BackgroundSecond messengers have a key role in linking environmental stimuli to physiological responses. One such messenger, guanosine 3′,5′-cyclic monophosphate (cGMP), has long been known to be an essential signaling molecule in many different physiological processes in higher plants, including biotic stress responses. To date, however, the guanylyl cyclase (GC) enzymes that catalyze the formation of cGMP from GTP have largely remained elusive in higher plants.Principal FindingsWe have identified an Arabidopsis receptor type wall associated kinase–like molecule (AtWAKL10) as a candidate GC and provide experimental evidence to show that the intracellular domain of AtWAKL10431–700 can generate cGMP in vitro. Further, we also demonstrate that the molecule has kinase activity indicating that AtWAKL10 is a twin-domain catalytic protein. A co-expression and stimulus-specific expression analysis revealed that AtWAKL10 is consistently co-expressed with well characterized pathogen defense related genes and along with these genes is induced early and sharply in response to a range of pathogens and their elicitors.ConclusionsWe demonstrate that AtWAKL10 is a twin-domain, kinase-GC signaling molecule that may function in biotic stress responses that are critically dependent on the second messenger cGMP.
Edited by Ulf-Ingo FlüggeKeywords: Affinity Cyclic voltammetry Guanylate cyclase H-NOX domain Nitric oxide Oxygen Square wave voltammetry Arabidopsis thaliana a b s t r a c t While there is evidence of nitric oxide (NO)-dependent signalling via the second messenger cyclic guanosine 3 0 ,5 0 -monophosphate (cGMP) in plants, guanylate cyclases (GCs), enzymes that catalyse the formation of cGMP from guanosine 5 0 -triphosphate (GTP) have until recently remained elusive and none of the candidates identified to-date are NO-dependent. Using both a GC and heme-binding domain specific (H-NOX) search motif, we have identified an Arabidopsis flavin monooxygenase (At1g62580) and shown electrochemically that it binds NO, has a higher affinity for NO than for O 2 and that this molecule can generate cGMP from GTP in vitro in an NO-dependent manner.
Cellular and physiological evidence suggests the presence of a novel class of systemically mobile plant molecules that are recognized by antibodies against vertebrate atrial natriuretic peptides (ANPs). In order to characterize the function of these immunoanalogues we have expressed the full-length recombinant (AtPNP-A[1^126]) and demonstrate that this molecule induces osmoticum-dependent H 2 O uptake into protoplasts at nanomolar concentrations and thus a¡ects cell volume. A similar response is also seen with a recombinant that does not contain the signal peptide (AtPNP-A[26^126]) as well as a short domain (AtPNP-A[33^66]) that shows homology to the vertebrate peptide. Taken together, these ¢ndings suggest that AtPNP-A has an important and systemic role in plant growth and homeostasis.
Plant natriuretic peptides (PNPs) belong to a novel class of systemically mobile molecules that are structurally similar to the N-terminal domain of expansins and affect physiological processes such as protoplast volume regulation at nano-molar concentrations. Here we demonstrate that AtPNP-A, a recombinant Arabidopsis thaliana PNP causes rapid H(+) influx in the elongation zone of A. thaliana roots but not in the mature zone. AtPNP-A also induces significant K(+) and Na(+) efflux and this effect is seen in the mature root zone only. These observations suggest that responses to AtPNP-A are developmental stage and tissue specific and point to a complex role in plant growth and homeostasis.
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