Glutamate dehydrogenase is differentially regulated in seeded and parthenocarpic tomato fruits during crop development and postharvest storage

Highlights

• The two fruit kinds followed different patterns of GDH gene transcript accumulation during their development.

• No differences were detected in the localization of the α-GDH protein and of GHD enzyme activity.

• Cold storage at 5 °C increased the transcript accumulation of GDH genes in both fruit types.

• Storage at 10 °C mostly decreased the transcript accumulation of GDH genes in both fruit types.

Abstract

Cherry tomatoes are more susceptible than larger fruit varieties to producing auxin-induced parthenocarpic fruits indistinguishable from seeded fruits in most organoleptic characteristics. In this study, the effects of auxin-induced parthenocarpy and of the short-term cold storage on the expression of GDH genes, one of the main regulatory enzymes of nitrogen metabolism, were examined. Seeded and parthenocarpic fruits exhibited differences in the pattern of expression of the genes coding for α- and β-subunits of GDH during fruit development. Short-term cold storage at 5 °C significantly increased transcript accumulation of GDH genes, while storage at 10 °C for five days mostly caused a reduction in the expression of the genes in both fruit types. Finally, no significant differences between the two fruit types were detected in the localization of GDH protein and GDH enzyme activity.

Introduction

Farmers produce parthenocarpic tomatoes by artificial fruit setting using auxins in order to overcome poor fertilization when the environmental conditions are unfavourable to a satisfactory fruit set (de Jong et al., 2009). Cherry tomatoes, in contrast to many larger fruiting varieties, produce well-formed fruits with no considerable differences in shape and colour compared to seeded tomatoes (Cuartero et al., 1987). Interestingly, while seeded and parthenocarpic tomatoes show insignificant disparities in some of the major quality indexes, they exhibit considerable variations in ascorbic acid and sugar metabolism (Tsaniklidis et al., 2012, Tsaniklidis et al., 2014, Rounis et al., 2014). These findings suggest that further differences between the two fruit types could exist in other major metabolic routes, such as in nutrient assimilation and turnover.

Nitrogen is a critical macronutrient that must be in reduced state in order to be used by plants. Ammonium is a reduced nitrogen form, easily accessible to plans for assimilation into amino acids and proteins (Masclaux-Daubresse et al., 2006). Ammonium, however, is noxious to plants, causing proton extrusion, pH turbulence, uncoupling of photophosphorylation, etc. (Kronzucker et al., 2001). Glutamate dehydrogenase (GDH-EC 1.4.1.2) regulates to a great extent glutamic acid (Glu), one of the proteinogenic amino acids, which serves as an intermediate molecule during the synthesis of complex amino acids and, thus, plays a pivotal role in ammonium assimilation and amino acid metabolism (Dubois et al., 2003, Masclaux-Daubresse et al., 2006, Bernard and Habash, 2009). Glutamate dehydrogenase is involved in ammonium ion transfer by catalyzing the reversible deamination of Glu to α-ketoglutarate (Ferraro et al., 2012). Hence, this enzyme provides a link between carbohydrate and amino acid metabolism. In ripe tomato fruits, glutamic acid is found at high concentrations, giving them the distinguished ‘umami’ taste (Sorrequieta et al., 2010). In plants, the higher Km for ammonium in GDH, compared to glutamine synthase (GS), is considered to be involved almost exclusively in Glu breakdown. This biochemical pathway produces α-ketoglutarate and consequently provides carbon skeletons for the tricarboxylic acid cycle (Dubois et al., 2003, Mungur et al., 2005). Moreover, under several conditions where the TCA cycle is inhibited this pathway could be important as an alternative route to 2-oxoglutarate (Sweetlove et al., 2010). GDH enzyme has a hexameric structure consisting of two polypeptides (α- and β-subunit) that fluctuate slightly in mass and charge (Skopelitis et al., 2006). GDH enzyme can be separated by PAGE electrophoresis into seven different anodal-migrating isoenzymes. GDH gene families coding for α- and β-subunits of the enzyme have been characterized in Arabidopsis thaliana (Turano et al., 1997) and Nicotiana plumbaginifolia (Ficarelli et al., 1999). Only recently, a third GDH polypeptide with a minor participation in the hexamere structure of the GDH complex protein was characterized in Arabidopsis (Fontaine et al., 2013). In addition, GDH is located in mitochondria, chloroplasts and cytosol (Dubois et al., 2003). GDH is often associated to senescence, stress tolerance and balancing the N/C status of plants (Skopelitis et al., 2006, Labboun et al., 2009, Fontaine et al., 2012). Although GDH function is well characterized, little is known regarding its regulation at the transcriptional level during fruit development. Furthermore, responses to abiotic stress such as cold induction (under post-harvest storage) are vastly uncharted and have not yet been fully investigated. This study attempts to investigate the localization and expression of GDH genes, during the development and post harvest storage of cherry tomato fruits, since it is a key enzyme in amino acid composition. In order to investigate further the effects of parthenocarpy on tomato fruit metabolism, the localization of GDH protein and enzyme activity, and transcription of the genes coding for the α- and β-subunits of GDH were studied in seeded and parthenocarpic fruits, during their development and following post harvest storage.