In Search of Enzymes with a Role in 3', 5'-Cyclic Guanosine Monophosphate Metabolism in Plants

Abstract

In plants, nitric oxide (NO)-mediated 3', 5'-cyclic guanosine monophosphate (cGMP) synthesis plays an important role during pathogenic stress response, stomata closure upon osmotic stress, the development of adventitious roots and transcript regulation. The NO-cGMP dependent pathway is well characterized in mammals. The binding of NO to soluble guanylate cyclase enzymes (GCs) initiates the synthesis of cGMP from guanosine triphosphate. The produced cGMP alters various cellular responses, such as the function of protein kinase activity, cyclic nucleotide gated ion channels and cGMP-regulated phosphodiesterases. The signal generated by the second messenger is terminated by 3', 5'-cyclic nucleotide phosphodiesterase (PDEs) enzymes that hydrolyze cGMP to a non-cyclic 5'-guanosine monophosphate. To date, no homologues of mammalian cGMP-synthesizing and degrading enzymes have been found in higher plants. In the last decade, six receptor proteins from Arabidopsis thaliana have been reported to have guanylate cyclase activity in vitro. Of the six receptors, one was shown to be a NO dependent guanylate cyclase enzyme (NOGC1). However, the role of these proteins in planta remains to be elucidated. Enzymes involved in the degradation of cGMP remain elusive, albeit, PDE activity has been detected in crude protein extracts from various plants. Additionally, several research groups have partially purified and characterized PDE enzymatic activity from crude protein extracts. In this review, we focus on presenting advances toward the identification of enzymes involved in the cGMP metabolism pathway in higher plants.

Keywords: cGMP; guanylate cyclase; nitric oxide; phosphodiesterases; plants; signaling.

FIGURE 1

A schematic depiction of the nitric oxide-induced cGMP signaling pathway in developmental, abiotic and biotic stress processes. During water stress, an increase in the hormone abscisic acid (ABA) activates the synthesis of nitric oxide (NO). Subsequently, NO stimulates the NO-dependent guanylate cyclase to produce cGMP (Dubovskaya et al., 2011; Joudoi et al., 2013). Concurrently, ABA also activates the production of H₂O₂ which reacts with NO to produces reactive nitrogen species (RNS; Joudoi et al., 2013). A reaction between the RNS and cGMP produce 8-Nitro-cGMP which in turn activates the accumulation of cytoplasmic calcium, [Ca2⁺]_cyt and the SLOW ANION CHANNEL 1 (SLAC1) which results in stomatal closure. Similarly, during a pathogenic attack, plants close their stomata; however, the NO-cGMP pathway is initiated by the hormone salicylic acid (SA; Hao et al., 2010). Furthermore, during a pathogenic attack, NO-cGMP signaling cascade activates the transcription of the pathogenic marker, PAL in an SA-independent manner (Durner et al., 1998). Furthermore, NO-cGMP signaling pathway is important during pollen tubule development. NO-cGMP signaling pathway is also involved in adventitious root formation stimulated by exogenous and endogenous chemicals, for example, N-Acyl-homoserine-lactones (AHLs) produced by gram negative rizobacteria. AHLs promote polar auxin transport which activates the NO-cGMP dependent signaling cascade leading to the development of adventitious root formation (Pagnussat et al., 2003a; Lanteri et al., 2006; Bai et al., 2012). Similarly, gravitropism bending requires the auxin induced NO-cGMP signaling pathway (Hu et al., 2005).

FIGURE 2

Phylogenetic analysis of biochemically characterized 3′, 5′-cyclic nucleotide monophosphate phosphodiesterase (PDEs) found in prokaryotes, lower and higher eukaryotes. The protein sequences of characterized PDEs from Class I, II, and III were retrieved from GenBank. The PDEs belong to Chlamydomonas reinhardtii (algae), Homo sapiens (human), Dictyostelium discoideum (Slime mold), Saccharomyces cerevisiae (fungi), Candida Albicans (fungi), Arthrobacter (proteobacteria), Escherichia coli (proteobacteria), Mycobacterium tuberculosis and Vibrio fischeri. A multiple alignment with the retrieved PDEs was performed in a protein alignment program, Muscle (Edgar, 2004). The phylogenetic tree was constructed in MEGA6 (Tamura et al., 2013) using the Neighbor-joining statistical method with a bootstrap replication number of 1000. Members of the Class I PDEs are found in lower and higher eukaryotes and Class II PDEs are found in lower eukaryotes as well as bacteria. Finally, Class III PDEs are found exclusively in bacteria.