Physiological and molecular analysis of tolerance to phosphate toxicity in jarrah (Eucalyptus marginata) seedlings inoculated with mycorrhizal fungi

[Truncated abstract] Jarrah (Eucalyptus marginata) is an Australian native tree with the capacity to form arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associations. ECM symbioses are the main mycorrhizal type for mature eucalypts while AM symbioses are thought to be transitory event early in eucalypt seedling growth. In previous research, where eucalypt seedlings of several species have been exposed to isolated strains of either AM or ECM fungi, plants in an ECM symbiosis had better P nutrition and growth responses than those in an AM symbiosis. However, little research has been carried out either exploring ECM and AM associations of jarrah in detail, or the effect on plant growth and nutrition, of a dual (AM and ECM) symbiosis. In addition, jarrah is among many Australian native plants that develop toxicity symptoms upon exposure to high doses of phosphate (Pi). Crucial knowledge gaps exist in understanding the molecular basis for P sensitivity of native species and how mycorrhizal symbioses function in the deficiency-toxicity continuum. However, unpublished earlier research has suggested a correlation between mycorrhizal status and tolerance to artificially high levels of Pi in soil. To better understand the role of both AM and ECM symbioses in the plant’s response to Pi and its analogues phosphite (Phi) and arsenate (AsV), research in controlled conditions has been undertaken. Herbaceous plants in an AM symbiosis are known to down-regulate the expression of some Pi transporter (class PHT1) genes in roots. Therefore, I hypothesize that first, ECM fungi are better symbiotic partners than AM in conferring growth and nutritional benefits to jarrah plants and second, mycorrhizal associations have the potential to induce tolerance against Pi, Phi and AsV toxicities, which could be linked with fungal ability to downregulate the expression of plant PHT1 genes. Therefore, this research was conducted to i) investigate AM, ECM and dual (AM & ECM) mycorrhizal associations of jarrah and their potential growth and P nutritional benefits, ii) clarify if mycorrhizal symbioses can induce tolerance to jarrah plants exposed to Pi, Phi and AsV toxicity conditions and iii) reveal any possible relationship between the expression of PHT1 genes in jarrah roots and either P sensitivity or any mycorrhiza-mediated tolerance to Pi, Phi and AsV toxicities. I developed a nurse-pot system to investigate mycorrhizal colonisation of jarrah by AM and ECM fungal species both separately and in dual AM+ECM culture (Kariman et al., 2012, Chapter 2). The nurse-pot system was effective at initiating colonisation of functioning AM (Scutellospora calospora) or putative ECM (Scleroderma sp.) systems. The presence of S. calospora, Austroboletus occidentalis and Scleroderma sp. individually significantly increased the shoot biomass of seedlings compared to nonmycorrhizal (NM) controls whereas no positive response was observed with Rhizophagus irregularis. In addition, the non-responsive AM fungus R. irregularis suppressed the ECM symbiosis in a dual pot culture also containing Scleroderma sp. inoculum. Also noteworthy, despite the positive growth and nutritional responses in the A. occidentalis treatment, ECM structures were not observed in either nurse or test seedlings...