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17 th International Congress on Nitrogen Fixation Fremantle, Western Australia 27 November – 1 December 2011 Session Details: Wednesday 30 November 2011 Plenary Session 6 1330 - 1500 Author: Murray Unkovich Agriculture, Food and Wine, University of Adelaide, PMB 1, Glen Osmond, SA 5064 Presentation Title: Nitrogen fixation and nitrogen isotope fractionation in legumes Presentation Time: 1400 - 1430 Isotope fractionation occurs when there are changes in the partitioning of isotopes between a source and a product. For biological N2 fixation it equates to differences in the relative abundance of N isotopes between atmospheric N2 and fixed NH4 + in bacteria. The literature is somewhat confusing on this topic and a range of fractionation factors has been proposed. For N2 fixing legume symbioses, this fractionation becomes important when using natural variations in 15 N abundance to quantify N2 fixation where there is a need to be clear about the extent and source of fractionation. On a whole plant basis there is very little evidence for any isotope fractionation, and indeed if one considers the biochemistry and kinetics of the N2 fixation process one would not expect any fractionation. Evidence is presented that sources of variation in apparent isotope fractionation may have arisen from a number of factors including errors in measurement and processes other than N2 fixation contributing to apparent fractionation. For legumes, differences in the efficiency of symbiont bacteria and differences in plant age appear to influence shoot 15 N. It is important that these factors are teased out so that isotope fractionation can be reliably measured and better exploited in N cycle studies at plant and ecosystem scales. Careful attention needs to be paid to standardisation of mass spectrometric analysis, particularly since the adoption of continuous flow machines which have tended to divert focus to relative and precise rather than accurate measurement, such that measurement aganst atmospheric N2, the primary standard is no longer undertaken, even though it is readily available. More accurate rather than more precise measurement of 15 N will permit a better understanding of the N exchanges which occur between between legumes and their microsymbionts, which are still only partly understood, and also between ecosystem components. 80 2011
17 th International Congress on Nitrogen Fixation Fremantle, Western Australia 27 November – 1 December 2011 Session Details: Wednesday 30 November 2011 Plenary Session 6 1330 - 1500 Authors: Xi Chen, Theja Shidore, Theresa Dinse, Sabrina Gemmer, Thomas Hurek and Barbara Reinhold-Hurek Department of Microbe-Plant Interactions, University of Bremen, Postfach 33 04 40, 28334 Bremen. Presentation Title: Functional genomic analyses of endophyte – rice Interactions Presentation Time: 1430 -1500 Azoarcus sp. strain BH72, a mutualistic endophyte of rice and other grasses, is of agro-biotechnological interest because it supplies fixed nitrogen to its host and colonises plants in remarkably high numbers without eliciting disease symptoms. This raises the question of mechanisms of compatible interactions between host and bacterium. The complete genome of strain BH72 was sequenced (1), and the rice genome is also available. This allows application of functional genomic analyses of both partners during interaction. For symbiotic interactions such with rhizobia and arbuscular mykorrhiza (AM), common plant signalling cascades are now well characterized. The results of our analysis of differential gene expression in Azoarcus-infected rice roots and mutational analyses suggest that the cascades are not operating during endophytic interactions. Nevertheless, transcriptomic analysis demonstrated that both partners show extensive metabolic adaptations during endophytic interaction. Transcriptomic analyses were carried out for both partners, rice as well as exudates-exposed Azoarcus sp. They allowed identification of candidate genes which were shown by mutational analysis to be required for optimal rhizosphere fitness in Azoarcus sp. (1) Krause et al. 2006. Genomic insights into the lifestyle of the mutualistic, N2-fixing grass endophyte Azoarcus sp. strain BH72. Nature Biotechnol. 24: 1385-1391. 81 2011
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17 th <strong>International</strong> Congress on <strong>Nitrogen</strong> <strong>Fixation</strong><br />
Fremantle, Western Australia<br />
27 November – 1 December 2011<br />
Session Details: Wednesday 30 November 2011<br />
Plenary Session 6<br />
1330 - 1500<br />
Author: Murray Unkovich<br />
Agriculture, Food and Wine, University of Adelaide, PMB 1, Glen Osmond, SA 5064<br />
Presentation Title: <strong>Nitrogen</strong> fixation and nitrogen isotope fractionation in legumes<br />
Presentation Time: 1400 - 1430<br />
Isotope fractionation occurs when there are changes in the partitioning of isotopes between a source and a<br />
product. For biological N2 fixation it equates to differences in the relative abundance of N isotopes between<br />
atmospheric N2 and fixed NH4 + in bacteria. The literature is somewhat confusing on this topic and a range of<br />
fractionation factors has been proposed. For N2 fixing legume symbioses, this fractionation becomes important<br />
when using natural variations in 15 N abundance to quantify N2 fixation where there is a need to be clear about the<br />
extent and source of fractionation. On a whole plant basis there is very little evidence for any isotope<br />
fractionation, and indeed if one considers the biochemistry and kinetics of the N2 fixation process one would not<br />
expect any fractionation. Evidence is presented that sources of variation in apparent isotope fractionation may<br />
have arisen from a number of factors including errors in measurement and processes other than N2 fixation<br />
contributing to apparent fractionation. For legumes, differences in the efficiency of symbiont bacteria and<br />
differences in plant age appear to influence shoot 15 N. It is important that these factors are teased out so that<br />
isotope fractionation can be reliably measured and better exploited in N cycle studies at plant and ecosystem<br />
scales. Careful attention needs to be paid to standardisation of mass spectrometric analysis, particularly since<br />
the adoption of continuous flow machines which have tended to divert focus to relative and precise rather than<br />
accurate measurement, such that measurement aganst atmospheric N2, the primary standard is no longer<br />
undertaken, even though it is readily available. More accurate rather than more precise measurement of 15 N will<br />
permit a better understanding of the N exchanges which occur between between legumes and their<br />
microsymbionts, which are still only partly understood, and also between ecosystem components.<br />
80<br />
2011
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