Fungal–plant signalling in the Ustilago maydis–maize pathosystem

The smut fungus Ustilago maydis needs the host plant maize for completion of its sexual life cycle. Recent experiments highlight the importance of cAMP and mitogen-activated protein (MAP) kinase signalling for cell fusion as well as for subsequent stages of plant colonisation and induction of disease symptoms. During these distinct developmental stages the same signalling cascades must be differentially regulated and accommodate multiple inputs and outputs.     

Signalling between arbuscular mycorrhizal fungi and plants: identification of a gene expressed during early interactions by differential RNA display analysis

Plant and Soil - Although there is evidence for an interplay of signalling/recognition events at different stages during plant/fungal interactions in arbuscular mycorrhiza, the nature of signalling...     

Cold stress signalling in female reproductive tissues

Cold stress is a significant threat for plant productivity and impacts on plant distribution and crop production, particularly so when it occurs during the growth phase. A developmental stage at risk is that of flowering, since a single stress event during sensitive stages, such as the full‐bloom stage of fruit trees can be fatal for reproductive success. Although pollen development and fertilization are widely viewed as the most critical reproductive phases, the development and function of female reproductive tissues, which in Angiosperms are embedded in the gynoecium, are also affected by cold stress. Today however, we have essentially no understanding of the cold stress response pathways that act during floral organogenesis. In this review, we briefly summarize our current knowledge of cold stress signalling modules active in vegetative tissues that may provide a framework of general principles also transferable to female reproductive tissues. We then align these signalling cascades with those that govern gynoecium development to identify factors that may act in both processes and could thereby contribute to cold stress responses in female reproductive tissues.     

Of light and length: regulation of hypocotyl growth in Arabidopsis

At all stages, plant development results from a complex integration of multiple endogenous and environmental signals. The sedentary nature of plants strongly enhances the impact of the environment on plant development as compared to animal development. The embryonic and postembryonic seedling stem, called the hypocotyl, of the model species Arabidopsis (thale cress) has proved to be an excellent system for studying such signal interplay in the regulation of growth and developmental responses. The extension of the hypocotyl, which is regulated by a network of interacting factors, including light and plant hormones, is such a process. These regulatory factors often reciprocally regulate their biosynthesis and/or signalling. Here we present the current state of knowledge about the regulation of hypocotyl growth by a large repertoire of internal and external cues.     

Cysteine-rich peptides (CRPs) mediate diverse aspects of cell-cell communication in plant reproduction and development

Cell-cell communication in plants is essential for the correct co-ordination of reproduction, growth, and development. Studies to dissect this mode of communication have previously focussed primarily on the action of plant hormones as mediators of intercellular signalling. In animals, peptide signalling is a well-documented intercellular communication system, however, relatively little is known about this system in plants. In recent years, numerous reports have emerged about small, secreted peptides controlling different aspects of plant reproduction. Interestingly, most of these peptides are cysteine-rich, and there is convincing evidence suggesting multiple roles for related cysteine-rich peptides (CRPs) as signalling factors in developmental patterning as well as during plant pathogen responses and symbiosis. In this review, we discuss how CRPs are emerging as key signalling factors in regulating multiple aspects of vegetative growth and reproductive development in plants.     

Different NaCl-Induced Calcium Signatures in the Arabidopsis thaliana Ecotypes Col-0 and C24

A common feature of stress signalling pathways are alterations in the concentration of cytosolic free calcium ([Ca²⁺]_cyt), which allow the specific and rapid transmission of stress signals through a plant after exposure to a stress, such as salinity. Here, we used an aequorin based bioluminescence assay to compare the NaCl-induced changes in [Ca²⁺]_cyt of the Arabidopsis ecotypes Col-0 and C24. We show that C24 lacks the NaCl specific component of the [Ca²⁺]_cyt signature compared to Col-0. This phenotypic variation could be exploited as a screening methodology for the identification of yet unknown components in the early stages of the salt signalling pathway.     

Receptors and signalling components of plant hormones

Recent advances in understanding plant hormonal signalling has resulted in the identification of a variety of signalling components including receptor kinases with homology to the bacterial two component system as well as serine/threonine kinases and protein phosphatases. In addition, the existence of MAP kinase pathways in plants indicates a similar role of these signalling cascades in the relay of exogenous signals into the nucleus as has been disclosed in animal cells. The emerging signalling pathways of the plant hormone abscisic acid and ethylene are presented.     

