ABSTRACT
Until recently the role of cyclic nucleotide monophosphates (cNMPs) in plants had been controversial, with equivocal data about their concentrations, biosynthetic and degrading enzymes, and cellular targets. This review discusses the current knowledge in this field, with focus on the largest class of cNMP targets in plant cells, the cyclic nucleotide-gated channels (CNGCs). Aspects of structure and function are addressed, with reference to studies in heterologous systems and in planta. The picture emerging, albeit still fragmented, is of proteins with diverse functions in the control of ion homeostasis, development, and defense against biotic and abiotic threats.
Subject(s)
Ion Channels/metabolism , Nucleotides, Cyclic/metabolism , Plant Proteins/metabolism , Plants/metabolism , Amino Acid Sequence , Arabidopsis/genetics , Arabidopsis/metabolism , Arabidopsis Proteins/chemistry , Arabidopsis Proteins/genetics , Arabidopsis Proteins/metabolism , Calcium/metabolism , Calmodulin/metabolism , Cyclic Nucleotide-Gated Cation Channels , Ion Channels/chemistry , Ion Channels/genetics , Models, Molecular , Molecular Sequence Data , Phylogeny , Plant Development , Plant Proteins/chemistry , Plant Proteins/genetics , Plants/genetics , Sequence Homology, Amino Acid , Signal TransductionABSTRACT
External physical and chemical stimuli are transduced via second messengers, following primary interaction with specific membrane or soluble receptors. Ca(2+) is an important second messenger in plants as in other eukaryotes, mediating responses to numerous environmental stimuli and affecting a multitude of cellular processes including gene expression. However, there is yet very little information concerning the cis-elements that mediate Ca(2+)-responsive gene expression. In this article we discuss a recent investigation combining bioinformatics with experimental data, revealing DNA regulatory elements that convey specific cytosolic Ca(2+) transients to the transcription machinery.
ABSTRACT
The regulation of gene expression by cellular calcium is crucial for plant defense against biotic and abiotic stresses. However, the number of genes known to respond to specific transient calcium signals is limited, and as yet there is no definition of a calcium-responsive cis element in plants. Here, we generated specific cytosolic calcium transients in intact Arabidopsis thaliana seedlings and linked them to early transcriptome changes, followed by bioinformatic analysis of the responsive genes. A cytosolic calcium transient induced by calmodulin antagonists and blocked by lanthanides was characterized using aequorin-based luminometry and photon imaging. Analysis of transcriptome changes revealed 230 calcium-responsive genes, of which 162 were upregulated and 68 were downregulated. These include known early stress-responsive genes as well as genes of unknown function. Analysis of their upstream regions revealed, exclusively in the upregulated genes, a highly significant occurrence of a consensus sequence (P < 10(-13)) comprising two abscisic acid-specific cis elements: the abscisic acid-responsive element (ABRE; CACGTG[T/C/G]) and its coupling element ([C/A]ACGCG[T/C/G]) [corrected] Finally, we show that a tetramer of the ABRE cis element is sufficient to confer transcriptional activation in response to cytosolic Ca(2+) transients. Thus, at least for some specific Ca(2+) transients and motif combinations, ABREs function as Ca(2+)-responsive cis elements.
Subject(s)
Arabidopsis/metabolism , Calcium/metabolism , Cytosol/drug effects , RNA, Messenger/genetics , Arabidopsis/drug effects , Arabidopsis/genetics , Base Sequence , Calcium Channel Blockers/pharmacology , Calcium Signaling , Calmodulin/antagonists & inhibitors , Cytosol/metabolism , DNA Primers , Genes, Plant , Promoter Regions, Genetic , Regulatory Sequences, Nucleic AcidABSTRACT
Inositol 1,4,5-trisphosphate 3-kinase, and more generally inositol polyphosphate kinases (Ipk), play important roles in signal transduction in animal cells; however, their functions in plant cells remain to be elucidated. Here, we report the molecular cloning of a cDNA (AtIpk2beta) from a higher plant, Arabidopsis. Arabidopsis AtIpk2beta is a 33-kD protein that exhibits weak homology ( approximately 25% identical amino acids) with Ipk proteins from animals and yeast and lacks a calmodulin binding site, as revealed by sequence analysis and calmodulin binding assays. However, recombinant AtIpk2beta phosphorylates inositol 1,4,5-trisphosphate to inositol 1,4,5,6-tetrakisphosphate and also converts it to inositol 1,3,4,5,6-pentakisphosphate [Ins(1,3,4,5,6)P(5)]. AtIpk2beta also phosphorylates inositol 1,3,4,5-tetrakisphosphate to Ins(1,3,4,5,6)P(5). Thus, the enzyme is a D3/D6 dual-specificity inositol phosphate kinase. AtIpk2beta complements a yeast ARG82/IPK2 mutant lacking a functional ArgR-Mcm1 transcription complex. This complex is involved in regulating Arg metabolism-related gene expression and requires inositol polyphosphate kinase activity to function. AtIpk2beta was found to be located predominantly in the nucleus of plant cells, as demonstrated by immunolocalization and fusion to green fluorescent protein. RNA gel blot analysis and promoter-beta-glucuronidase reporter gene studies demonstrated AtIpk2beta gene expression in various organs tested. These data suggest a role for AtIpk2beta as a transcriptional control mediator in plants.
