initiation of fruit clusters instead of tendrils, for the first two or three tendril locations at the base of a fruiting cane.


By some mechanism, the vine creates grape clusters early in the season and then switches to creating tendrils when the cane gets longer. This offers the possibility that if the silencing mechanism for fruit cluster initiation were suppressed, grapes


would bear fruit along the whole length of a cane instead of tendrils. These asexual (somatic) changes become heritable without changing the fundamental DNA coding of the cells.


Asexual cell division takes place by the familiar mechanism of mitosis. Occasional mutations occur but most growth is true to the exact expression of genes from the parent cell. On the other hand, when a seed is formed, the cells are created by the union of male and female DNA, each of which is a single strand of DNA created by a meiotic cell division. All the modifications that have occurred during somatic growth are wiped out.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24