Long-distance stress and developmental signals associated with abscisic acid signaling in environmental responses

Flowering plants consist of highly differentiated organs, including roots, leaves, shoots and flowers, which have specific roles: root system for water and nutrient uptake, leaves for photosynthesis and gas exchange and reproductive organs for seed production. The communication between organs through the vascular system, by which water, nutrient and signaling molecules are transported, is essential for coordinated growth and development of the whole plant, particularly under adverse conditions. Here, we highlight recent progress in understanding how signaling pathways of plant hormones are associated with long-distance stress and developmental signals, with particular focus on environmental stress responses. In addition to the root-to-shoot peptide signal that induces abscisic acid accumulation in leaves under drought stress conditions, we summarize the diverse stress-responsive peptide signals reported to date to play a role in environmental responses.     

Signal transduction cascades regulating fungal development and virulence.

Cellular differentiation, mating, and filamentous growth are regulated in many fungi by environmental and nutritional signals. For example, in response to nitrogen limitation, diploid cells of the yeast Saccharomyces cerevisiae undergo a dimorphic transition to filamentous growth referred to as pseudohyphal differentiation. Yeast filamentous growth is regulated, in part, by two conserved signal transduction cascades: a mitogen-activated protein kinase cascade and a G-protein regulated cyclic AMP signaling pathway. Related signaling cascades play an analogous role in regulating mating and virulence in the plant fungal pathogen Ustilago maydis and the human fungal pathogens Cryptococcus neoformans and Candida albicans. We review here studies on the signaling cascades that regulate development of these and other fungi. This analysis illustrates both how the model yeast S. cerevisiae can serve as a paradigm for signaling in other organisms and also how studies in other fungi provide insights into conserved signaling pathways that operate in many divergent organisms.     

Signal Transduction Cascades Regulating Fungal Development and Virulence

Cellular differentiation, mating, and filamentous growth are regulated in many fungi by environmental and nutritional signals. For example, in response to nitrogen limitation, diploid cells of the yeast Saccharomyces cerevisiae undergo a dimorphic transition to filamentous growth referred to as pseudohyphal differentiation. Yeast filamentous growth is regulated, in part, by two conserved signal transduction cascades: a mitogen-activated protein kinase cascade and a G-protein regulated cyclic AMP signaling pathway. Related signaling cascades play an analogous role in regulating mating and virulence in the plant fungal pathogen Ustilago maydis and the human fungal pathogens Cryptococcus neoformans and Candida albicans. We review here studies on the signaling cascades that regulate development of these and other fungi. This analysis illustrates both how the model yeast S. cerevisiae can serve as a paradigm for signaling in other organisms and also how studies in other fungi provide insights into conserved signaling pathways that operate in many divergent organisms.     

Hormonal control of nitrogen acquisition: roles of auxin, abscisic acid, and cytokinin

Nitrogen is the mineral nutrient that often limits plant growth and development. In response to changes in nitrogen supply, plants display elaborate responses at both physiological and morphological levels to adjust their growth and development. Because higher plants consist of multiple organs with different functions and nutritional requirements, they rely on local and long-distance signalling pathways to coordinate the responses at the whole-plant level. Phytohormones have been considered as signalling substances of such pathways. Amongst phytohormones, abscisic acid, auxin, and cytokinins have been closely linked to nitrogen signalling. Recent evidence has provided some insights into how nitrogen and the phytohormone signals are integrated to bring about changes in physiology and morphology. In this review, the evidence is summarized, mostly focusing on examples related to nitrogen acquisition.     

The emerging role of GABA as a transport regulator and physiological signal

While the proposal that γ-aminobutyric acid (GABA) acts a signal in plants is decades old, a signaling mode of action for plant GABA has been unveiled only relatively recently. Here, we review the recent research that demonstrates how GABA regulates anion transport through aluminum-activated malate transporters (ALMTs) and speculation that GABA also targets other proteins. The ALMT family of anion channels modulates multiple physiological processes in plants, with many members still to be characterized, opening up the possibility that GABA has broad regulatory roles in plants. We focus on the role of GABA in regulating pollen tube growth and stomatal pore aperture, and we speculate on its role in long-distance signaling and how it might be involved in cross talk with hormonal signals. We show that in barley (Hordeum vulgare), guard cell opening is regulated by GABA, as it is in Arabidopsis (Arabidopsis thaliana), to regulate water use efficiency, which impacts drought tolerance. We also discuss the links between glutamate and GABA in generating signals in plants, particularly related to pollen tube growth, wounding, and long-distance electrical signaling, and explore potential interactions of GABA signals with hormones, such as abscisic acid, jasmonic acid, and ethylene. We conclude by postulating that GABA encodes a signal that links plant primary metabolism to physiological status to fine tune plant responses to the environment.

γ-Aminobutyric acid (GABA) encodes a plant signal that links primary metabolism to physiological status to fine tune plant responses to the environment.     

