br Acknowledgments br Introduction Guanosine cyclic monophos
Introduction Guanosine 3′,5′-cyclic monophosphate (cGMP) is a signaling molecule with key roles in diverse (patho) physiological responses and processes in numerous prokaryotes and all eukaryotes. The number of reports stating the role of cGMP in different processes in plants is continuously growing. Cyclic GMP has been implicated in K+ and Na+ cation transport (Maathuis, 2006), gibberellic acid-dependent α-amylase synthesis (Penson et al., 1996), seed germination (Teng et al., 2010), ABA-induced stomatal closure (Dubovskaya et al., 2011), IBA-induced stomatal opening (Cousson, 2001), gravitropism (Hu et al., 2005), photoperiodic flower induction (Szmidt-Jaworska et al., 2004, Szmidt-Jaworska et al., 2009), and the response to many abiotic (Maathuis and Sanders, 2001, Donaldson et al., 2004) and biotic stresses (Meier et al., 2009). The temporal and spatial differences in cGMP concentration within the cell are controlled by the opposing action of guanylyl cyclases (GCs) and phospodiesterases (PDEs). Guanylyl cyclases are enzymes that are responsible for the conversion of GTP to cGMP and pyrophosphate (PPi). In animal cells there are two types of GC isoenzyme, one located in cytosol referred to as soluble guanylyl cyclases (sGCs) with the second membrane-bounded GC isoenzyme termed transmembrane or particulate guanylyl cyclases (tmGCs or pGCs). In contrast, little information relating to the role and structure of plant guanylyl cyclases is available. Despite the physiological and biochemical evidence for the presence of GCs in plants, knowledge about this group of enzymes is still limited. This may be related to the completely different structure and activation mechanisms of plant guanylyl cyclases in comparison with well-known animal GCs. In 2003 the first putative plant GC (AtGC1) was discovered in Arabidopsis thaliana (Ludidi and Gehring, 2003). The amino CVT 10216 australia sequence analysis indicates that it is a soluble form of GC. The activity of purified recombinant protein is magnesium ion-dependent and nitric oxide-independent. Later on guanylyl cyclases in Zea mays and Pharbitis nil were identified (Yuan et al., 2008, Szmidt-Jaworska et al., 2009). The similarity in the ZmGC1 gene structure and the predicted amino acid sequence of GC from A. thaliana is striking, especially within their catalytic and kinase-like domains. ZmGC1 is associated with the resistance in the fungal pathogen Fusarium graminearum (Yuan et al., 2008). PnGC-1 recombinant protein from P. nil was able to convert GTP to cGMP in the presence of Mn2+ ions and similarly to AtGC1 is not regulated by nitric oxide. Results from the expression analysis of the PnGC1gene indicates that GC can be involved in light/dark-dependent processes, including phytochrome-controlled flower induction (Szmidt-Jaworska et al., 2008, 2009). Based on these results, it seems obvious that the structure and activation mechanism of the discovered plant GCs is completely different from their animal counterparts. More recently, a novel class of cyclases that contain a functional GC domain and cytosolic kinase domain has been discovered. This group includes several receptor kinases such as the brassinosteroid receptor insensitive1 (BRI1), phytosulfokine1 receptor (PSKR1), pathogen peptide1 receptor (PePR1) and a wall associated kinase like 10 (WAKL10), which all share a similar kinase-GC domain structure (Kwezi et al., 2007, Kwezi et al., 2011, Qi et al., 2010, Meier et al., 2010). These proteins belong to the group of enzymes called “moonlighting kinases with guanylyl cyclase activity” (Irving et al., 2012). Plant kinase-GCs are highly unusual as they contain two catalytic regions in the same domain of the protein where the GC catalytic center is embedded in the kinase domain adjacent to the P activation loop. A more common structure of enzymes with multiple catalytic functions, observed in some animal GCs, is the presence of a number of domains separated by linker regions (Irving et al., 2012).