Understanding the Role of Secondary Metabolites and Endophytic Fungi in White Spruce Defenses Against Eastern Spruce Budworm
Creators: Ullah, Aziz
Publisher: name: University of Alberta Library
Year: 2023

<p>As a boreal tree species, white spruce (Picea glauca Moench Voss) is under a constant threat by outbreaks of defoliators (foliar feeders) including eastern spruce budworm (Choristoneura fumiferana). Historically, applications of biological insecticides and silvicultural treatments have been used to improve white spruce resistance against the eastern spruce budworm. More recently, variations in chemical defenses, such as monoterpenes, phenolics, and fungal endophytes of white spruce have received a particular attention due to their potential roles in herbivory resistance. However, how such variations in white spruce defenses affect insect herbivores is largely unknown in part due to limitations in replicating the complete defense profile of trees under controlled conditions. Furthermore, little is known about the role of fungal endophytes alone or in combination with defense compounds in tree resistance to insect herbivores.<br>
In the first study (Chapter II), I sampled the foliage of 80 white spruce families and clustered them into two chemotypes (chemotypic phenotypes) based on their foliar monoterpene concentrations. I investigated the role of phenolics and monoterpenes in white spruce defense by observing how budworms respond to media amended with these metabolites. I found that the higher proportions and concentrations of monoterpenes and phenolics in the white spruce foliage negatively affected the survival of eastern spruce budworms. Families that had greater proportions and concentrations of monoterpenes were more fatal to the budworm, while families that contained large proportions of phenolics and low monoterpene concentrations were less fatal. I concluded that both monoterpenes and phenolics play essential roles in the defenses of white spruce, with monoterpenes being responsible for the overall toxicity and phenolics inhibiting the growth of the budworms. This study shows that different classes of defense metabolites are being coordinated against the same insect herbivore.<br>
In the second study (Chapter III), I characterized the composition of fungal endophytes and terpenes (monoterpenes and sesquiterpenes) in the needles of 30 white spruce genotypes at two progeny locations. I found that the fungal community and terpene compositions varied between locations and among spruce genotypes, and those with higher sesquiterpene abundance also had higher fungal endophyte abundance. I also tested the effects of selected fungal morphotypes on the eastern spruce budworm performance and found that some fungi reduced the feeding and caused significant mortality to the budworms. Interestingly, fungal metabolites contained the same class of terpenes as those characterized in spruce trees including monoterpenes and sesquiterpenes. Overall, I concluded that foliar fungal endophytes play a pivotal role in enhancing white spruce defenses through the modification of host defense metabolites and providing direct anti-herbivore resistance. I suggested that the plant-associated fungal endophytes are crucial in the arms-race between host plants and herbaceous insects. <br>
In the third study (Chapter IV), I grew seedlings from 30 different white spruce families (20 trees/family) from seeds in a growth chamber. After ten months of vegetative growth, I inoculated five fungal endophytes into the needles of half of the seedlings in each family, Cladosporium halotolerans, Cladosporium cladosporioides, Chalara sp., Geopyxis sp., and Didymella sp. The remaining half of the seedlings were not inoculated and served as the control. After two months of inoculation, I harvested the seedlings and weighed the above ground fresh biomass from both inoculated and control plants. I found that the fungal inoculation was successful since the fungal endophytic communities were different between inoculated and control seedlings. Overall, spruce families with the higher endophytic fungal abundance also had higher monoterpene and sesquiterpene concentrations and needle biomass. All together, these results demonstrate that fungal endophytes can increase the concentrations of the defense metabolites of spruce trees along with improving growth.</p>