Tansley Review No. 106: Cyclic nucleotides in higher plants: the enduring paradox.

For three decades, hypotheses relating to the occurrence and function of cyclic nucleotides in higher plants have been highly controversial. Although cyclic nucleotides had been shown to have key regulatory roles in animals and bacteria, investigations with higher plants in the 1970s and early 1980s were criticized on the basis of (i) a lack of specificity of effects apparently elicited by cyclic nucleotides, (ii) the equivocal identification of putative endogenous cyclic nucleotides and (iii) ambiguity in the identification of enzymes connected with cyclic nucleotide. More recent evidence based on more rigorous identification procedures has demonstrated conclusively the presence of cyclic nucleotides, nucleotidyl cyclases and cyclic nucleotide phosphodiesterases in higher plants, and has identified plant processes subject to regulation by cyclic nucleotides. Here we review the history of the debate, the recent evidence establishing the presence of these compounds and their role; future research objectives are discussed. contents Summary I. background 427 II. the history of the debate on camp in plants 431 III. evidence for the natural occurrence of camp in plants 432 IV. adenylyl cyclase 435 V. phosphodiesterases 437 VI. camp-dependent protein kinases, creb, camp-binding proteins and cyclicnucleotide-gated channels 439 VII. putative functions of camp 442 VIII. camp and other cyclic nucleotides 445 IX. future directions 447 References 449.