Authors: Lu Yin, Avinash Karn, Lance Cadle-Davidson, Cheng Zou, Anna Underhill, Paul Atkins, Erin Treiber, Daniel Voytas, Matthew Clark
Frontiers in Plant Science 12 (2021):150 DOI: 10.3389/fpls.2021.587640
Summarized by Mattew Clark and Lu Yin.
Figure 1. The visual scale used to rate trichome density in the hybrid grape populations. Shown is an example on leaf stem.
The Takeaway
- Leaf hair density in grape varies by genetic background (species).
- Analysis confirmed genetic control for leaf hairs in maturing leaves on chromosome 1.
- Fine-mapping using a large population narrowed the region to several candidate genes in a diverse genetic background.
Background. Leaf and stem trichomes (hairs) are often diagnostic for ampelography, a term scientists use to denote the use of leaf characteristics to classify and identify grapevines. For example, Vitis labrusca and its derived cultivars are known for very dense leaf hairs, which can be a distinguishing feature. Research by Barba et al. (2019) also showed that the hairs on leaves can play a role in harboring beneficial mites that feed on fungal spores. Our research in leaf trichomes came from the observation during a budbreak experiment that a mapping population of 125 individuals (named GE1025) had variable amounts and density of leaf hairs. Lu Yin, PhD, a student at the time studying leaf phylloxera resistance, was curious to see if trichrome density played a role in resistance to foliar grape phylloxera that was also observed in this population. Our field observations of Elmer Swenson materials (V. labrusca based, dense trichomes), showed a specific resistance response to leaf phylloxera feeding. ‘Edelweiss’ has dense trichomes, and shows resistance to foliar phylloxera, while ‘Frontenac’, which has a limited density of leaf hairs, is highly susceptible to phylloxera and based in V. ripaira genetics.
We chose to study the leaves at a stage where they were expanding and transitioning to a mature leaf. In earlier leaf stages observations are difficult as the leaf is folded and often encased in short lived, transient trichomes on their surface or other trichomes from the bud.
Experiments. We made initial observations of trichome density on dormant cuttings of grape population GE1025 in the greenhouse after budbreak. This replicated experiment lent itself to additional observations for trichomes. Trichomes were scored on the oldest non-mature leaf of each individual in two years using a 0 to 6 scale (Figure 1) for ribbon and simple types using a dissecting microscope. A third experiment was conducted on leaf tissues collected directly from the field. Leaf images were digitized by placing representative leaves on a flatbed scanner so that multiple raters could evaluate the photographs for the trichomes on a computer at a later time.
Quantitative trait locus (QTL) mapping was conducted on GE1025, the DNA of which had been sequenced using genotype-by-sequencing during the VitisGen1 project, and the genetic regions associated with the variation in trichome density were identified. To further narrow down these regions, additional QTL mapping was done on a different population of 1000 individuals called GE1783 that has the same parents as the original population.
Results. Variation in trichome density was observed in GE1025. Regions on chromosome 1 and 10 were identified as important in controlling trichome density (Figure 2). This result confirmed that trichome density was controlled separately from phylloxera resistance, which was found on chromosome 14 using the same population (Clark et al. 2018). The region on chromosome 1 was fine mapped into a region containing 45 candidate genes using the larger population. Barba et al. (2019) also found a QTL for trichome density on chromosome 1, which is slightly different from our region. Through genetic sequencing and analysis of the parents at this region, variation at one of the candidate genes was found important in cucumber trichomes and the resulting variation in trichome density was responsive to plant hormones. Even though the initial intent was to observe the relationship between trichome density and phylloxera resistance, our results showed that there was a small yet significant negative correlation, meaning that when an individual has fewer trichome on the leaves, this vine tended to have a more phylloxera galls.
Figure 2. Regions on the grape genome responsible for variation in trichome density on different leaf positions.
Conclusions. Variation in leaf hair density was observed in a population that also segregated for phylloxera resistance. A region on chromosome 1 is responsible for the variation observed in trichome density, different from that of phylloxera resistance. However, a small negative correlation was observed between both traits which warrants further investigation. The region on chromosome 1 was fine mapped to a region with 45 candidate genes, including one target that has been shown to regulate trichome development in cucumber. Future research my focus on studying trichome development at an earlier stage as younger leaves are often the target for phylloxera feeding site establishment.
Matthew Clark is Asst. Professor in the Department of Horticultural Science at the University of Minnesota-Twin Cities, where he focuses on grape breeding and enology research.
Lu Yin is a PhD Candidate in the Dept. of Horticultural Science and is studying the phylloxera-grape foliar interaction.