Increasing number of experimental data
Increasing number of experimental data reveal that both ROS and RNS are multifunctional molecules playing a substantial role in plant physiology and particularly in seed biology (Bailly et al., 2008; Šírová et al., 2011; Yu et al., 2014). Dual function of RNS can be described by the model of “nitrosative door” (similar to “oxidative window” designed by Bailly et al. (2008) for ROS) which “opens” for seed germination when intracellular NO level reaches optimal concentration (Krasuska et al., 2015a). The mode of action of RNS depends on modification of amino Fluoxymesterone residues in proteins (posttranslational modification) e.g. nitration or S-nitrosation (Tichá et al., 2016; Yu et al., 2014). S-nitrosation of proteins or peptides occurs when nitroso group is attached to a cysteine residue. This posttranslational modification has a strong impact on protein structure and/or function (Tichá et al., 2016). In plant cells, NO is synthesized via oxidative or reductive pathways (Gupta et al., 2011). During seed dormancy alleviation, increased NO level may come from enzymatic or non-enzymatic sources. Till today there is no strong molecular evidence for the presence of NO synthase (NOS) in higher plants (Jeandroz et al., 2016), however arginine (Arg)-dependent NOS-like activity has been detected in several higher plants (Corpas and Barroso, 2017), also in axes of germinating apple embryos (Krasuska et al., 2016, 2017b). It is suggested that non-enzymatic sources of NO are more relevant in seed dormancy breakage, especially during initial hours of imbibition. One of the important NO donor is nitrite (NO2−), which liberates NO under acidic pH (Yamasaki, 2000) or acts as acceptor of electrons (Igamberdiev et al., 2010). Alteration in NO2− concentration is linked to dormancy loss and stimulation of germination of apple embryos (Krasuska et al., 2017b). S-nitrosoglutathione (GSNO) is considered as a cellular NO reservoir (Broniowska et al., 2013; Dürner and Klessig, 1999). This molecule is the most abundant S-nitrosated derivative thiol of many physiological function (Broniowska et al., 2013). GSNO was detected in various plant tissues (Barroso et al., 2006) and quite recently also in seeds (Ma et al., 2016). This molecule is more stable than S-nitrosocysteine (Bartberger et al., 2000, 2001) and in specific conditions (presence of transition metal ions e.g. copper) can liberate NO (review by Broniowska et al., 2013). Transnitrosation is the physiologically relevant mechanism of GSNO decay when the nitroso functional group is donated to another thiol (Hogg, 1999). GSNO reductase (GSNOR) (EC 220.127.116.11) is a glutathione-dependent enzyme, a member of class III alcohol dehydrogenase family (Jensen et al., 1998; Tichá et al., 2016). In plants growing under optimal conditions activity of GSNOR is linked to physiological development and fertility (Lee et al., 2008). In Arabidopsis (Arabidopsis thaliana (L.) Heynh.) it was demonstrated that this enzyme was not expressed at the same level in all parts of the seedling and the activity varied depending on organ and developmental stage (Espunya et al., 2006). Most data describing GSNOR in planta are focused on its role in reaction to abiotic and biotic stresses (Barroso et al., 2006; Kubienová et al., 2014). Reduction of GSNO by GSNOR leads to formation of oxidised form of glutathione (GSSG) and NH2OH or alternatively ends with the rearrangement and then spontaneous hydrolyzation to GSO2H and NH3 (Jensen et al., 1998). GSNOR activity is not connected with NO liberation, rather with effective scavenging from the free “NO pool”, thus this enzyme regulates nitrosylated glutathione level (Broniowska et al., 2013; Ma et al., 2016). Activity of this enzyme has been confirmed in many plants (Airaki et al., 2012; Barroso et al., 2006; Chaki et al., 2011; Krasuska et al., 2017a; Kubienová et al., 2013, 2014). In Arabidopsis GSNOR is encoded by one gene, not expressed only in mature pollen (Leterrier et al., 2011).