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Christmas Island, Australia, is a remote tropical island that relies predominantly on phosphate mining income. Given that mining operations will decline during the next decade, there is an urgent need to explore alternative industries to maintain local employment. This need could be met through activities such as a transition from mining to the production of high-value crops established in post-mined quarries. However, agriculture post-mining presents a series of challenges to growing crops, such as the lack of appropriate nutrients and organic matter, and the potential for soil compaction and the presence of heavy metals. Industrial hemp (Cannabis sativa L.) grows well in contaminated substrates, making it an ideal candidate for evaluation in the transition from mining to agriculture on Christmas Island. Hemp is a crop gaining increasing attention due to its wide range of commercial products and the relaxing of legal restrictions associated with its secondary metabolites. Preliminary experiments with hemp on Christmas Island identified a range of challenges to plant growth, such as the early onset of flowering, stunted growth, nutrient deficiency symptoms, low yield, and fluctuations in cannabinoid concentrations. Although some research has been carried out on hemp globally, there is limited information on optimal conditions for flowering, growth and nutrition, and cannabinoid production under tropical/subtropical environmental conditions. The current study examined a range of hemp varieties in terms of time to flowering, growth, biomass and seed yields, as well as cannabinoid production, in response to a series of abiotic stressors present under the tropical conditions of Christmas Island in postphosphate mined soil. Before undertaking these experiments, an improved method for extraction and detection of cannabinoids was developed. Following this, to investigate the challenges associated with growing hemp on Christmas Island, selected tropical/subtropical varietie

Legume growth, nutrient uptake, nodulation, and symbiotic nitrogen fixation can be hampered by soil acidity (pH CaCl2 < 5.0). Chickpea (Cicer arietinum L.) is sensitive to acidic soils, but the mechanism of inhibition and relative sensitivity of the symbiosis is not defined. Moreover, there is limited genetic diversity among chickpea for a range of traits including acid tolerance. This thesis explored whether wild Cicer species contained germplasm with greater acid soil tolerance than cultivated chickpea. A range of solution and soil experiments was carried out to investigate nutrient uptake, nodulation pattern and N2 fixation in acidic conditions, using two cultivated chickpea (Cicer arietinum) cultivars and two wild Cicer species (C. reticulatum (C. reti) and C. echinospermum (C. echi). Aluminium toxicity screening experiments with cultivated chickpea and Cicer genotypes showed that the haematoxylin stain of roots was unable to distinguish sensitive from more tolerant germplasm. In dilute nutrient solution at pH 4.2 and low Al concentrations (7.5, 15, 30 µM), C. echi contained higher concentrations in root and shoot of Al, P and S, but marginally lower Mg, K and Ca. By contrast, C. reti that maintained higher relative root length at 15 and 30 µM Al contained lower Al in roots but higher shoot Ca concentration. In solution culture, inoculated PBA HatTrick, PBA Striker and C. reti produced nodules at pH 4.2 but at pH 5.2 nodulation and N2 fixation increased. Genotype PBA Striker performed better in nodule formation, while both PBA Striker and C. reti contained higher shoot N concentration at pH 5.2 than pH 4.2. In an acid soil, Mesorhizobium inoculated plants formed nodules in both wild Cicer species at pH 5.6 while nodule formation was inhibited at pH 4.0 and 4.5. When soil and solution pH were > 5.0, PBA Striker and C. reti effectively forms symbiosis with Mesorhizobium while N-treated plants absorb higher shoot N concentration. This study has shown there is genot

Legumes are important components for both smallholder and commercial agriculture in Zimbabwe in relation to food and income security and improvement of soil fertility through a symbiotic association with rhizobia. The efficiency of biological nitrogen fixation is largely unknown in most situations in Zimbabwe. While rhizobia inoculant is available for many legumes, only soybean is consistently inoculated. Native soybean rhizobia have not been genetically characterized or taxonomically identified. The inoculation response of cowpea, groundnut, lablab, sunn hemp, pigeon pea and soybean was investigated under Zimbabwean field conditions, together with effects on a subsequent maize crop. Separately, soybean microsymbionts were obtained from soils with known inoculation histories from nine smallholder and three commercial farms to isolate naturalized inoculant strains and native rhizobia. Isolates were genetically characterized using partial recA gene sequences. Phylogenetic analysis of representative isolates was undertaken using recA, glnII, and 16S rDNA sequences. Symbiotic genes nifH and nodC were analysed. Isolates were screened for nitrogen fixation efficiency and the two best fixers per species were tested for compatibility with three soybean varieties under glasshouse conditions. The best isolate of each species was tested across different field sites in Zimbabwe. Inoculation generally increased grain yield, shoot biomass and nitrogen accumulation. Maize biomass was higher when succeeding inoculated legumes than when succeeding uninoculated legumes. Partial recA gene sequencing grouped the isolates into four species: Bradyrhizobium diazoefficiens (13%), B. japonicum (21%), B. elkanii (61%) and B. ottawaense (5%). B. ottawaense had the widest host range across 13 legumes, followed by B. elkanii, B. diazoefficiens and B. japonicum. Phylogenetic analyses were consistent with vertical transmission of core genes and horizontal transfer of symbiotic genes. Based on sym

