Trichomes and unique gene expression confer insect herbivory resistance in Vitis labrusca grapevines

Grapevine (Vitis) is one of the world's most valuable fruit crops, but insect herbivory can decrease yields. Understanding insect herbivory resistance is critical to mitigating these losses. Vitis labrusca, a wild North American grapevine species, has been leveraged in breeding programs to generate hybrid grapevines with enhanced abiotic and biotic stress resistance, rendering it a valuable genetic resource for sustainable viticulture. This study assessed the resistance of V. labrusca acc. 'GREM4' and Vitis vinifera cv. 'PN40024' grapevines to Popillia japonica (Japanese beetle) herbivory and identified morphological and genetic adaptations underlying this putative resistance.

'GREM4' displayed greater resistance to beetle herbivory compared to 'PN40024' in both choice and no-choice herbivory assays spanning periods of 30 min to 19 h. 'GREM4' had significantly higher average leaf trichome densities than 'PN40024' and beetles preferred to feed on the side of leaves with fewer trichomes. When leaves from each species that specifically did not differ in trichome densities were fed on by beetles, significantly less leaf area was damaged in 'GREM4' (3.29mm²) compared to 'PN40024' (9.80mm²), suggesting additional factors beyond trichomes contributed to insect herbivory resistance in 'GREM4'. Comparative transcriptomic analyses revealed 'GREM4' exhibited greater constitutive (0 h) expression of defense response and secondary metabolite biosynthesis genes compared to 'PN40024', indicative of heightened constitutive defenses. Upon herbivory, 'GREM4' displayed a greater number of differentially expressed genes (690) compared to 'PN40024' (502), suggesting a broader response. Genes up-regulated in 'GREM4' were enriched in terpene biosynthesis, flavonoid biosynthesis, phytohormone signaling, and disease defense-related functions, likely contributing to heighted insect herbivory defense, while genes differentially expressed in 'PN40024' under herbivory were enriched in xyloglucan, cell wall formation, and calcium ion binding. The majority of genes implicated in insect herbivory defense were orthologs with specific expression patterns in 'GREM4' and 'PN40024', but some paralogous and genome-specific genes also likely contributed to conferring resistance.

Our findings suggest that 'GREM4' insect herbivory resistance was attributed to a combination of factors, including trichomes and unique constitutive and inducible expression of genes implicated in terpene, flavonoid, and phenylpropanoid biosynthesis, as well as pathogen defense.