Research article Salt stress affects glutamine synthetase activity and mRNA accumulation on potato plants in an organ-dependent manner Jorge Teixeira * , Fernanda Fidalgo Faculty of Sciences of the University of Porto, Botany Department, Edifı ´cio FC4, Rua do Campo Alegre, s/n  4169–007 Porto, Portugal article info Article history: Received 20 September 2008 Accepted 12 May 2009 Available online 23 May 2009 Keywords: Gene expression Glutamine synthetase Nitrogen metabolism Potato Salt stress abstract Ammonium assimilation into glutamine and glutamate is vital for plant growth as these are precursors for almost all nitrogenous compounds. Ammonium can be assimilated onto nitrogenous organic compounds by the concerted action of two enzymes that compose the glutamine synthetase (GS, EC 6.3.1.2) – glutamate synthase (Fd-GOGAT, EC 1.4.7.1; NADH–GOGAT, EC 1.4.1.14) cycle. Ammonium may also be directly incorporated into glutamate by the glutamate dehydrogenase (GDH, EC 1.4.1.2) aminating reaction. However, as GDH reversibly deaminates glutamate, its physiological role in vivo remains controversial. Potato has been classified as moderately tolerant to salinity. Potato GS is encoded by a small multigene family which is differentially regulated in an organ and age-dependent way. In this study, the effect of increasing concentrations of salinity in the soil in GS activity and gene-specific mRNA accumulation levels were studied on potato leaves and roots, as well as the biochemical parameters protein, chlorophyll, lipid peroxidation and proline levels, in order to evaluate the severity of the imposed stress. The data obtained suggests that when potato plants are subjected to salt stress, increased ammonium assimilation occurs in roots, due to an increased GS accumulation, along with a decreased assimilation in leaves. Regarding GS gene-specific mRNA accumulation, an organ-dependent response was also observed that contributes for the detected alteration in the ammonium assimilatory metabo- lism. This response may be a key feature for future genetic manipulations in order to increase crop productivity in salty soils. The possible contribution of GDH for ammonia assimilation was also investigated. Ó 2009 Elsevier Masson SAS. All rights reserved. 1. Introduction Environmental stresses such as drought, salinity and low or high temperatures are important factors which limit plant distribution and productivity [1]. On dry land, the exchange of oxygen and carbon dioxide is directly linked to the loss of water from the body of a plant because the water vapour concentration is greater in the leaf than in the atmosphere. Plants, with their roots growing in salty water, losing enormous volumes of water to the dry air face a tremendous challenge [2]. Salt stress in most glycophytes leads to extensive changes in metabolism accompanied by substantial changes in gene expression with increases in some gene products and decreases in others. Better characterisation of the molecular signals involved in stress perception and the molecular events that specify the expression of stress tolerance will provide a sound basis foundation to improve agricultural productivity [1]. The effects of salinity on plant nitrogen metabolism studied to date revealed increased protein degradation, inhibition of protein synthesis and accumulation and/or depletion of protein and non- protein amino acids in a small group of dicots and monocots [3,4]. Ammonium derived from the primary nitrate reduction, as well as other metabolic pathways, including root uptake, photorespi- ration and amino acid catabolism, is converted first to glutamine by glutamine synthetase (GS, EC 6.3.1.2) and then to glutamate by glutamate synthase (Fd-GOGAT, EC 1.4.7.1 and NADH–GOGAT, EC 1.4.1.14). In higher plants, two GS isoenzymes can be found:one is localised in the plastid stroma (GS2) and is responsible for assim- ilating the ammonium derived from photorespiration and from the reduction of nitrate, being the major GS enzyme present in leaves and virtually absent in non-phosynthesising organs of C3 plants; the other isoenzyme is localised to the cytosol (GS1). In leaves, GS1 is mainly associated to the vascular bundle [5,6] and is the main GS isoenzyme present in non-photosynthetic organs. GS1 has been implicated in several physiological processes ranging from phloem loading to the reassimilation of nitrogen derived from protein breakdown during senescence and seed germination [7]. The GS1 enzyme that accumulates specifically in the phloem companion Abbreviations: GDH, Glutamate dehydrogenase; GOGAT, Glutamate synthase; GS, Glutamine synthetase. * Corresponding author. Tel.: þ351 220 402 703; fax: þ351 220 402 799 . E-mail address: agteixei@fc.up.pt (J. Teixeira). Contents lists available at ScienceDirect Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy 0981-9428/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.plaphy.2009.05.002 Plant Physiology and Biochemistry 47 (2009) 807–813