Systematic review of the chloroplastic redox network and new insights into the function of Trx h2 in Arabidopsis thaliana L.

Abstract

As sessile organisms, plants suffer great influence of environmental condition. To maintain homeostasis, plants needs a precise and refine metabolism that must be fast and precise in responding to constraining environmental changes. In this context, the redox metabolism is pivotal to maintain the homeostasis in response to environmental changes through genetic expression reprogramming and post-translation modifications such as redox regulation mediated by thioredoxins (Trx). Proteins that contain at least two conserved cysteine (Cys) residues are candidates to suffer redox regulation, given that two oxidized Cys residues form a disulfide bridge which is cleaved by the activity of Trxs. Trxs are thus important enzymes for the redox control of metabolism, in which the reduction of disulfide bridge (de)activate target proteins. Trxs has been described to be located mainly at chloroplast, cytosol and mitochondria, with predominance of higher number of isoforms in chloroplasts. The discovery of new Trx targets has long been investigated. In this vein, the chloroplastic Trx network is the most studied organelle so far, in which the Trx-mediated regulation of proteins from Calvin-Benson cycle, starch metabolism, chlorophyll biosynthesis and ATP synthesis is already well-established. However, our comprehension on Trx network is coordinated with other components from chloroplastic redox metabolism remains unclear. This is especially due the fact that chloroplastic Trxs are often studied and reviewed isolated, which hampers our understanding regarding how the entire chloroplastic redox network act in synchrony to maintain homeostasis of chloroplast metabolism. By contrast of the chloroplastic Trx network, the role of mitochondrial Trxs and the function of Trx h isoforms, which are beyond the chloroplast, remain unclear, especially in the case of the Trx h2 isoform. This isoform is located at both cytosol and mitochondria. Trx h2 was recently proposed to regulates the TCA cycle succinate dehydrogenase and fumarase enzymes plus alternative oxidase of the electron transport rate. However, no studies have confirmed whether this also occur in vivo, which is especially due the lack of trxh2 mutant characterization. Here, we thus characterized trxh2 mutant lines aiming to provide information regarding the role of this isoform for plant growth and leaf metabolism. This thesis is divided in two chapters. In the first, we provided a precise and refine review of chloroplastic redox metabolism, evidencing its main characters and the importance of redox metabolism on cellular homeostasis. After that, we described the results from experimental characterization of trxh2 mutant lines.