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Plants have sophisticated stress-sensing and stress-coping mechanisms which enable them to survive and be productive under abiotic and biotic stress conditions. According to the type and severity of the stress, different adaptive and/or tolerance mechanisms are involved in defense processes. However, these processes may overlap or be active together in order to make it easier for plants to fight against different or combined stresses in their natural environment at the same time. Osmotic adjustment and protection of membrane integrity are the two important and mutual components of overall defense processes. The bioactive molecules, glycinebetaine, trehalose, and proline, have been reported to maintain both osmotic adjustment and membrane integrity either by exogenous treatments or by their increased biosynthesis. Glycinebetaine is a quaternary ammonium molecule which is well-known to be a compatible solute. Trehalose is a non-reducing disaccharide with water replacement and glass formation characteristics. The proteinogenic amino acid proline is also a compatible osmoprotectant molecule with a variety of stress-related functions. Although there is a vast amount of information available on the protective role of these molecules, their mode of action is still unclear. Differentiation in the expression of defense-related genes is usually found out to be correlated with the increased or decreased levels of these molecules. Here we collected and reviewed the latest information available on glycinebetaine, trehalose, and proline by means of their influence and impact on the transcription and regulation of the stress-responsive genes.

A distinct feature of selenocysteine (Sec) biosynthesis is that this amino acid is synthesized on its tRNA, designated tRNA[Ser]Sec. Sec is then inserted into protein in response to the codon, UGA, as the 21st proteinogenic amino acid. In eukaryotes and archaea, Sec biosynthesis involves several steps. Transfer RNA[Ser]Sec is first aminoacylated by seryl-tRNA synthetase with serine. O-Phosphoseryl-tRNA[Ser]Sec kinase phosphorylates seryl-tRNA[Ser]Sec forming O-phosphoseryl-tRNA[Ser]Sec that in turn reacts with Sec synthase (SEPSECS) in the presence of selenophosphate yielding Sec-tRNA[Ser]Sec. Selenophosphate is generated by selenophosphate synthetase 2 (SPS2) from selenide and/or other selenium metabolites and ATP. Interestingly, sulfide can replace selenide in the reaction involving SPS2 yielding thiophosphate which can then form cysteine- (Cys)-tRNA[Ser]Sec in the presence of SEPSECS. The Cys moiety on Cys-tRNA[Ser]Sec can donate Cys to protein in response to UGA codons at internal positions of mammalian selenoprotein mRNAs. Cys/Sec replacement occurs naturally in vivo and the amount of replacement is dependent on the level of selenium in the diet.

While the existence of the rare 21st proteinogenic amino acid, selenocysteine (Sec, U) in cellular proteins has been known for over 40 years, recent advances in peptide chemistry have supported its importance not only in the biological function of natural selenoproteins, but also in synthetic systems. Besides its obvious applications in the synthesis of selenoproteins, fundamental differences between the chemistry of cysteine’s thiol and Sec’s selenol moieties have added a surprising number of technologies to the protein chemist’s toolbox. In this chapter, we discuss Sec’s impact on chemical protein synthesis, folding of challenging disulfide-rich proteins, and the chemistry of the little-understood selenoproteins, SEP15 and SELM. Additional important aspects on Sec will be the subjects of other chapters in this book.