Auxin perception and signal transduction

The action of auxin on whole plants is very complex, but we are starting to understand how some of the earliest events are signalled in single cells. There is now good evidence that auxin induces rapid events at the plasma membrane by binding to a population of the auxin-binding protein ABPI, which is associated with a membrane-spanning docking protein, possibly a G-protein-coupled receptor (GPCR). ABPI is targeted to the endoplasmic reticulum (ER) lumen, but it does not appear to bind auxin within the ER and its function (if any) in this location is unknown. It is also not known how the protein reaches the cell surface, but it is possible that it is exported together with its docking protein. Binding of auxin causes a conformational change affecting the C-terminus of ABPI and it is likely that this change serves to activate the receptor at the plasma membrane. The signal transduction pathway appears to involve activation of phospholipase A₂(PLA₂) leading to the production of lipid second messengers which activate the plasma membrane proton ATPase (H⁻-ATPase) by a phosphorylation-dependent mechanism. Branch points exist that could potentially lead from this pathway to responses in the nucleus, but there is not yet any firm evidence that ABP1 is involved in such responses. Since intracellular auxin concentrations are correlated with sensitivity in some cases, it is possible that there is also a site of auxin perception inside the cell.     

UV-B signal transduction pathway in Arabidopsis

Plants experience a variety of environmental stresses such as cold, drought, freezing, flooding, wounding, heat and UV-B, all of which result in decreased productivity. Among abiotic stresses, UV-B stress is considered to be a critical factor affecting the rate of plant growth because the amount of UV-B reaching the Earth’s surface is constantly increasing. While high fluence rates of UV-B trigger stress-related processes, low fluence rates of UV-B induce photomorphogenesis, a crucial developmental process at the early seedling stage in plants. Among the signaling components involved in UV-B-mediated cellular response, a clade composed of UVR8-COP1-HY5 has been shown to be a central sequence that effectively transduces the pathway from the primary signal to adaptation response. This review summarizes the most recent progress in studies of UVR8-COP1-HY5 as the key players participating in the UV-B signal transduction pathway. The current understanding of additional UV-B signaling components including substrate receptors of multi-subunit E3 ubiquitin ligase is also discussed.     

Two-component signal transduction pathways in Arabidopsis

The two-component system, consisting of a histidine (His) protein kinase that senses a signal input and a response regulator that mediates the output, is an ancient and evolutionarily conserved signaling mechanism in prokaryotes and eukaryotes. The identification of 54 His protein kinases, His-containing phosphotransfer proteins, response regulators, and related proteins in Arabidopsis suggests an important role of two-component phosphorelay in plant signal transduction. Recent studies indicate that two-component elements are involved in plant hormone, stress, and light signaling. In this review, we present a genome analysis of the Arabidopsis two-component elements and summarize the major advances in our understanding of Arabidopsis two-component signaling.     

Auxin is required for leaf vein pattern in Arabidopsis

To investigate possible roles of polar auxin transport in vein patterning, cotyledon and leaf vein patterns were compared for plants grown in medium containing polar auxin transport inhibitors (N-1-naphthylphthalamic acid, 9-hydroxyfluorene-9-carboxylic acid, and 2,3,5-triiodobenzoic acid) and in medium containing a less well-characterized inhibitor of auxin-mediated processes, 2-(p-chlorophynoxy)-2-methylpropionic acid. Cotyledon vein pattern was not affected by any inhibitor treatments, although vein morphology was altered. In contrast, leaf vein pattern was affected by inhibitor treatments. Growth in polar auxin transport inhibitors resulted in leaves that lacked vascular continuity through the petiole and had broad, loosely organized midveins, an increased number of secondary veins, and a dense band of misshapen tracheary elements adjacent to the leaf margin. Analysis of leaf vein pattern developmental time courses suggested that the primary vein did not develop in polar auxin transport inhibitor-grown plants, and that the broad midvein observed in these seedlings resulted from the coalescence of proximal regions of secondary veins. Possible models for leaf vein patterning that could account for these observations are discussed.     

The ethylene signal transduction pathway in Arabidopsis

The gaseous hormone ethylene is an important regulator of plant growth and development. Using a simple response of etiolated seedlings to ethylene as a genetic screen, genes involved in ethylene signal transduction have been identified in Arabidopsis. Analysis of two of these genes that have been cloned reveals that ethylene signalling involves a combination of a protein (ETR1) with similarity to bacterial histidine kinases and a protein (CTR1) with similarity to Raf-1, a protein kinase involved in multiple signalling cascades in eukaryotic cells. Several lines of investigation provide compelling evidence that ETR1 encodes an ethylene receptor. For the first time there is a glimpse of the molecular circuitry underlying the signal transduction pathway for a plant hormone.     

