Comparative study of salt stress-induced physiological and molecular responses in tomato (Solanum lycopersicum L.)

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Salinity is one of the main abiotic stress factors reducing the crop productivity. Salinity affects all dicotyledonous crops and among them tomato (Solanum lycopersicum L.) is a nominated model to investigate salt stress. Tomato is moderately sensitive to salinity, thus we were able to study the effects of both sublethal- and lethal salt stress. Salt stress includes ionic-, osmotic- and oxidative stress, whereas lethal salt stress induces programmed cell death (PCD). PCD is a well-defined, genetically controlled process. PCD has essential role in developmental processes and mediates plant responses to environmental stresses including salinity (Shabala 2009). Ordered series of events have been detected during salt stress-induced PCD. Excess of Na+ results ion imbalance, reduced water potential and induces ROS burst. Disruption of ion homeostasis and ROS production collectively induces the activity of proteases and together resulting PCD. Protein degradation is a key consequence of salt stress-induced PCD. We applied activity-based protein profiling (ABPP) to investigate the two main protein degradation pathways (protease-based and proteasome-based degradation pathways) in tomato under different salt conditions. Cys proteases are considered to participate in the regulation of PCD. In animals different stress conditions trigger the activation of caspases (Cys containing Asp-specific proteases). Although, their homologs have not been found in plant genomes, some plant proteases have caspase-like activity including Cys proteases and proteasome catalytic subunits (Han et al., 2012). The stress proteasome in the animal kingdom facilitates faster conversion of oxidized proteins during stress conditions by incorporating different catalytic β subunits. Plants deal with similar kinds of stresses and also carry multiple paralogous genes encoding for each of the three catalytic β subunits. In contrast to Arabidopsis thaliana, tomato has a simplified proteasome gene set with single genes encoding each β subunit except for two genes encoding β2. Using proteasome activity profiling on tomato roots during salt stress we discovered a transient modification of the catalytic subunits of the proteasome coinciding with a loss of viability. This stress-induced active proteasome disappears at later time points and coincides with the need to degrade oxidized proteins during salt stress. This stress-induced proteasome may play an important role in PCD during abiotic stress. Finally, we investigated the response of an abscisic acid (ABA)-defective, sitiens mutant of tomato exposed to sublethal- and lethal salt stress. The mutant is deficient in functional abscisic aldehyde oxidase activity catalysing the final step in ABA biosynthesis, thus the tissues accumulate about 10% ABA compared to wild type. ABA is produced under water-deficit conditions and under salt stress and regulates plant water balance and osmotic stress tolerance. Importantly, ABA deficiency highly increased susceptibility to salt stress in sitiens roots. Little is known about the role of ABA in protein degradation. rpn10, a mutant in regulatory particle of 26S proteasome, was highly sensitive to ABA by the selective stabilization of the short-lived ABA-signaling protein ABI5 (abscisic acid insensitive) in Arabidopsis (Smalle et al., 2003). However, maturation process of cystein proteases was delayed and accumulation of their inactive pro-protease form was prolonged by ABA in tomato endosperm (Trobacher et al., 2013). Interestingely, activity of certain Cys proteases increased in the absence of ABA indicating that ABA might promote survive via suppress proteolytic activity under high salinity.

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