Cyclic nucleotides in plants: revealing universal principles of cellular signalling

Cyclic nucleotides (cNMPs), particularly cGMP and cAMP, have constituted a fundamental component of cellular signaling research in animals for several decades. Although their presence in plants was postulated as early as the 1970s, the concept of a functional role for cNMPs in plant biology remained marginal for a long time. Low concentrations of these molecules, methodological challenges in their detection, and the apparent absence of classical cyclase homologs or effectors known from animal systems fostered skepticism regarding their existence and physiological relevance. Consequently, despite these obstacles, discoveries in cNMP signaling in plants lagged rapidly advancing knowledge derived from animal models for many years. However, over the last two decades, this field has undergone a significant reassessment. The identification of plant guanylyl and adenylyl cyclases with atypical structures and their enzymatic activity within complex, multidomain, and multifunctional proteins revealed a distinct molecular architecture of the cNMP pathway in plant cells. Rather than employing the classical, specialized enzymes and effectors characteristic of animal systems, plants utilize modular solutions that integrate cyclic nucleotide production with signal perception and cellular response. This suggests functional convergence rather than simple evolutionary conservation of pathway components. Currently, cyclic nucleotides are considered essential components of signaling networks that regulate plant responses to stress factors and growth and developmental processes. cGMP and cAMP function in concert with key mediators such as Ca²⁺, reactive oxygen species (ROS), and nitric oxide (NO), forming integrated signaling pathways with high plasticity. Increasing evidence indicates that, despite structural differences, the organizational logic of these pathways, rapid secondary messenger production, signal localization, and precise degradation, exhibits remarkable similarities across biological kingdoms. This presentation will outline the historical development of research on cyclic nucleotides and their metabolism in plants, the current state of knowledge regarding their biosynthesis, degradation, perception, and putative biological functions, and the methodological challenges associated with their quantification and data interpretation. Comparative aspects will also be discussed to better understand how diverse organisms utilize a shared class of molecules to regulate complex adaptive processes. In this context, cyclic nucleotides in plants emerge not as a marginal evolutionary relic but as a long-underappreciated yet fundamental element integrating environmental signals with cellular responses.