The ethylene signal transduction pathway in Arabidopsis

The gaseous hormone ethylene is an important regulator of plantgrowth and development. Using a simple response of etiolatedseedlings to ethylene as a genetic screen, genes involved inethylene signal transduction have been identified in Arabidopsis.Analysis of two of these genes that have been cloned revealsthat ethylene signalling involves a combination of a protein(ETR1) with similarity to bacterial histidine kinases and aprotein (CTR1) with similarity to Raf-1, a protein kinase involvedin multiple signalling cascades in eukaryotic cells. Severallines of investigation provide compelling evidence that ETR1encodes an ethylene receptor. For the first time there is aglimpse of the molecular circuitry underlying the signal transductionpathway for a plant hormone. Key words: Ethylene, plant growth, plant development, regulation, signal transduction, Arabidopsis     

Auxin transport-dependent,stage-specific dynamics of leaf vein formation

For centuries, the formation of vein patterns in the leaf has intrigued biologists, mathematicians and philosophers. In leaf development, files of vein-forming procambial cells emerge from seemingly homogeneous subepidermal tissue through the selection of anatomically inconspicuous preprocambial cells. Although the molecular details underlying the orderly differentiation of veins in the leaf remain elusive, gradually restricted transport paths of the plant hormone auxin have long been implicated in defining sites of vein formation. Several recent advances now appear to converge on a more precise definition of the role of auxin flow at different stages of vascular development. The picture that emerges is that of vein formation as a self-organizing, reiterative, auxin transport-dependent process.Key words: arabidopsis, leaf development, polar auxin transport, procambium, vascular patterningThe vascular system of plants is a branching array of cell files extending through all organs.¹ In dicot leaves, these vascular strands, or ‘veins’, are arranged in a ramified pattern that largely reflects the shape of the leaf (Fig. 1A).^2,3 ‘Lateral veins’ branch from a conspicuous central vein (‘midvein’) that is continuous with the stem vasculature. In many species, lateral veins extend along the leaf edge to form ‘marginal veins’, which connect to adjacent lateral veins to form prominent closed loops. Finally, a series of ‘higher-order veins’ branch from midvein and loops and can either terminate in the lamina (‘free-ending veins’) or join two veins (‘connected veins’).Open in a separate windowFigure 1Conceptual summary of dicot leaf vein formation. (A) Schematics of a simplified mature leaf illustrating midvein (M), first, second and third loops (L1, L2 and L3, respectively)—each derived from corresponding lateral (LV) and marginal (MV) veins—free-ending (FV) and connected (CV) higher-order veins, hydathodes (H) and middle-to-margin positions (decreasing green gradient) as used in the text. (B) State transitions in leaf subepidermal cell differentiation. Available evidence suggests that the vein patterning process is limited to ground meristem cells (white), while subepidermal cells that have begun to acquire mesophyll characteristics are incapable of responding to vein-inducing signals.^11,13,19,38 Expression of preprocambial (blue) and mesophyll emergence markers seem to identify two mutually exclusive and typically irreversible cell states, one leading to procambium (pink) and the other to mature mesophyll (green) formation. The transition from ground meristem to differentiated mesophyll could conceivably occur through a cell state that is formally equivalent to the preprocambial state in vascular differentiation. However, the existence of such a ‘premesophyll’ state (faded gray), the extent of its stability, its mutual exclusivity or competition with the preprocambial state and its responsiveness to vein-inducing signals still remain open questions. (C) Stage-specific dynamics of leaf vein patterning and their dependency on auxin levels and transport as exemplified for loop formation, but in general equally applicable to all veins. Upper series: PIN1-labeled auxin transport paths corresponding to preprocambial cell selection zones (yellow). Note how loops are composed of a lateral PIN1 expression domain (LD) and an initially free-ending marginal PIN1 expression domain (MD). Further, note slightly expanded PIN1 expression domains in a fraction of hydathode-associated third loops during normal development, broad PIN1 domains on the side of local auxin application (arrowhead) and nearly ubiquitous PIN1 expression upon systemic auxin transport inhibition. Middle series: directions of Athb8/J1721-marked preprocambial strand formation (blue arrows). Note middle-to-margin progression of preprocambial strand formation during normal loop development. Further, note margin-to-middle preprocambial strand extension in a fraction of third loops during normal development and in all loops forming on the side of auxin application. Finally, note co-existence of middle-to-margin and margin-to-middle polarities of preprocambial strand extension during the formation of individual loops in response to auxin transport inhibition. Lower series: gradual appearance of procambial cell identity acquisition (pink to magenta). Note simultaneous differentiation of lateral and marginal procambial strands in normal loop development. Further, note successive formation of lateral and marginal procambial strands in a fraction of third loops during normal development and in all loops formed on the side of auxin application and under conditions of reduced auxin transport. Arrows temporally connect successive stages of vein formation. See text for additional details.Vascular cells mature from procambial cells: narrow, cytoplasmdense cells, characteristically arranged in continuous strands.⁴ Leaf procambial strands differentiate from files of isodiametric preprocambial cells, which are selected from the anatomically homogeneous subepidermal tissue of the leaf primordium, the ground meristem (Fig. 1B).^5,6 The mechanism by which ground meristem cells are specified to procambial cell fate is unknown, but an instrumental role for auxin transport and resulting auxin distribution patterns in this process has increasingly gained support.^7–13 This brief essay summarizes a recent group of articles that emphasizes the importance of auxin transport in leaf vein formation.     

