Expression of the β‐glucosidase genePgβglu‐1underpins natural resistance of white spruce against spruce budworm

Abstract

Periodic outbreaks of spruce budworm (SBW) affect large areas of ecologically and economically important conifer forests in North America, causing tree mortality and reduced forest productivity. Host resistance against SBW has been linked to growth phenology and the chemical composition of foliage, but the underlying molecular mechanisms and population variation are largely unknown. Using a genomics approach, we discovered a β-glucosidase gene, Pgβglu-1, whose expression levels and function underpin natural resistance to SBW in mature white spruce (Picea glauca) trees. In phenotypically resistant trees, Pgβglu-1 transcripts were up to 1000 times more abundant than in non-resistant trees and were highly enriched in foliage. The encoded PgβGLU-1 enzyme catalysed the cleavage of acetophenone sugar conjugates to release the aglycons piceol and pungenol. These aglycons were previously shown to be active against SBW. Levels of Pgβglu-1 transcripts and biologically active acetophenone aglycons were substantially different between resistant and non-resistant trees over time, were positively correlated with each other and were highly variable in a natural white spruce population. These results suggest that expression of Pgβglu-1 and accumulation of acetophenone aglycons is a constitutive defence mechanism in white spruce. The progeny of resistant trees had higher Pgβglu-1 gene expression than non-resistant progeny, indicating that the trait is heritable. With reported increases in the intensity of SBW outbreaks, influenced by climate, variation of Pgβglu-1 transcript expression, PgβGLU-1 enzyme activity and acetophenone accumulation may serve as resistance markers to better predict impacts of SBW in both managed and wild spruce populations.

A Conifer UDP-Sugar Dependent Glycosyltransferase Contributes to Acetophenone Metabolism and Defense against Insects

“…The ready glycosylation of pungenol may indicate that the formation of acetophenone glycosides is important for rapidly sequestering the potentially biologically more active acetophenone aglycon. The aglycon appears only in resistant white spruce genotypes that express the PgbGLU1 gene, the product of which acts as the glycosidase that cleaves acetophenone glycosides (Mageroy et al, 2015).…”

“…The gray arrows represent proposed enzyme-catalyzed reactions that remain to be confirmed. The enzyme activity of PgbGLU1 was described previously by Mageroy et al (2015). B, PgUGT5b utilizes pungenol, quercetin, and kaempferol as substrates.…”

“…The formation of these metabolites did not appear to be inducible by herbivore attack but may be under developmental control (Mageroy et al, 2015). We hypothesized that candidate UGTs with a role in acetophenone metabolism could be identified if their expression was correlated with the time course of acetophenone glycoside accumulation.…”

“…In recent previous work on the acetophenones involved in white spruce resistance to SBW, we characterized the b-glycosidase PgbGLU-1, which releases the aglycons pungenol and piceol from their glycosides, pungenin and picein (Mageroy et al, 2015). Here, we describe the discovery and characterization of a counteracting acetophenone UGT that can produce the acetophenone glycoside pungenin.…”

“…Flushing shoots were collected once per week from undamaged trees with four replicates per time point over the time course of 7 weeks. In addition, foliage from five different white spruce genotypes, previously described by Mageroy et al (2015), was collected from a plantation in Saint-Cyrille-de-Wendover, Quebec, Canada (45°930N, 72°520W).…”