Nitrate-specific and cytokinin-mediated nitrogen signaling pathways in plants

Inorganic nitrogen is a limiting factor in plant growth and development. Plants constantly sense changes in nitrogen availability and respond appropriately by modulating gene expression. Plants employ multiple routes for the long-distance signaling and communication of nitrogen status. One of these depends on nitrate itself (nitrate-specific signaling), while another uses cytokinin as a messenger. Recent studies suggest that nitrate-specific signaling functions predominantly in the context of the synthesis of amino acids and nucleic acids. This pathway includes the control of the expression of a wide variety of genes. On the other hand, cytokinin-mediated signaling is related mainly to the control of nitrogen partitioning and development. Nitrogen-dependent cytokinin accumulation and the involvement of His-Asp phosphorelay systems are characteristic of this pathway. The coordination of both regulatory pathways seems to be crucially important for the integration of nitrogen signals at the whole plant level.     

Plant abiotic stress response and nutrient use efficiency

Abiotic stresses and soil nutrient limitations are major environmental conditions that reduce plant growth, productivity and quality.Plants have evolved mechanisms to perceive these environmental challenges, transmit the stress signals within cells as well as between cells and tissues, and make appropriate adjustments in their growth and development in order to survive and reproduce. In recent years, significant progress has been made on many fronts of the stress signaling research, particularly in understanding the downstream signaling events that culminate at the activation of stress-and nutrient limitation-responsive genes, cellular ion homeostasis, and growth adjustment. However, the revelation of the early events of stress signaling, particularly the identification of primary stress sensors, still lags behind. In this review, we summarize recent work on the genetic and molecular mechanisms of plant abiotic stress and nutrient limitation sensing and signaling and discuss new directions for future studies.     

Integration of C/N-nutrient and multiple environmental signals into the ABA signaling cascade

Due to their immobility, plants have developed sophisticated mechanisms to robustly monitor and appropriately respond to dynamic changes in nutrient availability. Carbon (C) and nitrogen (N) are especially important in regulating plant metabolism and development, thereby affecting crop productivity. In addition to their independent utilization, the ratio of C to N metabolites in the cell, referred to as the “C/N balance”, is important for the regulation of plant growth, although molecular mechanisms mediating C/N signaling remain unclear. Recently ABI1, a protein phosphatase type 2C (PP2C), was shown to be a regulator of C/N response in Arabidopsis plants. ABI1 functions as a negative regulator of abscisic acid (ABA) signal transduction. ABA is versatile phytohormone that regulates multiple aspects of plant growth and adaptation to environmental stress. This review highlights the regulation of the C/N response mediated by a non-canonical ABA signaling pathway that is independent of ABA biosynthesis, as well as recent findings on the direct crosstalk between multiple cellular signals and the ABA signaling cascade.     

Sugar Repression of a Gibberellin-Dependent Signaling Pathway in Barley Embryos

Increasing evidence shows that sugars can act as signals affecting plant metabolism and development. Some of the effects of sugars on plant growth and development suggest an interaction of sugar signals with hormonal regulation. We investigated the effects of sugars on the induction of [alpha]-amylase by gibberellic acid in barley embryos and aleurone layers. Our results show that sugar and hormonal signaling interact in the regulation of gibberellic acid-induced gene expression in barley grains. The induction of [alpha]-amylase by gibberellic acid in the aleurone layer is unaffected by the presence of sugars, but repression by carbohydrates is effective in the embryo. [alpha]-Amylase expression in the embryo is localized to the scutellar epithelium and is hormone and sugar modulated. The effects of glucose are independent from the effects of sugars on gibberellin biosynthesis. They are not due to an osmotic effect, they are independent of abscisic acid, and only hexokinase-phosphorylatable glucose analogs are able to trigger gene repression. Overall, the results suggest the existence of an interaction between the hormonal and metabolic regulation of [alpha]-amylase genes in barley grains.     

Why no feeding frenzy? Mechanisms of nutrient acquisition and utilization during infection by the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is a devastating pathogen of rice and wheat. It is a hemibiotroph that exhibits symptomless biotrophic growth for the first 4 to 5 days of infection of susceptible cultivars before becoming necrotrophic. Here, we review recent advances in our understanding of how M. oryzae is able to grow, acquire nutrients, and interact with the plant cell during infection. In particular, we describe direct mechanisms (such as the integration of carbon and nitrogen metabolism by trehalose-6-phosphate synthase 1) and indirect mechanisms (such as the suppression of host responses) that allow M. oryzae to utilize available host nutrient. We contrast the ability of M. oryzae to voraciously metabolize a wide range of carbon and nitrogen sources in vitro with the carefully orchestrated development it displays during the biotrophic phase of in planta growth and ask how the two observations can be reconciled. We also look at how nutrient acquisition and effector biology might be linked in order to facilitate rapid colonization of the plant host.     

