Plants growing in close association with N(2)-fixing bacteria are able to overcome growth limitations in N-depleted soils. The molecular mechanism by which free-living, N(2)-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 NH(+)(4)-transporter gene, LEAMT1;2 (L. esculentum ammonium transporter 1;2), indicating that bacterial NH(+)(4) 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 N(2)-fixing bacteria do excrete NH(+)(4) 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 > or = 1 microM external NH(+)(4). While peak expression was obtained with 2-5 microM NH(+)(4), a further increase in NH(+)(4) 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 NH(+)(4) concentration or the plant N status. Since micromolar concentrations of NH(+)(4) strongly increased the LEAMT1;2-mRNA levels, the transported NH(+)(4) ion itself could represent a key signal in the associative interaction between higher plants and N(2)-fixing micro-organisms.

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