Pathological hormone imbalances

Plant hormones play important roles in regulating developmental processes and signalling networks involved in plant responses to a wide range of biotic and abiotic stresses. Salicylic acid (SA), jasmonates (JA) and ethylene (ET) are well known to play crucial roles in plant disease and pest resistance. However, the roles of other hormones such as abscisic acid (ABA), auxin, gibberellin (GA), cytokinin (CK) and brassinosteroid (BL) in plant defence are less well known. Much progress has been made in understanding plant hormone signalling and plant disease resistance. However, these studies have mostly proceeded independently of each other, and there is limited knowledge regarding interactions between plant hormone-mediated signalling and responses to various pathogens. Here, we review the roles of hormones other than SA, JA and ET in plant defence and the interactions between hormone-mediated signalling, plant defence and pathogen virulence. We propose that these hormones may influence disease outcomes through their effect on SA or JA signalling.     

Stromuli. Plastiden‐Brücken im Netzwerk der Zelle

Plastidic bridges in the plant cell network: Stromules Stromules are mobile protrusions emanating from plastids. They might form bridges between plastids and connect them also with other compartments of the plant cell. They could be involved in coordination of plastid activities and in signalling. Stromules have been first observed in the water fern Selaginella more than 100 years ago. Later improved light microscopy enabled the visualization of stromules in higher plant plastids. 15 years ago, since plants accumulating the green fluorescing proteins (GPF) in the stroma became available they have been newly detected and are now studied intensively. Formation of stromules differs among plant tissues, developmental stages and environmental situations. Actin and myosin are required for the formation of stromules.     

Receptors and Signalling Components of Plant Hormones

Abstract

Recent advances in understanding plant hormonal signalling has resulted in the identification of a variety of signalling components including receptor kinases with homology to the bacterial two component system as well as serine/threonine kinases and protein phosphatases. In addition, the existence of MAP kinase pathways in plants indicates a similar role of these signalling cascades in the relay of exogenous signals into the nucleus as has been disclosed in animal cells. The emerging signalling pathways of the plant hormone abscisic acid and ethylene are presented.     

The role of nitric oxide in the germination of plant seeds and pollen

Two complex physiological processes, with opposite positions in the plant's life-cycle, seed and pollen germination, are vital to the accomplishment of successful plant growth and reproduction. This review summarizes the current state of knowledge of the intersection of NO signalling with the signalling pathways of ABA, GA, and ethylene; plant hormones that control the release of plant seeds from dormancy and germination. The cross-talk of NO and ROS is involved in the light- and hormone-specific regulation of seeds’ developmental processes during the initiation of plant ontogenesis. Similarly to seed germination, the mechanisms of plant pollen hydration, germination, tube growth, as well as pollen-stigma recognition are tightly linked to the proper adjustment of NO and ROS levels. The interaction of NO with ROS and secondary messengers such as Ca²⁺, cAMP and cGMP discovered in pollen represent a common mechanism of NO signalling. The involvement of NO in both breakpoints of plant physiology, as well as in the germination of spores within fungi and oomycetes, points toward NO as a component of an evolutionary conserved signalling pathway.     

Plant extracellular ATP signalling: new insight from proteomics

Complex signalling systems have evolved in multicellular organisms to enable cell-to-cell communication during growth and development. In plants, cell communication via the extracellular matrix (apoplast) controls many processes vital for plant survival. Secretion of ATP into the extracellular matrix is now recognised as a previously unknown stimulus for cell signalling with a role in many aspects of plant physiology. In the last decade, the secondary messenger molecules in extracellular ATP signalling were identified, but the downstream gene and protein networks that underpin plant responses to extracellular ATP are only beginning to be characterised. Here we review the current status of our knowledge of plant extracellular signalling and demonstrate how applying state-of-the art proteomic technologies is rapidly bringing new discoveries in extracellular ATP research. We discuss how monitoring of the global proteomic profile during responses to modulation of extracellular ATP signalling has led to novel insight into pathogen defence systems and plant programmed cell death regulation. On the basis of extensive proteomic, pharmacological, and reverse genetics data, extracellular ATP has been confirmed to constitute an important molecular switch that tightly controls organellar energy metabolism, reprogramming of primary metabolic pathways, and redirection of resources to protein networks that support adaptation of plants to stress.     

