Expression of the NH₄ ⁺-transporter gene LEAMT1;2 is induced in tomato roots upon association with N₂-fixing bacteria

Abstract.

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.