Abstract
Nitrogen (N) as well as Phosphorus (P) are key nutrients determining crop productivity. Legumes have developed strategies to overcome nutrient limitation by, for example, forming a symbiotic relationship with N-fixing rhizobia and the release of P-mobilizing exudates and are thus able to grow without supply of N or P fertilizers. The legume-rhizobial symbiosis starts with root release of isoflavonoids that act as signaling molecules perceived by compatible bacteria. Subsequently, bacteria release nod factors, which induce signaling cascades allowing the formation of functional N-fixing nodules. We report here the identification and functional characterization of a plasma membrane-localized MATE-type transporter (LaMATE2) involved in the release of genistein from white lupin roots. The LaMATE2 expression in the root is upregulated under N deficiency as well as low phosphate availability, two nutritional deficiencies that induce the release of this isoflavonoid. LaMATE2 silencing reduced genistein efflux and even more the formation of symbiotic nodules, supporting the crucial role of LaMATE2 in isoflavonoid release and nodulation. Furthermore, silencing of LaMATE2 limited the P-solubilization activity of lupin root exudates. Transport assays in yeast vesicles demonstrated that LaMATE2 acts as a proton-driven isoflavonoid transporter.
Highlights
One of the major challenges of sustainable agriculture comprises the production of high-quality plant material with preservation of soil components and reduced application of chemical fertilizers without penalizing yield
Former work from our laboratories showed that flavonoids are released mainly from the so-called juvenile, and immature cluster roots and that genistein and its derivative exudation was induced by phosphate (P) deficiency (Weisskopf et al, 2006)
To identify a flavonoid exporter possibly playing an important role in the establishment of nodules in Fabaceae plants, we looked first whether a link between the expression of LaMATE2 and release of flavonoids exists
Summary
One of the major challenges of sustainable agriculture comprises the production of high-quality plant material with preservation of soil components and reduced application of chemical fertilizers without penalizing yield. Around 100 Mio years ago, certain angiosperms evolved a bias toward the evolution of nodulation with the so-called N-fixing soil bacteria Among those angiosperms are legumes (Fabales) and one non-legume genus, Parasponia (Cannabaceae, Rosales) which can establish mutualistic symbioses with Rhizobia, a polyphyletic group of proteobacteria and diverse group of plants belonging to the orders Fagales, Rosales, and Cucurbitales which can associate symbiotically with filamentous actinobacteria of the genus Frankia (Martin et al, 2017). By forming symbiotic associations, plants obtain mineral nutrients They supply the symbiont with organic compounds, sugars and lipids in the case of mycorrhiza, mostly carboxylates to N-fixing bacteria (Udvardi and Poole, 2013; Jiang et al, 2017; Luginbuehl et al, 2017). Despite all the research performed on this symbiotic interaction, a transporter releasing flavonoids into the rhizosphere and initiating the first step of this symbiosis awaits its identification
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