Hysterangium mats and associated bacteria under eucalyptus gomphocephala in south-western Australia

Eucalyptus gomphocephala (tuart) is an ecologically and culturally important woodland and forest tree native to south-western Australia. Unfortunately, tuart has been markedly declining in recent decades, and the mortality rate is as high as 90% in some populations. The cause of tuart decline is still poorly understood and we know nothing about how the decline changes populations of organisms such as ectomycorrhizal (ECM) fungi and beneficial bacteria. Therefore, this study investigated aspects of mycorrhizal fungal mats under healthy tuart, including effects on forest soil properties, and presence of beneficial bacteria. A common mat morphotype (grey, hydrophobic) was chosen for this research and the ECM fungus was identified as a Hysterangium sp. based on fungal morphology, ECM structure and ITS gene region analysis. The effect of the ECM mats on soil moisture, nutrients, microbial biomass (ninhydrin-reactive N) and microbial community (Biolog Ecoplate) was investigated. Potential PGPR with ability to solubilize phosphate (hydroxylapatite-Ca5HO13P3), produce IAA and use ACC acid as the sole carbon source were screened. Hysterangium mats significantly improved soil moisture, soil pH, nutrient level and microbial biomass compared to non-mat soils, however, there was no evidence of any difference in microbial diversity and activity between mat and non-mat soils. Twenty eight IAA producing bacterial isolates, 7 phosphate solubilizing isolates and 2 ACC deaminase producing isolates were screened from mat and non-mat soils. Interactions among beneficial bacteria, ECM fungi and eucalypt seedlings were then investigated. ECM synthesis was also conducted between Hysterangium MURU6276 and E. gomphocephala seedlings in vitro. The plant growth promoting ability of bacteria to eucalypt seedlings was assessed through a bioassay using petri dishes containing MMN medium overlaid with cellophane. Half plate petri dishes were used to screen for mycorrhiza helper bacteria. ECM association between Hysterangium MURU6275 and E. gomphocephala seedlings was confirmed in vitro. Most bacterial isolates from under tuart had stimulating or inhibiting effects on the root growth of E. gomphocephala and/or E. grandis. The best 6 growth promoting isolates were T1M3, T1N2, T2N7, T3N4, T4N4 and T4N6. However, there was no evidence of the presence of mycorrhiza helper bacteria amongst the isolated bacteria. These findings provide the basis for further detailed research on biochemical processes and the diversity and functions of bacterial populations under Hysterangium mats associated with tuart. Areas for further study are suggested