Developing Fitter Cultivars

Diverse pathogens have an adverse affect on commercial grapevine yield impacting fruit and wine quality. European evolved vitis vinifera lacks genetic resistance to North American evolved pathogens. In order to achieve stable yields and good quality fruit, grapevines are farmed with the intensive application of fungicides during the growing season. Fungicide application has negative consequences on the environment, is associated with health hazards for field workers and consumers of grapes and wine.

The approach of replacing conventional grapevine cultivars with pathogen resistant cultivars is the most sustainable alternative for farmers to pursue, but doing so using conventional plant breeding techniques requires a substantial time commitment from growers (about 25-30 years per cultivar). Bringing a new cultivar to market can take decades, and is such a challenge that it tests the ingenuity, patience, and persistence of commercial plant breeders. Frequently the resulting disease resistant cultivars make mediocre wine making it difficult for growers to sell and creating negative incentives for farmers looking to achieve sustainability. Vegetative propagation is the simplest and most effective way of propagating grapevines, so the industry isn’t self-correcting (although it could).

Introgression

Wild grapevines co-evolved with pathogens in North America and developed desirable genetic resistance traits to those pathogens. In order to equip European varieties with equivalent genetic resistance to these pathogens, crossbreeding is used to transfer resistant genes from wild grapevine relatives to winemaking cultivars. Introgression involves crossing a wild vine with a desirable resistance trait with a winemaking cultivar. The recipient plant undergoes several backcrosses to the winemaking cultivar along with recurrent selection of the desired resistance trait to get the winemaking quality as close as possible to the original. For a woody crop like grapevine with a lengthy juvenile period, this approach is labor and time-intensive without the certainty of a desirable outcome. From the first cross until the release of a new grapevine variety, on average 25-30 years pass for a successful outcome.

The reason why self-fertilized grapevines rarely breed true.

Every seedling generated by cross breeding represents a new unique variety. For this reason, evaluating fundamental traits for prospective backcrosses are largely determined by the intended use of the new cultivar. The most common trait assessed to retain in progeny is the trait being bred for, resistance to major pathogens. Then, in corresponding order of the plant breeders priorities, retention of hermaphroditism for self-pollination to improve yield, morphological traits such as cluster architecture, shoot growth and axillary formation are also highly prized. Assuming several prospective candidates are produced from this sorting process, the selected plants are vegetatively propagated and the evaluation of wine quality determinants (aromatic compounds, secondary metabolites etc.) is carried out. Finally, during the last step before the new variety release, the most promising breeding lines undergo several trials to test the agronomic performance in different locations and different environments. This provides the breeders an opportunity to investigate the interactive relationships between genotype and environment.

Due to the heterozygosity of a lot of genes in the grapevine genome, each backcross (even with the exact same plant through self-fertilization) can scramble the genetics of the the resulting progeny. In fact it is so difficult to get back to the original cultivar that most breeders stop well short of that, resulting in mediocre wine making quality but retaining disease resistance. For a simple dihybrid cross, offspring phenotypes will conform to a Mendelian ratio of 9:3:3:1. If you are looking for a doubly recessive outcome only one in sixteen offspring will have that. Plant breeders have to sort through thousands of offspring to select for promising candidates. Domesticated grapevines have more than 30K genes, and highly heterozygous genomes.

New Breeding Technologies

It is becoming easier to sequence grapevine genomes and to analyze the impact of genes via cross breeding and comparative analysis to select for traits using Mendelian genetic analysis and developing and isolating quantitative trait loci to establish where in the genome a desirable trait is located. Analyzing traits is reasonably straightforward when they involve a single gene with a simple dominance/recessive pattern.

History of grapevine gene sequencing

With tools like CRISPR it is possible to precisely repair or replace only the genes/traits you want to impact (and leaving everything else the same) by introducing, for example, disease resistance traits into European wine making grapevines from genes that originally developed in North American grapevines. The scientific community has made tremendous strides in the last couple of decades understanding plant genomics. Some genes conferring traits are well understood.

A decade from now grapevine nurseries using crude techniques like root grafting European cultivars onto North American rootstocks will be a thing of the past. Farmers will no longer be spraying fungicide and pesticides during the growing season onto their vines. Table and wine grapes will be safer for workers to farm, and healthier for grape consumers to ingest. The entire viticulture industry will be more environmentally sustainable and provide a better habitat for animals, insects  and other soil microorganisms that live in vineyards worldwide.