Physiological roles of cNMPs (cyclic nucleotide monophosphates) in plants

•Photo transduction downstream of phytochrome, in the pathway that leads to activation of Chalcone Synthase (CS) and anthocyanin biosynthesis.

•The cGMP-dependent signaling pathway antagonizes the phytochrome signaling pathway mediated by calcium/calmodulin (Ca2+/CaM), which is involved in chloroplast development (and CAB expression). Similarly, the latter pathway is antagonistic to the cGMP-mediated pathway of CS.

•rbcS and CS genes are activated by Ca2+ and cGMP, respectively, and attenuated by cGMP and Ca2+, respectively.

•cNMPs mediate phytochrome signaling in the cyanobacterium Anabaena and also participate in phototransduction in the vertebrate retina. Hence, the role of cNMPs in phototransduction may be evolutionarily conserved from prokaryotes through plants to mammals.

•Cyclic nucleotides have been suggested to be involved in plant responses to both biotic and abiotic stresses. First, cyclic nucleotides have been found to mediate responses to pathogens, downstream of nitric oxide (NO). NO may activate cGMP pathways for both defense gene induction and potentiation of ROI-induced cell death.

•cNMPs also help protect plants against salt stress. cNMPs participate in various developmental processes in addition to photomorphogenesis. For example, cAMP acts as a second messenger in pollen. tube growth and reorientation and cGMP is a second messenger in auxin-induced adventitious rooting.

•cGMP play an important role in the response of cereal aleurone cells to gibberellic acid. Ample evidence for the role of cNMPs in controlling ion homeostasis in plants has also accumulated in the past decade and earlier. cAMP-stimulated carrot cells exhibited enhanced Ca2+ influx.

•Furthermore, salt and osmotic stress cause rapid increases in cGMP levels in Arabidopsis thaliana and Ca2+ transients seem to be dependent on cGMP elevation in response to ionic stress. cAMP and cGMP improve tolerance to salt stress. Interestingly, improved salt tolerance correlated with cNMP-dependent decrease of channel open probability, and reduced influx of Na+.

Role of cyclic nucleotides

•Inside the cell cyclic AMP stimulates a protein kinase, which in turn phosphorylates substrate proteins thereby altering their activity, before the signal is switched off by the hydrolysis of cyclic AMP to AMP by phosphodiesterase.

•cyclic GMP has been implicated as part of the signal transduction pathway involving NO-mediated responses such as guard cell closure, cell death, root development

•In wheat seeds ABA and cyclic AMP appears to promote synthesis of particular proteins, the synthesis of which is also promoted by drought

•In oat seedlings the concentration of cyclic AMP is influenced by light in a way that suggests it is part of a phytochrome signal pathway

•cyclic AMP and cyclic GMP appear to be involved in nodulation

•cyclic AMP play role in elicitation of phytoalexin synthesis

•a cAMP-gated K+ channel, made up of four proteins assembled in such a way that the membrane-spanning domains combine to form a central pore, is a key component in a signaling pathway where odorants bind to receptors linked to another form of G protein, Golf, which activates adenylyl cyclase

•Rap 1 activation involves binding to Epac (Exchange protein directly activated by cyclic AMP), after the latters activation by cAMP, Epac being a guanine nucleotide exchanger facilitating the release of GDP from Rap 1 and its replacement with GTP

•Incase of animals: In response to light activation of the visual pigment, an activated G-protein interacts with cyclic GMP phosphodiesterase, the resultant change in cyclic GMP levels altering the permeability of the eye rod cell membrane to sodium ions which in turn initiates a transmission along the optic nerve

Calcium-calmodulin cascade

Decoding of calcium signatures: calcium signaling and its sensors with associated transcription factors.