Indolyl-3-butyric acid-induced Arabidopsis stomatal opening mediated by 3',5'-cyclic guanosine-monophosphate

cGMP functions as an important messenger involved in SlSAMS1-regulated salt stress tolerance in tomato

Salt stress adversely affects the growth, development, and yield of tomato (Solanum lycopersicum). SAM Synthetase (SAMS), which is responsible for the biosynthesis of S-adenosylmethionine (SAM, a precursor of polyamine biosynthesis), participates in plant response to abiotic stress. However, the regulatory mechanism of SAMS-mediated salt stress tolerance remains elusive. In this study, we characterized a SAMS homologue SlSAMS1 in tomato. We found that SlSAMS1 is highly expressed in tomato roots, and its expression can be induced by salt stress. Crucially, overexpression of SlSAMS1 in tomato enhances salt stress tolerance. Through metabolomic profiling, we identified some differentially accumulated metabolites, especially, a secondary messenger guanosine 3',5'-cyclic monophosphate (cGMP) which may play a key role in SlSAMS1-regulated salt tolerance. A series of physiological and biochemical data suggest that cGMP alleviates salt stress-induced growth inhibition, and potentially acts downstream of the polyamine-nitric oxide (PA-NO) signaling pathway to trigger H2O2 signaling in response to salt stress. Taken together, the study reveals that SlSAMS1 regulates tomato salt tolerance via the PA-NO-cGMP-H2O2 signal module. Our findings elucidate the regulatory pathway of SlSAMS1-induced plant response to salt stress and indicate a pivotal role of cGMP in salt tolerance.

Predicted Three-Dimensional Structure of the GCR1 Putative GPCR in Arabidopsis thaliana and Its Binding to Abscisic Acid and Gibberellin A1

GCR1 has been proposed as a plant analogue to animal G-protein-coupled receptors that can promote or regulate several physiological processes by binding different phytohormones. For instance, abscisic acid (ABA) and gibberellin A1 (GA1) have been shown to promote or regulate germination and flowering, root elongation, dormancy, and biotic and abiotic stresses, among others. They may act through binding to GCR1, which would put GCR1 at the heart of key signaling processes of agronomic importance. Unfortunately, this GPCR function has yet to be fully validated due to the lack of an X-ray or cryo-EM 3D atomistic structure for GCR1. Here, we used the primary sequence data from Arabidopsis thaliana and the GEnSeMBLE complete sampling method to examine 13 trillion possible packings of the 7 transmembrane helical domains corresponding to GCR1 to downselect an ensemble of 25 configurations likely to be accessible to the binding of ABA or GA1. We then predicted the best binding sites and energies for both phytohormones to the best GCR1 configurations. To provide the basis for the experimental validation of our predicted ligand-GCR1 structures, we identify several mutations that should improve or weaken the interactions. Such validations could help establish the physiological role of GCR1 in plants.

cGMP signalling in plants: from enigma to main stream

All living organisms communicate with their environment, and part of this dialogue is mediated by secondary messengers such as cyclic guanosine mono phosphate (cGMP). In plants, most of the specific components that allow production and breakdown of cGMP have now been identified apart from cGMP dependent phosphodiesterases, enzymes responsible for cGMP catabolism. Irrespectively, the role of cGMP in plant signal transductions is now firmly established with involvement of this nucleotide in development, stress response, ion homeostasis and hormone function. Within these areas, several consistent themes where cGMP may be particularly relevant are slowly emerging: these include regulation of cation fluxes, for example via cyclic nucleotide gated channels and in stomatal functioning. Many details of signalling pathways that incorporate cGMP remain to be unveiled. These include downstream targets other than a small number of ion channels, in particular cGMP dependent kinases. Improved genomics tools may help in this respect, especially since many proteins involved in cGMP signalling appear to have multiple and often overlapping functional domains which hampers identification on the basis of simple homology searches. Another open question regards the topographical distribution of cGMP signals are they cell limited? Does long distance cGMP signalling occur and if so, by what mechanisms? The advent of non-disruptive fluorescent reporters with high spatial and temporal resolution will provide a tool to accelerate progress in all these areas. Automation can facilitate large scale screens of mutants or the action of effectors that impact on cGMP signalling.

Involvement of secondary messengers and small organic molecules in auxin perception and signaling

Auxin is a major phytohormone involved in most aspects of plant growth and development. Generally, auxin is perceived by three distinct receptors: TRANSPORT INHIBITOR RESISTANT1-Auxin/INDOLE ACETIC ACID, S-Phase Kinase-Associated Protein 2A and AUXIN-BINDING PROTEIN1. The auxin perception is regulated by a variety of secondary messenger molecules, including nitric oxide, reactive oxygen species, calcium, cyclic GMP, cyclic AMP, inositol triphosphate, diacylglycerol and by physiological pH. In addition, some small organic molecules, including inositol hexakisphosphate, yokonolide B, p-chlorophenoxyisobutyric acid, toyocamycin and terfestatin A, are involved in auxin signaling. In this review, we summarize and discuss the recent progress in understanding the functions of these secondary messengers and small organic molecules, which are now thoroughly demonstrated to be pervasive and important in auxin perception and signal transduction.

