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1. Introduction
In higher plants, salinity can provoke alterations in the metabolism of proteins and nucleic acids, photosynthesis and respiration [1–3]. In addition, the production and participation of reactive oxygen species (ROS) during different plant stress conditions including salinity is also well documented [4–8], and more recently the involvement of nitric oxide (NO) and related molecules designated as reactive nitrogen species (RNS) seems to be also a complementary part of the mechanism of response of plants against environmental stresses [9] which can participate in a nitro-oxidative stress situation.
NADPH is a key cofactor in the cellular redox homeostasis, being an indispensable electron donor in numerous enzymatic reactions, biosynthetic pathways, and detoxification processes [10, 11]. In this sense, NADPH is necessary in the metabolism of ROS and RNS; for example, it is a reducing equivalent for the regeneration of reduced glutathione (GSH) by glutathione reductase (component of ascorbate-glutathione cycle) and for the activity of the NADPH-dependent thioredoxin system, two important cell antioxidants against oxidative damage. Moreover, NADPH is also required for the generation of superoxide radical by the NADPH oxidase (NOX) [12], but is also a necessary cofactor for the generation of nitric oxide (NO) by the L-arginine-dependent nitric oxide synthase activity [13]. The most important enzymes which have the capacity to generate reducing power in the form of NADPH in plants are the ferredoxin-NADP reductase as a component of photosystem I [14] and a group of NADP-dehydrogenases located in different subcellular compartments which includes the NADP-isocitrate dehydrogenase (NADP-ICDH), the glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) (both belonging to the pentose phosphate pathway), and the NADP-malic enzyme (ME) [15–17]. Among the different NADP-ICDH isoforms present in higher plants, it has been shown that the cytosolic NADP-ICDH represents more than 90% of the total cellular NADP-ICDH activity [18–21], and very recently in vitro assays have shown that the Arabidopsis cytosolic NADP-ICDH activity from Arabidopsis roots and leaves is differentially regulated by molecules involved in ROS and RNS metabolism [22] including H2O2, NO, and ONOO- indicating a metabolic interconnection among this enzyme and these molecules.
In the present work, using Arabidopsis as model plant, it is shown that under salinity (100 mM NaCl) stress there is a concomitant nitro-oxidative imbalance that is...