Rhizobia are soil bacteria capable of forming N2-fixing symbioses with legumes, with highly effective strains often selected in agriculture as inoculants to maximize symbiotic N2 fixation. When rhizobia in the genus Mesorhizobium have been introduced with exotic legumes into farming systems, horizontal transfer of symbiosis Integrative and Conjugative Elements (ICEs) from the inoculant strain to soil bacteria has resulted in the evolution of ineffective N2-fixing rhizobia that are competitive for nodulation with the target legume. In Australia, Cicer arietinum (chickpea) has been inoculated since the 1970's with Mesorhizobium ciceri sv. ciceri CC1192, a highly effective strain from Israel. Although the full genome sequence of this organism is available, little is known about the mobility of its symbiosis genes and the diversity of cultivated C. arietinum-nodulating organisms. Here, we show the CC1192 genome harbors a 419-kb symbiosis ICE (ICEMcSym1192) and a 648-kb repABC-type plasmid pMC1192 carrying putative fix genes. We sequenced the genomes of 11 C. arietinum nodule isolates from a field site exclusively inoculated with CC1192 and showed they were diverse unrelated Mesorhizobium carrying ICEMcSym1192, indicating they had acquired the ICE by environmental transfer. No exconjugants harboured pMc1192 and the plasmid was not essential for N2 fixation in CC1192. Laboratory conjugation experiments confirmed ICEMcSym1192 is mobile, integrating site-specifically within the 3' end of one of the four ser-tRNA genes in the R7ANS recipient genome. Strikingly, all ICEMcSym1192 exconjugants were as efficient at fixing N2 with C. arietinum as CC1192, demonstrating ICE transfer does not necessarily yield ineffective microsymbionts as previously observed.Importance Symbiotic N2 fixation is a key component of sustainable agriculture and in many parts of the world legumes are inoculated with highly efficient strains of rhizobia to maximise fixed N2 inputs into farming systems. Sy

© 2019, Springer Nature B.V. The article entitled “Rethinking soil water repellency and its management”, which is part of the special issue on “Applying microbial communities to improve restoration and conservation outcomes” was published prematurely in Volume 220, Issue 10, October 2019.

Legumes play an integral role in increasing agricultural productivity, particularly in low input agricultural systems in Australia, due to their ability to form symbiotic interactions with a group of soil bacteria called rhizobia. However, in medium-to-low rainfall areas of southern Australia, there is a lack of suitable annual pasture legumes, which is limiting agricultural productivity and profitability in these farming systems. Scorpiurus muricatus is an annual legume from the Mediterranean which possesses high nutritive value and palatability for livestock, is high yielding, capable of self-seeding and is well-adapted to hot and dry summers. As such, S. muricatus is currently being evaluated as a new pasture legume for southern Australia. Crucial to the success of introducing this legume will be the availability of a highly effective rhizobial inoculant strain. This thesis therefore sought to characterise the phylogeny, free-living and symbiotic phenotype of a range of bacteria isolated from Scorpiurus spp. A total of 19 strains were investigated, with 16s rRNA sequencing demonstrating that 18 of these strains belonged to the genus Mesorhizobium, with the remaining strain (WSM1184) most closely related to Agrobacterium tumefaciens. Analysis of nifH and nodC symbiosis genes further showed that the characterised Mesorhizobium strains generally shared highly similar sequences for these loci, indicating a comparatively high degree of genetic similarity. In particular, WSM1343 (isolated from Scorpiurus sulcatus growing in Morocco) and WSM1386 (isolated from S. sulcatus in Manjimup, Western Australia) were shown to share highly similar symbiosis genes, but divergent 16S rRNA genes, suggesting the possibility that these strains may contain symbiosis genes on mobile Integrative and Conjugative Elements (ICEs). While the temperature tolerance and apparent optimum growth temperature of the test strains of 28°C was consistent with that commonly reported for Mesorhizobium s