Expression of the NH4 +-transporter gene LEAMT1;2 is induced in tomato roots upon association with N2-fixing bacteria
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Plants growing in close association with N2-fixing bacteria are able to overcome growth limitations in N-depleted soils. The molecular mechanism by which free-living, N2-fixing bacteria promote plant growth is still a matter of debate. By inoculating N-depleted tomato (Lycopersicon esculentum Mill.) plants with Azospirillum brasilense or Azoarcus sp. we could demonstrate the induction of the root NH4 +-transporter gene, LEAMT1;2 (L. esculentum ammonium transporter 1;2), indicating that bacterial NH4 + might be used as an N source under these conditions. Azospirillum brasilense (nif –) mutants, which lack the structural nifDK genes, failed to induce LEAMT1;2 expression. This suggests that root-associated N2-fixing bacteria do excrete NH4 + to levels that can be sensed by tomato roots and is in agreement with the induction of expression of LEAMT1;2 with as low as ≥1 µM external NH4 +. While peak expression was obtained with 2–5 µM NH4 +, a further increase in NH4 + reduced LEAMT1;2-mRNA levels in a concentration-dependent manner. The inhibition of LEAMT1;2 expression by glutamine and the glutamine synthetase blocker L-methionine sulfoximine (MSX) provided evidence for the control of LEAMT1;2 expression by cytoplasmic NH4 + concentration or the plant N status. Since micromolar concentrations of NH4 + strongly increased the LEAMT1;2-mRNA levels, the transported NH4 + ion itself could represent a key signal in the associative interaction between higher plants and N2-fixing micro-organisms.
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