Modulation of nitric oxide bioactivity by plant haemoglobins

Nitric oxide (NO) is a highly reactive signalling molecule that has numerous targets in plants. Both enzymatic and non-enzymatic synthesis of NO has been detected in several plant species, and NO functions have been characterized during diverse physiological processes such as plant growth, development, and resistance to biotic and abiotic stresses. This wide variety of effects reflects the basic signalling mechanisms that are utilized by virtually all mammalian and plant cells and suggests the necessity of detoxification mechanisms to control the level and functions of NO. During the last two years an increasing number of reports have implicated non-symbiotic haemoglobins as the key enzymatic system for NO scavenging in plants, indicating that the primordial function of haemoglobins may well be to protect against nitrosative stress and to modulate NO signalling functions. The biological relevance of plant haemoglobins during specific conditions of plant growth and stress, and the existence of further enzymatic and non-enzymatic NO scavenging systems, suggest the existence of precise NO modulation mechanisms in plants, as observed for different NO sources.     

An effector of a necrotrophic fungal pathogen targets the calcium-sensing receptor in chloroplasts to inhibit host resistance

SsITL, a secretory protein of the necrotrophic phytopathogen Sclerotinia sclerotiorum, was previously reported to suppress host immunity at the early stages of infection. However, the molecular mechanism that SsITL uses to inhibit plant defence against S. sclerotiorum has not yet been elucidated. Here, we report that SsITL interacted with a chloroplast-localized calcium-sensing receptor, CAS, in chloroplasts. We found that CAS is a positive regulator of the salicylic acid signalling pathway in plant immunity to S. sclerotiorum and CAS-mediated resistance against S. sclerotiorum depends on Ca²⁺ signalling. Furthermore, we showed that SsITL could interfere with the plant salicylic acid (SA) signalling pathway and SsITL-expressing transgenic plants were more susceptible to S. sclerotiorum. However, truncated SsITLs (SsITL-NT1 or SsITL-CT1) that lost the ability to interact with CAS do not affect plant resistance to S. sclerotiorum. Taken together, our findings reveal that SsITL inhibits SA accumulation during the early stage of infection by interacting with CAS and then facilitating the infection by S. sclerotiorum.     

Reactive oxygen and nitrogen species and glutathione: key players in the legume-Rhizobium symbiosis

Several reactive oxygen and nitrogen species (ROS/RNS) are continuously produced in plants as by-products of aerobic metabolism or in response to stresses. Depending on the nature of the ROS and RNS, some of them are highly toxic and rapidly detoxified by various cellular enzymatic and non-enzymatic mechanisms. Whereas plants have many mechanisms with which to combat increased ROS/RNS levels produced during stress conditions, under other circumstances plants appear to generate ROS/RNS as signalling molecules to control various processes encompassing the whole lifespan of the plant such as normal growth and development stages. This review aims to summarize recent studies highlighting the involvement of ROS/RNS, as well as the low molecular weight thiols, glutathione and homoglutathione, during the symbiosis between rhizobia and leguminous plants. This compatible interaction initiated by a molecular dialogue between the plant and bacterial partners, leads to the formation of a novel root organ capable of fixing atmospheric nitrogen under nitrogen-limiting conditions. On the one hand, ROS/RNS detection during the symbiotic process highlights the similarity of the early response to infection by pathogenic and symbiotic bacteria, addressing the question as to which mechanism rhizobia use to counteract the plant defence response. Moreover, there is increasing evidence that ROS are needed to establish the symbiosis fully. On the other hand, GSH synthesis appears to be essential for proper development of the root nodules during the symbiotic interaction. Elucidating the mechanisms that control ROS/RNS signalling during symbiosis could therefore contribute in defining a powerful strategy to enhance the efficiency of the symbiotic interaction.     

Signalling via plasmodesmata—the neglected pathway

This review considers recent studies on the role of plasmodesmata in the conduction of small solutes and signalling molecules between plant cells. The substructure of plasmodesmata is described in relation to the potential pathways available for symplastic signalling between cells. At least two discrete pathways are available for transport through plasmodesmata, the cytoplasmic sleeve between the desmotubule and the plasmalemma, and the endoplasmic reticulum which connects contiguous cells via the central desmotubule. This latter pathway has been shown recently to function as a dynamic continuum for the movement of lipids and lipid-signalling molecules between plant cells. The role of plasmodesmata in the conduction of hormones and electrical signals is also considered, as is the potential for movement of macromolecular signalling molecules via the symplast. The factors which regulate plasmodesmatal conductance and the significance of symplast 'domains' are discussed in relation to the control of movement of signalling molecules in the symplast.