Sub-theme 3d - Improve rhizobia effectiveness and perseverance in hostile soils
Specific sub-theme contributors:
In Australia, two Mesorhizobium strains have been used as commercial inoculants for chickpea; the current strain CC1192 has been used since 1977 (Bullard et al., 2005). Despite inoculum consistency, strain CC1192 has been poorly represented in chickpea nodules (~53%) (Elias & Herridge, 2014). In addition, nodule surveys indicate nodule FW and effective N2-fixation is moderate or low in chickpea despite a very high adoption of inoculation (Elias & Herridge, 2014). This suggests poorly effective or adapted strains, or external factors that influence N2-fixation (soil nitrate). Chickpea mesorhizobia are also susceptible to abiotic stress including heat, drought, acidity and salinity (Elias & Herridge, 2014; Laranjo et al., 2014; Drew et al., 2012; Casteriano et al., 2013). How abiotic stresses impact Mesorhizobium strain diversity and effectiveness is poorly understood.
We will characterise the diversity and effectiveness of Australian chickpea mesorhizobial populations and their tolerance to hostile soils (both northern and southern regions) using phylogenetic analysis of core and accessory (symbiotic island) genes (Sullivan & Ronson, 1998). Data will be analysed to determine the contribution of chickpea genotype and environment to mesorhizobial diversity. We will also examine the long-term stability and rate of evolution of commercial inoculant strains (nodulation, N2-fixation) across different soils and abiotic stress treatments (desiccation, drought, heat and salinity). Mesorhizobium transcriptomes will be analysed using RNA sequencing and subsequent mutagenesis to identify molecular determinants of effectiveness (both as free living and when in symbiosis) and their physiological tolerance to stress. These studies will be analysed in conjunction with those in Theme 3b. In addition, we will study the response of rhizobia to chickpea root exudates, in particular flavonoid exudates that determine nod gene expression in rhizobia to optimise nodulation.
Understanding genotype x environment interactions on Mesorhizobium effectiveness and community structure in Australian soils to enable better estimation of the likely success of inoculation and strain stability over time. Molecular markers for effectiveness and abiotic stress tolerance in isolated mesorhizobia will assist future strain selection.