Functional cross-talk between two-component and phytochrome B signal transduction in Arabidopsis

The A-type response regulator ARR4 is an element in the two-component signalling network of Arabidopsis. ARR4 interacts with the N-terminus of the red/far-red light photoreceptor phytochrome B (phyB) and functions as a modulator of photomorphogenesis. In concert with other A-type response regulators, ARR4 also participates in the modulation of the cytokinin response pathway. Here evidence is presented that ARR4 directly modulates the activity state of phyB in planta, not only under inductive but also under extended irradiation with red light. Mutation of the phosphorylatable aspartate to asparagine within the receiver domain creates a version of ARR4 that negatively affects photomorphogenesis. Additional evidence suggests that ARR4 activity is regulated by a phosphorelay mechanism that depends on the AHK family of cytokinin receptors. Accordingly, the ability of ARR4 to function on phyB is modified by exogenous application of cytokinin. These results implicate a cross-talk between cytokinin and light signalling mediated by ARR4. This cross-talk enables the plant to adjust light reponsiveness to endogenous requirements in growth and development.     

Phytohormones participate in an S6 kinase signal transduction pathway in Arabidopsis

Addition of fresh medium to stationary cells of Arabidopsis suspension culture induces increased phosphorylation of the S6 ribosomal protein and activation of its cognate kinase, AtS6k. Analysis of the activation response revealed that medium constituents required for S6 kinase activation were the phytohormones 1-naphthylacetic acid (auxin) and kinetin. Pretreatment of cells with anti-auxin or PI3-kinase drugs inhibited this response. Consistent with these findings, LY294002, a PI3-kinase inhibitor, efficiently suppressed phytohormone-induced S6 phosphorylation and translational up-regulation of ribosomal protein S6 and S18A mRNAs without affecting global translation. These data indicate that (1) activation of AtS6k is regulated by phytohormones, at least in part, via a lipid kinase-dependent pathway, that (2) the translational regulation of ribosomal proteins appears to be conserved throughout the plant and animal kingdom, and that (3) these events are hallmarks of a growth-related signal transduction pathway novel in plants.     

Calcium signatures and signal transduction schemes during microbe interactions in Arabidopsis thaliana

Journal of Plant Biochemistry and Biotechnology - Calcium (Ca2+) is considered as crucial second messenger in all living organisms. Ca2+ signaling regulates a diverse array of different biological...     

The ABA signal transduction mechanism in commercial crops: learning from Arabidopsis

The phytohormone abscisic acid (ABA) affects a wide range of stages of plant development as well as the plant's response to biotic and abiotic stresses. Manipulation of ABA signaling in commercial crops holds promising potential for improving crop yields. Several decades of research have been invested in attempts to identify the first components of the ABA signaling cascade. It was only in 2009, that two independent groups identified the PYR/PYL/RCAR protein family as the plant ABA receptor. This finding was followed by a surge of studies on ABA signal transduction, many of them using Arabidopsis as their model. The ABA signaling cascade was found to consist of a double-negative regulatory mechanism assembled from three protein families. These include the ABA receptors, the PP2C family of inhibitors, and the kinase family, SnRK2. It was found that ABA-bound PYR/RCARs inhibit PP2C activity, and that PP2Cs inactivate SnRK2s. Researchers today are examining how the elucidation of the ABA signaling cascade in Arabidopsis can be applied to improvements in commercial agriculture. In this article, we have attempted to review recent studies which address this issue. In it, we discuss various approaches useful in identifying the genetic and protein components involved. Finally, we suggest possible commercial applications of genetic manipulation of ABA signaling to improve crop yields.     

Primary evidence for involvement of IP3 in heat-shock signal transduction in Arabidopsis

The role of inositol 1,4,5-trisphosphate （IP3） in transducing heat-shock （HS） signals was examined in Arabidopsis. The whole-plant IP3 level increased within 1 min of HS at 37℃. After 3 min of HS, the IP3 level reached a maximum 2.5 fold increase. Using the transgenic Arabidopsis plants that have AtHsp 18.2 promoter-β-glucuronidase （GUS） fusion gene, it was found that the level of GUS activity was up-regulated by the addition of caged IP3 at both non-HS and HS temperatures and was down-regulated by the phospholipase C （PLC） inhibitors {1-[6-（（ 1713-3-Methoxyestra-1,3,5（10）-trien- 7-yl）amino）hexyl]-2,5-pyrrolidinedione } （U-73122）. The intracellular-free calcium ion concentration （[Ca^2＋]i） increased during HS at 37℃ in suspension-cultured Arabidopsis cells expressing apoaequorin. Treatment with U-73122 prevented the increase of [Ca^2＋]i to some extent. Above results provided primary evidence for the possible involvement of IP3 in HS signal transduction in higher plants.