Hyperpolization-activated Ca(2+) channels in guard cell plasma membrane are involved in extracellular ATP-promoted stomatal opening in Vicia faba

Extracellular ATP (eATP) plays essential roles in plant growth, development, and stress tolerance. Extracellular ATP-regulated stomatal movement of Arabidopsis thaliana has been reported. Here, ATP was found to promote stomatal opening of Vicia faba in a dose-dependent manner. Three weakly hydrolysable ATP analogs (adenosine 5'-O-(3-thio) triphosphate (ATPγS), 3'-O-(4-benzoyl) benzoyl adenosine 5'-triphosphate (Bz-ATP) and 2-methylthio-adenosine 5'-triphosphate (2meATP)) showed similar effects, indicating that ATP acts as a signal molecule rather than an energy charger. ADP promoted stomatal opening, while AMP and adenosine did not affect stomatal movement. An ATP-promoted stomatal opening was blocked by the NADPH oxidase inhibitor diphenylene iodonium (DPI), the reductant dithiothreitol (DTT) or the Ca(2+) channel blockers GdCl3 and LaCl3. A hyperpolarization-activated Ca(2+) channel was detected in plasma membrane of guard cell protoplast. Extracellular ATP and weakly hydrolyzable ATP analogs activated this Ca(2+) channel significantly. Extracellular ATP-promoted Ca(2+) channel activation was markedly inhibited by DPI or DTT. These results indicated that eATP may promote stomatal opening via reactive oxygen species that regulate guard cell plasma membrane Ca(2+) channels.

Breakthroughs spotlighting roles for extracellular nucleotides and apyrases in stress responses and growth and development

Animal and plant cells release nucleotides into their extracellular matrix when touched, wounded, and when their plasma membranes are stretched during delivery of secretory vesicles and growth. These released nucleotides then function as signaling agents that induce rapid increases in the concentration of cytosolic calcium, nitric oxide and superoxide. These, in turn, are transduced into downstream physiological changes. These changes in plants include changes in the growth of diverse tissues, in gravitropism, and in the opening and closing of stomates. The concentration of extracellular nucleotides is controlled by various phosphatases, prominent among which are apyrases EC 3.6.1.5 (nucleoside triphosphate diphosphohydrolases, NTPDases). This review provides phylogenetic and pHMM analyses of plant apyrases as well as analysis of predicted post-translational modifications for Arabidopsis apyrases. This review also summarizes and discusses recent advances in research on the roles of apyrases and extracellular nucleotides in controlling plant growth and development. These include new findings that document how apyrases and extracellular nucleotides control auxin transport, modulate stomatal aperture, and mediate biotic and abiotic stress responses, and on how apyrase suppression leads to growth inhibition.

Cyclic GMP is involved in auxin signalling during Arabidopsis root growth and development

The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) plays an important role in plant development and responses to stress. Recent studies indicated that cGMP is a secondary signal generated in response to auxin stimulation. cGMP also mediates auxin-induced adventitious root formation in mung bean and gravitropic bending in soybean. Nonetheless, the mechanism of the participation of cGMP in auxin signalling to affect these growth and developmental processes is largely unknown. In this report we provide evidence that indole-3-acetic acid (IAA) induces cGMP accumulation in Arabidopsis roots through modulation of the guanylate cyclase activity. Application of 8-bromo-cGMP (a cell-permeable cGMP derivative) increases auxin-dependent lateral root formation, root hair development, primary root growth, and gene expression. In contrast, inhibitors of endogenous cGMP synthesis block these processes induced by auxin. Data also showed that 8-bromo-cGMP enhances auxin-induced degradation of Aux/IAA protein modulated by the SCF(TIR1) ubiquitin-proteasome pathway. Furthermore, it was found that 8-bromo-cGMP is unable to directly influence the auxin-dependent TIR1-Aux/IAA interaction as evidenced by pull-down and yeast two-hybrid assays. In addition, we provide evidence for cGMP-mediated modulation of auxin signalling through cGMP-dependent protein kinase (PKG). Our results suggest that cGMP acts as a mediator to participate in auxin signalling and may govern this process by PKG activity via its influence on auxin-regulated gene expression and auxin/IAA degradation.

Peroxisomes as a source of auxin signaling molecules

Peroxisomes house many metabolic processes that allow organisms to safely sequester reactions with potentially damaging byproducts. Peroxisomes also produce signaling molecules; in plants, these include the hormones indole-3-acetic acid (IAA) and jasmonic acid (JA). Indole-3-butyric acid (IBA) is a chain-elongated form of the active auxin IAA and is a key tool for horticulturists and plant breeders for inducing rooting in plant cultures and callus. IBA is both made from and converted to IAA, providing a mechanism to maintain optimal IAA levels. Based on genetic analysis and studies of IBA metabolism, IBA conversion to IAA occurs in peroxisomes, and the timing and activity of peroxisomal import and metabolism thereby contribute to the IAA pool in a plant. Four enzymes have been hypothesized to act specifically in peroxisomal IBA conversion to IAA. Loss of these enzymes results in decreased IAA levels, a reduction in auxin-induced gene expression, and strong disruptions in cell elongation resulting in developmental abnormalities. Additional activity by known fatty acid β-oxidation enzymes also may contribute to IBA β-oxidation via direct activity or indirect effects. This review will discuss the peroxisomal enzymes that have been implicated in auxin homeostasis and the importance of IBA-derived IAA in plant growth and development.