Adenyl cyclases and cAMP in plant signaling - past and present
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REVIEW
Adenyl cyclases and cAMP in plant signaling - past and present Review
Chris Gehring
Abstract In lower eukaryotes and animals 3'-5'-cyclic adenosine monophosphate (cAMP) and adenyl cyclases (ACs), enzymes that catalyse the formation of cAMP from ATP, have long been established as key components and second messengers in many signaling pathways. In contrast, in plants, both the presence and biological role of cAMP have been a matter of ongoing debate and some controversy. Here we shall focus firstly on the discovery of cellular cAMP in plants and evidence for a role of this second messenger in plant signal transduction. Secondly, we shall review current evidence of plant ACs, analyse aspects of their domain organisations and the biological roles of candidate molecules. In addition, we shall assess different approaches based on search motifs consisting of functionally assigned amino acids in the catalytic centre of annotated and/or experimentally tested nucleotide cyclases that can contribute to the identification of novel candidate molecules with AC activity such as F-box and TIR proteins. Is there cAMP-dependent signaling in plants? By the mid 1970s, 3'-5'-cyclic adenosine monophosphate (cAMP) had been firmly established as an important signaling molecule and second messenger in both animals and lower eukaryotes [1-4]. It was understood that adenyl cyclases (also referred to as adenylyl or adenylate cyclases) (ACs) catalyse the conversion of ATP to cAMP and pyrophosphate, that cAMP affects many different physiological and biochemical processes including the activity of kinases and that cAMP levels decrease in the presence of phosphodiesterases [4]. Given the growing realization of the importance of ACs and cAMP, it is not surprising that plant scientists were keen to learn if this signaling system was universal and therefore operating in plants too. As a result a controversy ensued that is perhaps best summarized by a concluding statement in a review in 1977 - "Our present knowledge, or rather ignorance, of cyclic AMP in higher plants does not permit us to indulge in speculation on its function and thus to increase the disparity between available facts and conclusions, which are based solely on the conviction that plants, with respect to cyclic AMP, should behave like animals or certain bacteria" [5]. Plant scientists were cautioned not to jump to conclusions for two reasons, firstly, the cAMP * Correspondence: [email protected] 1 Division of Chemistry, Life Science & Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
levels in plants appeared to be low compared to that found in animals and secondly, vagaries of the assays were not conducive to firm conclusions [5]. Whilst more advanced analytical tools have since overcome the second point, the first remains [6]. Reported cAMP levels in plants are typically < 20 pmol/g fresh weight (e.g. ≤12 pmol/g fresh weight in rye grass endosperm cell cultures [7] and < 12 pmol/g fresh w
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