Response to nitrate/ammonium nutrition of tomato (Solanum lycopersicum L.) plants overexpressing a prokaryotic NH4+-dependent asparagine synthetase

Nitrogen availability is an important limiting factor for plant growth. Although NH4+ assimilation is energetically more favorable than NO3-, it is usually toxic for plants. In order to study if an improved ammonium assimilatory metabolism could increase the plant tolerance to ammonium nutrition, tomato (Solanum lycopersicum L. cv P-73) plants were transformed with an NH4+-dependent asparagine synthetase (AS-A) gene from Escherichia coli (asnA) under the control of a PCpea promoter (pea isolated constitutive promotor). Homozygous (Horn), azygous (Az) asnA and wild type (WT) plants were grown hydroponically for 6 weeks with normal Hoagland nutrition (NO3-/NH4+ = 6/0.5) and high ammonium nutrition (NO3-/NH4+ = 3.5/3). Under Hoagland's conditions, Horn plants produced 40-50% less biomass than WT and Az plants. However, under NO3-/NH4+ = 3.5/3 the biomass of Horn was not affected while it was reduced by 40-70% in WT and Az plants compared to Hoagland, respectively. The Horn plants accumulated 1.5-4 times more asparagine, glycine, serine and soluble proteins and registered higher glutamine synthetase (GS) and glutamate synthase (GOGAT) activities in the light-adapted leaves than the other genotypes, but had similar NH4+ and NO3- levels in all conditions. In the dark-adapted leaves, a protein catabolism occurred in the Horn plants with a concomitant 25-40% increase in organic acid concentration, while asparagine accumulation registered the highest values. The aforementioned processes might be responsible for a positive energetic balance as regards the futile cycle of the transgenic protein synthesis and catabolism. This explains growth penalty under standard nutrition and growth stability under NO3-/NH4+ = 3.5/3, respectively. (C) 2013 Elsevier GmbH. All rights reserved.