Can Water-Saving Traits Reduce Grapevine Irrigation Demand Without Compromising Yield and Quality? AVF project #2024-2811.

Summary of Progress:

Climate change is expected to increase irrigation demand at the same time as reducing irrigation supply, and developing plant material with lower irrigation needs is crucial to adapt the grape industry to future conditions.  Previous work suggests that altering stomatal traits to reduce maximum gas exchange rates or reduce gas exchange more strongly in response to water stress would increase grapevine water-use efficiency without compromising the carbon supply for growth and ripening.  The goal of our project is to use genetic engineering to generate plant material with water-saving stomatal traits, while leaving the rest of the genome intact, to experimentally test the effects of these traits on yield and ripening in economically important grape cultivars.  We have targeted four genes with known effects on stomatal traits and gas exchange for overexpression (OST1 and CLE25) or underexpression (knock-out mutations) (EPFL9 and EVE).  We are conducing these transformations in Thompson Seedless, as a table grape with a high success rate for genetic transformation, and Chardonnay, as the most-produced wine grape in California, including in hot, water-limited regions.  We have completed the bioinformatics work needed for the plant transformations and created plasmids with the target mutations, which we supplied to the UC Davis Plant Transformation facility to create the transformed plant material.  The facility delivered our first set of transformed plants, Thompson Seedless, with an overexpression of OST1, in December 2025.  OST1 regulates stomatal sensitivity to abscisic acid, a hormone produced under water stress, so the overexpression mutants are expected to conserve water by closing the stomata more rapidly under drought.  We are currently propagating new plants to generate plant material for future experiments.  The facility expects the rest of the transformed plants to be delivered by the end of 2026, when we will apply for new funding to test the impacts of the mutations on drought resistance and water-use efficiency.

Developing plant material that is less dependent on irrigation is critical to adapt the grape industry to climate change.  Previous work has suggested that selecting for stomatal traits that reduce gas exchange would conserve water without compromising the carbon supply for growth and ripening.  This project is using genetic engineering to generate plant material with new, more water-saving stomatal traits to experimentally test the impacts on yield and ripening in economically important cultivars.  We generated plasmids for two overexpression mutants (for genes OST1 and CLE25, which we expect to increase stomatal sensitivity to water stress) and two knock-out mutants (for genes EPFL9 and EVE, which we expect to reduce maximum transpiration rates).  We supplied the plasmids to the UC Davis Plant Transformation Facility to generate the plant material for further experiments.  We received the first set of plant material, Thompson Seedless mutants overexpressing OST1, in December 2025.  OST1 regulates stomatal sensitivity to abscisic acid, a hormone produced under water stress, so the mutants should close the stomata more rapidly in response to water stress, conserving water during drought.  We are currently propagating plants to use in drought experiments testing this hypothesis.  Overall, we expect this work to show whether selecting for water-saving stomatal traits in existing cultivars is an effective and worthwhile investment for the grape industry.