Differential regulation of three functional ammonium transporter genes by nitrogen in root hairs and by light in leaves of tomato

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Abstract
Summary: To elucidate the role of transporters in N nutrition of tomato, two new transporter genes were isolated from cDNA libraries of root hairs or leaves of tomato. While LeAMT1;2 is closely related to LeAMT1;1 (75.6% amino acid identity), LeAMT1;3 is more distantly related (62.8% identity) and possesses two short upstream open reading frames in the 5′ end of the mRNA and a particularly short N‐terminus of the protein as unique features. When expressed in yeast mutants defective in uptake, all three genes complemented uptake. In roots of hydroponically grown plants, transcript levels of LeAMT1;2 increased after or NO3– supply, while LeAMT1;1 was induced by N deficiency coinciding with low glutamine concentrations, and LeAMT1;3 was not detected. In aeroponic culture, expression of LeAMT1;1 and LeAMT1;2 was higher in root hairs than in the remaining root fraction. Growth of plants at elevated CO2 slightly decreased expression of LeAMT1;2 and LeAMT1;3 in leaves, but strongly repressed transcript levels of chloroplast glutamine synthetase and photorespiratory serine hydroxymethyl‐transferase. Expression of LeAMT1;2 and LeAMT1;3 showed a reciprocal diurnal regulation with highest transcript levels of LeAMT1;3 in darkness and highest levels of LeAMT1;2 after onset of light. These results indicate that in tomato at least two high‐affinity transporters, LeAMT1;1 and LeAMT1;2, are differentially regulated by N and contribute to root hair‐mediated acquisition from the rhizosphere. In leaves, the reciprocally expressed transporters LeAMT1;2 and LeAMT1;3 are supposed to play different roles in N metabolism, uptake and/or NH3 retrieval during photorespiration.

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