Tansley Review No. 106 - Cyclic nucleotides in higher plants: the enduringparadox

For three decades, hypotheses relating to the occurrence and function of cy clic nucleotides in higher plants have been highly controversial. Although cyclic nucleotides had been shown to have key regulatory roles in animals a nd bacteria, investigations with higher plants in the 1970s and early 1980s were criticized on the basis of (i) a lack of specificity of effects appar ently elicited by cyclic nucleotides, (ii) the equivocal identification of putative endogenous cyclic nucleotides and (iii) ambiguity in the identific ation of enzymes connected with cyclic nucleotide. More recent evidence bas ed on more rigorous identification procedures has demonstrated conclusively the presence of cyclic nucleotides, nucleotidyl cyclases and cyclic nucleo tide phosphodiesterases in higher plants, and has identified plant processe s subject to regulation by cyclic nucleotides. Here we review the history o f the debate, the recent evidence establishing the presence of these compou nds and their role; future research objectives are discussed.