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METALSYM Report Summary

Project ID: 335284
Funded under: FP7-IDEAS-ERC
Country: Spain

Mid-Term Report Summary - METALSYM (Metal transport in the tripartite symbiosis arbuscular mycorrhizal fungi-legume-rhizobia)

Plant nutrition is essential to understand any physiological process in plant biology, as well as to improve crops and agricultural practices. The root microbiome plays an important role in plant nutrition. The best characterized microbiome elements are two plant endosymbionts: arbuscular mycorrhizal fungi (AMF) and rhizobia. AMF are responsible for delivering most of the mineral nutrients required by the host plant. Similarly, rhizobia in legume nodules provide the vast majority of the nitrogen requirements. Given their importance for plant nutrition a significant effort in understanding macronutrient exchange among the symbionts has been made. However, very little is known about metal micronutrient exchange. This is in contrast to the role of metals as essential nutrients for life (30-50 % of the proteins are metalloproteins) and to the yield-limiting effect that low soil metal bioavailability has worldwide. AMF are a source of metals, transferring the incorporated metals to the host. Nitrogen-fixing rhizobia in mature nodules act as metal sinks, since the main enzymes required are highly expressed metalloproteins. We hypothesize that by changing the expression levels of the metal transporters involved, we will increase nitrogen fixation rates and increase plant metal uptake, resulting in higher crop production and fruit metal biofortification. Towards this goal, we will answer: i) How are metals incorporated from the AMF into the plant?, ii) How are metals delivered to the nodule?, iii) How are metals recovered from senescent nodules?, and iv) How does the natural variation of symbiotic-specific metal transporters affect yields and metal content of the fruit? In this project, we will use a multidisciplinary approach that involves metallotranscriptomics, plant physiology and molecular biology, and state-of-the-art synchrotron based X-ray fluorescence to study metal distributions.
In the first half of this project, we have identified several metal transporters located at the interfaces where metals are exchanged between model AMF Rhizophagus irregularis and model legume Medicago truncatula, as well as in the region of M. truncatula nodules where metals are released from the vasculature to feed nitrogen-fixing endosymbiotic rhizobia. Reducing the expression levels of the genes encoding these transporters results in a severe reduction of the metals exchanged with the endosymbionts and consequently a loss of the metabolic processes associated (for instance, nitrogen-fixing capabilities). In addition, our results indicate that metal recycling from senescent nodules is an unspecific process, since very few genes involved in metal transport are induced in the nodule senescent zone. The encoded proteins are located in the vasculature, as it would be expected by their role in delivering metals from the senescent zones to the shoot. During this period, we have also studied how nitrogen fixation capabilities and iron content varies in 100 different M. truncatula ecotypes under low-iron conditions. Transcriptomic analyses of the top nitrogen-fixer under low-iron conditions, show significant changes in the expression levels in genes involved in metal delivery to the nodules. Promoter region and putative transcription factors of these genes in the ecotypes is currently being studied.
The results obtained during this period have been published or submitted for publication, as well as presented in national and international conferences in the field.

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