Redox regulation, thioredoxins, and glutaredoxins in retrograde signalling and gene transcription.

Integration of reactive oxygen species (ROS)-mediated signal transduction pathways via redox sensors and the thiol-dependent signalling network is of increasing interest in cell biology for their implications in plant growth and productivity. Redox regulation is an important point of control in protein structure, interactions, cellular location, and function, with thioredoxins (TRXs) and glutaredoxins (GRXs) being key players in the maintenance of cellular redox homeostasis. The crosstalk between second messengers, ROS, thiol redox signalling, and redox homeostasis-related genes controls almost every aspect of plant development and stress response. We review the emerging roles of TRXs and GRXs in redox-regulated processes interacting with other cell signalling systems such as organellar retrograde communication and gene expression, especially in plants during their development and under stressful environments. This approach will cast light on the specific role of these proteins as redox signalling components, and their importance in different developmental processes during abiotic stress.

Thioredoxin TRXo1 is involved in ABA perception via PYR1 redox regulation.

Abscisic acid (ABA) plays a fundamental role in plant growth and development processes such as seed germination, stomatal response or adaptation to stress, amongst others. Increases in the endogenous ABA content is recognized by specific receptors of the PYR/PYL/RCAR family that are coupled to a phosphorylation cascade targeting transcription factors and ion channels. Just like other receptors of the family, nuclear receptor PYR1 binds ABA and inhibits the activity of type 2C phosphatases (PP2Cs), thus avoiding the phosphatase-exerted inhibition on SnRK2 kinases, positive regulators which phosphorylate targets and trigger ABA signalling. Thioredoxins (TRXs) are key components of cellular redox homeostasis that regulate specific target proteins through a thiol-disulfide exchange, playing an essential role in redox homeostasis, cell survival, and growth. In higher plants, TRXs have been found in almost all cellular compartments, although its presence and role in nucleus has been less studied. In this work, affinity chromatography, Dot-blot, co-immunoprecipitation, and bimolecular fluorescence complementation assays allowed us to identify PYR1 as a new TRXo1 target in the nucleus. Studies on recombinant HisAtPYR1 oxidation-reduction with wild type and site-specific mutagenized forms showed that the receptor underwent redox regulation involving changes in the oligomeric state in which Cys30 and Cys65 residues were implied. TRXo1 was able to reduce previously-oxidized inactive PYR1, thus recovering its capacity to inhibit HAB1 phosphatase. In vivo PYR1 oligomerization was dependent on the redox state, and a differential pattern was detected in KO and over-expressing Attrxo1 mutant plants grown in the presence of ABA compared to WT plants. Thus, our findings suggest the existence of a redox regulation of TRXo1 on PYR1 that may be relevant for ABA signalling and had not been described so far.

Autophagy Is Involved in the Viability of Overexpressing Thioredoxin o1 Tobacco BY-2 Cells under Oxidative Conditions.

Autophagy is an essential process for the degradation of non-useful components, although the mechanism involved in its regulation is less known in plants than in animal systems. Redox regulation of autophagy components is emerging as a possible key mechanism with thioredoxins (TRXs) proposed as involved candidates. In this work, using overexpressing PsTRXo1 tobacco cells (OEX), which present higher viability than non-overexpressing cells after H2O2 treatment, we examine the functional interaction of autophagy and PsTRXo1 in a collaborative response. OEX cells present higher gene expression of the ATG (Autophagy related) marker ATG4 and higher protein content of ATG4, ATG8, and lipidated ATG8 as well as higher ATG4 activity than control cells, supporting the involvement of autophagy in their response to H2O2. In this oxidative situation, autophagy occurs in OEX cells as is evident from an accumulation of autolysosomes and ATG8 immunolocalization when the E-64d autophagy inhibitor is used. Interestingly, cell viability decreases in the presence of the inhibitor, pointing to autophagy as being involved in cell survival. The in vitro interaction of ATG4 and PsTRXo1 proteins is confirmed by dot-blot and co-immunoprecipitation assays as well as the redox regulation of ATG4 activity by PsTRXo1. These findings extend the role of TRXs in mediating the redox regulation of the autophagy process in plant cells.

Decreased Levels of Thioredoxin o1 Influences Stomatal Development and Aperture but Not Photosynthesis under Non-Stress and Saline Conditions.

Salinity has a negative impact on plant growth, with photosynthesis being downregulated partially due to osmotic effect and enhanced cellular oxidation. Redox signaling contributes to the plant response playing thioredoxins (TRXs) a central role. In this work we explore the potential contribution of Arabidopsis TRXo1 to the photosynthetic response under salinity analyzing Arabidopsis wild-type (WT) and two Attrxo1 mutant lines in their growth under short photoperiod and higher light intensity than previous reported works. Stomatal development and apertures and the antioxidant, hormonal and metabolic acclimation are also analyzed. In control conditions mutant plants displayed less and larger developed stomata and higher pore size which could underlie their higher stomatal conductance, without being affected in other photosynthetic parameters. Under salinity, all genotypes displayed a general decrease in photosynthesis and the oxidative status in the Attrxo1 mutant lines was altered, with higher levels of H2O2 and NO but also higher ascorbate/glutathione (ASC/GSH) redox states than WT plants. Finally, sugar changes and increases in abscisic acid (ABA) and NO may be involved in the observed higher stomatal response of the TRXo1-altered plants. Therefore, the lack of AtTRXo1 affected stomata development and opening and the mutants modulate their antioxidant, metabolic and hormonal responses to optimize their adaptation to salinity.