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Efficient integration of various external and internal signals is required to maintain adaptive cellular function. Numerous distinct signal transduction systems have evolved to allow cells to receive these inputs, to translate their codes and, subsequently, to expand and integrate their meanings. Two of these, cyclic AMP and cyclic GMP, together referred to as the cyclic nucleotide signaling system, are between them. The cyclic nucleotides regulate a vast number of processes in almost all living organisms. Once synthesized by adenylyl or guanylyl cyclases, cyclic nucleotides transduce signals by acting through a number of cellular effectors. Because the activities of several of these effectors are altered simultaneously in response to temporal changes in cyclic nucleotide levels, agents that increase cAMP/cGMP levels can trigger multiple signaling events that markedly affect numerous cellular functions. In this mini review, we summarize recent evidence supporting the existence of cNMP effectors in plant cells. Specifically, we highlight cAMP-dependent protein kinase A (PKA), cGMP-dependent kinase G (PKG), and cyclic nucleotide phosphodiesterases (PDEs). Essentially this manuscript documents the progress that has been achieved in recent decades in improving our understanding of the regulation and function of cNMPs in plants and emphasizes the current gaps and unanswered questions in this field of plant signaling research.

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Proteins with a CyaB, thiamine triphosphatase domain (CYTH domain) may play a central role at the interface between nucleotide and polyphosphate metabolism. One of the plant CYTH domain‐containing proteins from Brachypodium distachyon, BdTTM3, is annotated in NCBI databases as an ‘adenylyl cyclase (AC)’ or a ‘triphosphate tunnel metalloenzyme’. The divergent nomenclature and the search for plant ACs induced us to experimentally confirm the enzymatic activity of BdTTM3. Based on in vitro analysis, we have shown that the recombinant form of BdTTM3 is a protein with high triphosphatase activity (binding both tripolyphosphate and ATP) and low AC activity. Furthermore, the analysis of BdTTM3 transcriptional activity indicates its involvement in the mechanism underlying responses to wounding stress in B. distachyon leaves.