Article,
Expression of the NH -transporter gene is induced in tomato roots upon association with N -fixing bacteria
Planta, 215 (3): 424-429 (2002)579lp Times Cited:38 Cited References Count:21.
DOI: 10.1007/s00425-002-0773-x
Abstract
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 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 N-2-fixing bacteria do excrete NH4+ levels that can be sensed by tomato roots and is in agreement with the induction of expression of LEAMT1:2 with as low as greater than or equal to1 muM external NH4+. While peak expression was obtained with 2-5 muM 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 N-2-fixing micro-organisms.
