Graham O'Hara

The symbiotic performance between legumes and rhizobia relies on the plant-bacteria genetic compatibility and on the symbiotic partner’s capacity to overcome environmental stresses. Symbiosis contributes nitrogen to the plants, which, among other things, increases the number of chloroplasts, and the number and size of cells per leaf. Hyperspectral imagery can detect vegetation changes combining information stored in the image. The symbiotic performance ¡s affected by some abiotic stress factors such as low clay content and low soil water holding capacity. These soil features can be estimated using ground penetrating radar (GPR), a geophysics instrument based on energy pulses interacting with soil layers. The aim of this work was to investigate whether integrated remote sensing techniques are able to estimate the interaction of field pea inoculated separately with five strains of Rhizobium leguminosarum bv. viceae with different nitrogen fixation effectiveness levels. The experiment was carried out firstly in a glasshouse to assess the pure symbiotic performance and then in an agricultural area to assess the interaction with abiotic factors. Hyperspectral images and GPR measurements were captured to cover the glasshouse and field site experiments. The plant sample analyses consisted of plant dry weight, nitrogen content and nodulation score. The plant samples showed significant differences in nitrogen levels, nodule score and dry weight across strains. The analyses of the spectral band combinations confirmed the presence of outstanding indices sensitive to the differential symbiotic performance and were correlated with plant analyses. The GPR data also revealed a mixed composition of soil properties associated with variable water availability that affected root and plant growth. It is concluded that remote sensing can be a valuable tool for estimating legume nitrogen fixation in fields, and GPR for estimating below ground properties that affect plant growth in field experiments.

Root nodule bacteria isolated from Zambian Lotononis angolensis (1) and southern USA Lupinus texensis (2) form a group that is distinct from other named and described legume root nodule bacteria. A phylogenetic tree based on the sequence of nearly full-length portions of the 16S rRNA gene indicates these isolates are affiliated to the α-proteobacterial genus Microvirga. Microvirga spp isolated from soil, air and thermal waters or hot springs have previously been formally described but none has been reported as capable of symbiotic nitrogen fixation. These isolates therefore represent a new lineage of nitrogen-fixing legume symbionts. We present here a polyphasic description of these novel species. A phylogenetic tree based on rpoB sequences supports the topography of the 16S rRNA tree in affiliating these isolates with Microvirga. Sequences of nifD and nifH are closely related in the L. angolensis and L. texensis strains, and there is no indication of horizontal gene transfer. In contrast, the nodA sequence of a L. angolensis strain grouped with Burkholderia tuberum and Methylobacterium nodulans nodA sequences, while that of a L. texensis strain was placed within a different group of rhizobia. The 16S rRNA phylogenetic tree indicates that L. texensis strains do not form a single lineage that is phylogenetically divergent from the African L. angolensis strains; rather, multiple lineages of these root nodule bacteria seem to be distributed across both geographic regions. DNA:DNA hybridization, fatty acid composition and % G+C data are consistent with these isolates forming several novel species. This study presents additional characterisation of these isolates, such as morphology, physiology, substrate utilisation, antibiotic resistance and legume host range that differentiates them from other Microvirga spp. These novel root nodule bacteria tolerate comparatively high temperatures and may have potential as inoculants in hot climates.

Root nodule bacteria isolated from native legumes in various biogeographical areas have demonstrated that rhizobia are more phylogenetically diverse than originally supposed. We present here an overview of our studies on novel species, isolated from nodules of legume hosts in Australia and Africa, which are affiliated to Burkholderia, Methylobacterium and Microvirga. The microsymbionts’ physiological adaptations to their environment, host specificity and phylogeny of nodulation and nitrogen fixation genes were examined, along with the modes of plant infection and nodule formation. Important findings include the apparent adaptation of Burkholderia spp. to infertile soils of wide pH range, the confirmation of specificity in the non-root hair-mediated Lotononis/Methylobacterium symbiosis and the potential of legume nodule morphology as a taxonomic aid. These species’ inclusion in the Genomic Encyclopedia for Bacteria and Archaea sequencing project will aid elucidation of the diverse rhizobial genomic architecture that underlies symbiotic ability and specificity.

Exopolysaccharides (EPS) represent an energy expensive composition of high molecular weight sugar polymers. Many physiological roles have been associated with EPS including pathogenesis, biofilm maturation, stress tolerance and symbiotic performance. Coincidentally, many of these functions are paralleled by the stringent response, a phase defined by a metabolic downshift in response to nutrient-limiting conditions. In many rhizobia, the synthesis of EPS and specific stationary phase regulators are required for the successful symbiosis with their legume host. Considerable research has examined the genetic and physical basis by which EPS is produced to initiate plant root infection, however, the physiological conditions that affect and coordinate these genes is not dearly understood. This work aimed to define the roles of EPS and the relationship to stationary phase in Sinorhizobiurn medicae whilst determining specific nutritional elements that affect the level of EPS synthesis.