Long-distance ABA Signaling and Its Relation to Other Signaling Pathways in the Detection of Soil Drying and the Mediation of the Plant’s Response to Drought

In this article we review evidence for a variety of long-distance signaling pathways involving hormones and nutrient ions moving in the xylem sap. We argue that ABA has a central role to play, at least in root-to-shoot drought stress signaling and the regulation of functioning, growth, and development of plants in drying soil. We also stress the importance of changes in the pH of the leaf cell apoplast as influenced both by edaphic and climatic variation, as a regulator of shoot growth and functioning, and we show how changes in xylem and apoplastic pH can affect the way in which ABA regulates stomatal behavior and growth. The sensitivity to drought of the pH/ABA sensing and signaling mechanism is emphasized. This allows regulation of plant growth, development and functioning, and particularly shoot water status, as distinct from stress lesions in growth and other processes as a reaction to perturbations such as soil drying.     

Cell type specificity of plant hormonal signals: Case studies and reflections on ethylene

In the light of increasing evidence that plant growth and development depend on signals perceived in distinct cell types where hormonal inputs are transformed into orchestrated responses triggering a plethora of physiological processes, we reflect on the case of ethylene signaling. Experimental approaches to address cell type-specificity of the ethylene response are discussed and future challenges in ethylene signaling studies are outlined.     

Receptor-like protein kinases: the keys to response

Plants are constantly challenged by changes in temperature, light, nutrient conditions, and exposure to pathogens and by other fluctuations in their environment. The molecular basis of how plants respond to these external factors is an active area of investigation. Plant cells often use receptors at the cell surface to sense environmental changes, and then transduce this information via activated signaling pathways to trigger adaptive responses. In Arabidopsis, the receptor-like protein kinase (RLK) gene family contains more than 600 members, many of which are likely to respond to the external challenges presented by an ever-changing environment. RLKs are involved in hormonal response pathways, cell differentiation, plant growth and development, self-incompatibility, and symbiont and pathogen recognition.     

生长素介导环境信号调控植物的生长发育

植物是一类营固着生活的自养型生物, 如何更好地适应周围环境对植物的生存至关重要。生长素是调控植物生长发育的重要激素之一。近年来的研究发现, 生长素不仅能够响应内在的发育信号, 而且能够介导各种环境信号, 参与植物生长发育和生长反应的调控。该文主要从光信号、温度信号、重力信号、营养元素和金属离子信号等方面重点阐述生长素如何介导上述各种不同的环境信号, 从而调控植物的生长发育。     

The role of strigolactones in root development

Strigolactones (SLs) and their derivatives were recently defined as novel phytohormones that orchestrate shoot and root growth. Levels of SLs, which are produced mainly by plant roots, increase under low nitrogen and phosphate levels to regulate plant responses. Here, we summarize recent work on SL biology by describing their role in the regulation of root development and hormonal crosstalk during root deve-lopment. SLs promote the elongation of seminal/primary roots and adventitious roots (ARs) and they repress lateral root formation. In addition, auxin signaling acts downstream of SLs. AR formation is positively or negatively regulated by SLs depending largely on the plant species and experimental conditions. The relationship between SLs and auxin during AR formation appears to be complex. Most notably, this hormonal response is a key adaption that radically alters rice root architecture in response to nitrogen- and phosphate-deficient conditions.     

Genome-wide patterns of carbon and nitrogen regulation of gene expression validate the combined carbon and nitrogen (CN)-signaling hypothesis in plants

Background  

Carbon and nitrogen are two signals that influence plant growth and development. It is known that carbon- and nitrogen-signaling pathways influence one another to affect gene expression, but little is known about which genes are regulated by interactions between carbon and nitrogen signaling or the mechanisms by which the different pathways interact.     

Autophagy in cellular growth control

Cell growth is regulated by two antagonistic processes: TOR signaling and autophagy. These processes integrate signals including growth factors, amino acids, and energy status to ensure that cell growth is appropriate to environmental conditions. Autophagy responds indirectly to the cellular milieu as a downstream inhibitory target of TOR signaling and is also directly controlled by nutrient availability, cellular energy status, and cell stress. The control of cell growth by TOR signaling and autophagy are relevant to disease, as altered regulation of either pathway results in tumorigenesis. Here we give an overview of how TOR signaling and autophagy integrate nutritional status to regulate cell growth, how these pathways are coordinately regulated, and how dysfunction of this regulation might result in tumorigenesis.     

Regulation of plant nitrate assimilation: from ecophysiology to brain proteins

Nitrogenous inorganic compounds impact plants as nutrients, signals and toxins. We are dissecting a regulatory network that controls nitrate assimilation at the level of nitrate reductase (NR) activity. The identification of protein kinase cascades, protein phosphatases and 14-3-3 proteins as regulators of NR are giving clues about how plants sense their nutrient availability, and use the information to signal changes in their metabolism and developmental strategies to cope with supplies. We hope that understanding these controls might lead to the design of transgenic plants with deregulated signalling networks, which would make them more efficient in using nitrogen fertilizers, and improving quality and yield of crops. There are circumstantial indications that gaseous anthropogenic nitrogenous emissions might also have complex regulatory influences on plant growth and development.