Nucleotides and nucleotide derivatives as signal molecules in plants

In reaction to a stimulus, signaling molecules are made, generate a response, and are then degraded. Nucleotides are classically associated with central metabolism and nucleic acid biosynthesis, but there are a number of nucleotides and nucleotide derivatives in plants to which this simple definition of a signaling molecule applies in whole or at least in part. These include cytokinins and chloroplast guanosine tetraposphate (ppGpp), as well as extracellular canonical nucleotides such as extracellular ATP (eATP) and NAD⁺ (eNAD⁺). In addition, there is a whole series of compounds derived from NAD⁺ such as ADP ribose (ADPR), and ATP-ADPR dinucleotides and their hydrolysis products (e.g. pRib-AMP) together with different variants of cyclic ADPR (cADPR, 2′-cADPR, 3′-cADPR), and also cyclic nucleotides such as 3′,5′-cAMP and 2′,3′-cyclic nucleoside monophosphates. Interestingly, some of these compounds have recently been shown to play a central role in pathogen defense. In this review, we highlight these exciting new developments. We also review nucleotide derivatives that are considered as candidates for signaling molecules, for example purine deoxynucleosides, and discuss more controversial cases.