Epsilon Open Archive

Abstract

Ectomycorrhizal associations of fungi with forest trees play an important role in the nutrition, growth and health of temperate and boreal forests. The ecology and physiology of ectomycorrhizal associations with Pinus spp are well documented but little is known about the molecular mechanisms behind these mutualistic interactions with gymnosperms compared to angiosperms. Through a complex and finely tuned molecular cross-talk between the two organisms, new lateral roots are initiated and new structures formed. The outcome is a symbiotic organ capable of exchanging nutrients essential for the survival of both partners. The main aim of this thesis was to identify the genes involved in the early stages of ectomycorrhizal development using Pinus sylvestris as a gymnosperm and Laccaria bicolor as the ectomycorrhizal fungus. Micro-array transcript profiling was used to ascertain the specificity in the response of conifer tissues to ectomycorrhizal fungi, using pathogenic (Heterobasidion annosum) and saprotrophic (Trichoderma aureoviride) fungi as non-mututalistic models. The profiles revealed differential responses of P. sylvestris roots to pathogenic, saprotrophic and ectomycorrhizal fungi within 24 h. We identified 236 differentially expressed genes potentially important for the initiation and regulation of biochemical, physiological and morphological changes during ectomycorrhizal establishment. The molecular regulation of ectomycorrhizal development appears largely comparable to that in angiosperm hosts, but differences exist in the timing and spatial scale of gene regulation. Of particular interest is the regulatory pattern of genes encoding stress related and also cell wall modification proteins whose down-regulation presumably enabled the development of the fungus within root tissues. Additionally, the expression and regulatory pattern of a subset of four symbiosis associated genes identified in the micro-array study was characterised in seedling roots; after exposure to exogenous auxin (indole-3-butyric acid) alone, L. bicolor alone, or L. bicolor in combination with the auxin transport inhibitor 2,3,5-triiodobenzoic acid. Knowledge of the genes regulated by auxin and involved in initiation of ectomycorrhizal symbiosis is scarce. The results identified a homolog to the receptor-like kinase Clavata1-like gene, two genes homologous to nodulin-21 and a gene of unknown function named Auxin-Induced1. This study is the first attempt to characterize the plant transcriptome in the Pinus sylvestris - Laccaria bicolor model system.