There are definite benefits to being a microorganism like powdery mildew. Among them are rapid reproduction, quick evolution, and being able to migrate easily via air currents or water droplets. Unfortunately for vineyard managers, these qualities also make powdery mildew ubiquitous and more likely to evolve immunity to fungicides. Fungicides, which have been successful in dealing with this disease for many years, are therefore becoming less useful. To circumvent this issue, disease-resistance genes have been identified and incorporated into grape breeding populations, but there is a caveat to this approach as well. When a grapevine only possesses one disease resistance gene, powdery mildew can still out-evolve that gene and keep infecting the plant.
A useful visual aid to understand how powdery mildew gets past a disease resistant gene is to think about Swiss cheese (bear with me!) If you were to hold up one slice of Swiss cheese, you would be able to pass a small object through any of its holes. However, if you were to overlap the first slice with a second, a few holes might line up, but most would be covered. It would be challenging to pass an object through, although it might be done with enough determination. If you were to add a third slice to the stack, it would become increasingly difficult. The more slices you stacked, the fewer holes would line up and the longer it would take to pass an object through them. In plant breeding, instead of stacking cheese slices, breeders stack genes in a process called “gene pyramiding”.
Gene pyramiding helps breeders to combine multiple desirable traits into one plant. In the case of breeding multiple disease resistance genes into a population of grapevines, this helps the vines retain their resistance to a disease over a longer time period. When multiple genes are creating more durable resistance, this resistance is said to be “stable”.
There are different factors at play in actually enacting that genetic resistance within the plant, and a wide variety of hormones and regulators that control how the plant responds to pressures in its environment. One part of the VitisGen3 project involves researchers looking at the incredible wealth of data about the various resistance genes found during VitisGen and VitisGen2 to see whether they have a regulatory mechanism in common. If they are able to find a “universal regulator” of disease resistance, it could help breeders develop a “SuperGrape”, or a grapevine with a superior ability to evade disease!