MtNIP5;1 , a novel Medicago truncatula boron diffusion facilitator induced under deficiency
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(2020) 20:552
RESEARCH ARTICLE
Open Access
MtNIP5;1, a novel Medicago truncatula boron diffusion facilitator induced under deficiency Sara Granado-Rodríguez, Luis Bolaños and Maria Reguera*
Abstract Background: Legumes comprise important crops that offer major agronomic benefits, including the capacity of establishing symbiosis with rhizobia, fixing atmospheric N2. It has been proven that legumes are particularly susceptible to boron (B) stress, which leads to important yield penalties. Boron (B) deficiency or toxicity in plants causes the inhibition of growth and an altered development. Under such conditions, the participation of two distinct protein families (the major intrinsic protein family MIP and the Boron transporter family BOR) is required to minimize detrimental effects caused by B stress. However, in legumes, little is known about the transport mechanisms responsible for B uptake and distribution, especially under deficiency. Results: A Medicago truncatula protein, MtNIP5;1 (Medtr1g097840) (homologous to the Arabidopsis thaliana AtNIP5; 1) was identified as a novel legume B transporter involved in B uptake under deficiency. Further analyses revealed that this M. truncatula aquaporin expression was boron-regulated in roots, being induced under deficiency and repressed under toxicity. It localizes at the plasma membrane of root epidermal cells and in nodules, where B plays pivotal roles in symbiosis. Furthermore, the partial complementation of the nip5;1–1 A. thaliana mutant phenotype under B deficiency supports a functional role of MtNIP5;1 as a B transporter in this legume model plant. Conclusions: The results here presented support a functional role of MtNIP5;1 in B uptake under deficiency and provides new insights into B transport mechanisms in legume species. Keywords: Boron deficiency, Medicago truncatula, Aquaporins, MtNIP5;1, Boron transport, Legumes
Background Boron (B) is an essential micronutrient for plants as it plays a structural role in plant cell walls, crosslinking two pectin polysaccharides rhamnogalacturonan-II [1, 2]. When B concentrations are below optimum, Bdeficiency symptoms appear and include the inhibition of root elongation and a reduced leaf expansion and fertility, which results in substantial agronomical losses around the world [3–5].
* Correspondence: [email protected] Departamento de Biología, Universidad Autónoma de Madrid, c/Darwin 2, Campus de Cantoblanco, 28049 Madrid, Spain
In soils, B is mainly found as uncharged boric acid [B (OH)3] (Ka = 5.80X10–10 at 25 °C (pKa = 9.24)) [6]. At optimal concentrations, B enters the plant through passive diffusion. However, under deficiency, B is transported into and within the plant by the coordinated action of proteins that belong to two protein families: the major intrinsic protein family MIP and the Boron transporter family BOR [7–10]. MIPs are a superfamily of aquaporins made up of several subfamilies, including the nodulin-26-like intrinsic proteins (NIPs) [11], which are permeable channels to small solutes including [B
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