Cyclic Nucleotides and Nucleotide Cyclases in Plant Stress Responses

The cyclic nucleotides monophosphates (cNMP) and in particular adenosine 3’,5’-cyclic monophosphate (cAMP) and guanosine 3’,5’-cyclic monophosphate (cGMP) are cyclic catalytic products of adenosine 5'-triphosphate (ATP) and guanosine 5’-triphosphate (GTP) respectively. These cNMPs are universal second messengers with key roles in many and diverse physiological responses and processes in prokaryotes, and in both higher and lower eukaryotes. Cyclic AMP is arguably one of the most extensively studied second messengers in animals, lower eukaryotes and bacteria where it has critical roles in signaling the metabolic status. In bacteria, cAMP is involved in the positive regulation of the lac operon where in an environment of a low glucose, cAMP accumulates and binds to the allosteric site of the cAMP receptor protein (CRP), a transcription activator protein. Once the CRP is in its active configuration, it binds to a cis-element upstream of the lac promoter and activates transcription. At high glucose concentrations, cAMP concentration decreases and CRP disengages from the lac operon promoter (Meiklejohn & Gralla, 1985). Cyclic AMP signaling is also critical for many aspects of the development of the slime mold Dictyostelium discoideum that grows unicellularly, but develops as multicellular organism (Kimmel & Firtel, 2004; McMains et al., 2008). Cyclic AMP has a role in chemotaxis and cAMP oscillations act as intracellular feedback loops in the transcriptional regulation of many regulatory pathways. Post aggregation, cAMP-dependent signals mediate cell sorting, pattern formation, and morphogenetic changes and cAMP receptor affinity can control wave dynamics, geometry and morphogenesis (Dormann et al., 2001). In animals, cAMP can be produced e.g. by Gs-coupled activation of adenylate cyclases (ACs) which triggers a signal cascade that includes the modification of Ca2+ channels, the phosphorylation of target proteins including enzymes involved in regulating glycogen metabolism and eventually the cAMP-dependent activation of the transcription factor, cAMP response element-binding (CREB) (Bolwell, 1995; Karin & Smeal, 1992). Cyclic AMP also plays a role in excitation-