Cell-specific mechanisms and systemic signalling as emerging themes in light acclimation of C3 plants

Chloroplasts perform essential signalling functions in light acclimation and various stress responses in plants. Research on chloroplast signalling has provided fundamental information concerning the diversity of cellular responses to changing environmental conditions. Evidence has also accumulated indicating that different cell types possess specialized roles in regulation of leaf development and stress acclimation when challenged by environmental cues. Leaf veins are flanked by a layer of elongated chloroplast-containing bundle sheath cells, which due to their central position hold the potential to control the flux of information inside the leaves. Indeed, a specific role for bundle sheath cells in plant acclimation to various light regimes is currently emerging. Moreover, perception of light stress initiates systemic signals that spread through the vasculature to confer stress resistance in non-exposed parts of the plant. Such long-distance signalling functions are related to unique characteristics of reactive oxygen species and their detoxification in bundle sheath cells. Novel techniques for analysis of distinct tissue types, together with Arabidopsis thaliana mutants with vasculature-specific phenotypes, have proven instrumental in dissection of structural hierarchy among regulatory processes in leaves. This review emphasizes the current knowledge concerning the role of vascular bundle sheath cells in light-dependent acclimation processes of C3 plants.     

Light intensity-dependent retrograde signalling in higher plants

Plants are able to acclimate to highly fluctuating light environment and evolved a short- and long-term light acclimatory responses, that are dependent on chloroplasts retrograde signalling. In this review we summarise recent evidences suggesting that the chloroplasts act as key sensors of light intensity changes in a wide range (low, high and excess light conditions) as well as sensors of darkness. They also participate in transduction and synchronisation of systemic retrograde signalling in response to differential light exposure of distinct leaves. Regulation of intra- and inter-cellular chloroplast retrograde signalling is dependent on the developmental and functional stage of the plastids. Therefore, it is discussed in following subsections: firstly, chloroplast biogenic control of nuclear genes, for example, signals related to photosystems and pigment biogenesis during early plastid development; secondly, signals in the mature chloroplast induced by changes in photosynthetic electron transport, reactive oxygen species, hormones and metabolite biosynthesis; thirdly, chloroplast signalling during leaf senescence. Moreover, with a help of meta-analysis of multiple microarray experiments, we showed that the expression of the same set of genes is regulated specifically in particular types of signals and types of light conditions. Furthermore, we also highlight the alternative scenarios of the chloroplast retrograde signals transduction and coordination linked to the role of photo-electrochemical signalling.     

Signalling crosstalk in light stress and immune reactions in plants

The evolutionary history of plants is tightly connected with the evolution of microbial pathogens and herbivores, which use photosynthetic end products as a source of life. In these interactions, plants, as the stationary party, have evolved sophisticated mechanisms to sense, signal and respond to the presence of external stress agents. Chloroplasts are metabolically versatile organelles that carry out fundamental functions in determining appropriate immune reactions in plants. Besides photosynthesis, chloroplasts host key steps in the biosynthesis of amino acids, stress hormones and secondary metabolites, which have a great impact on resistance against pathogens and insect herbivores. Changes in chloroplast redox signalling pathways and reactive oxygen species metabolism also mediate local and systemic signals, which modulate plant resistance to light stress and disease. Moreover, interplay among chloroplastic signalling networks and plasma membrane receptor kinases is emerging as a key mechanism that modulates stress responses in plants. This review highlights the central role of chloroplasts in the signalling crosstalk that essentially determines the outcome of plant–pathogen interactions in plants.     

Hydrogen peroxide: a Jekyll and Hyde signalling molecule

Reactive oxygen species (ROS) are a group of molecules produced in the cell through metabolism of oxygen. Endogenous ROS such as hydrogen peroxide (H₂O₂) have long been recognised as destructive molecules. The well-established roles they have in the phagosome and genomic instability has led to the characterisation of these molecules as non-specific agents of destruction. Interestingly, there is a growing body of literature suggesting a less sinister role for this Jekyll and Hyde molecule. It is now evident that at lower physiological levels, H₂O₂ can act as a classical intracellular signalling molecule regulating kinase-driven pathways. The newly discovered biological functions attributed to ROS include proliferation, migration, anoikis, survival and autophagy. Furthermore, recent advances in detection and quantification of ROS-family members have revealed that the diverse functions of ROS can be determined by the subcellular source, location and duration of these molecules within the cell. In light of this confounding paradox, we will examine the factors and circumstances that determine whether H₂O₂ acts in a pro-survival or deleterious manner.     

Oxidant and antioxidant signalling in plants: a re-evaluation of the concept of oxidative stress in a physiological context

While the chemical nature of reactive oxygen species (ROS) dictates that they are potentially harmful to cells, recent genetic evidence suggests that in planta purely physicochemical damage may be much more limited than previously thought. The most potentially deleterious effect of ROS under most conditions is that at high concentrations they trigger genetically programmed cell suicide events. Moreover, because plants use ROS as second messengers in signal transduction cascades in processes as diverse as mitosis, tropisms and cell death, their accumulation is crucial to plant development as well as defence. Direct ROS signal transduction will ensue only if ROS escape destruction by antioxidants or are otherwise consumed in a ROS cascade. Thus, the major low molecular weight antioxidants determine the specificity of the signal. They are also themselves signal-transducing molecules that can either signal independently or further transmit ROS signals. The moment has come to re-evaluate the concept of oxidative stress. In contrast to this pejorative or negative term, implying a state to be avoided, we propose that the syndrome would be more usefully described as 'oxidative signalling', that is, an important and critical function associated with the mechanisms by which plant cells sense the environment and make appropriate adjustments to gene expression, metabolism and physiology.     

Tetrapyrrole‐based drought stress signalling

Tetrapyrroles such as chlorophyll and heme play a vital role in primary plant metabolic processes such as photosynthesis and respiration. Over the past decades, extensive genetic and molecular analyses have provided valuable insights into the complex regulatory network of the tetrapyrrole biosynthesis. However, tetrapyrroles are also implicated in abiotic stress tolerance, although the mechanisms are largely unknown. With recent reports demonstrating that modified tetrapyrrole biosynthesis in plants confers wilting avoidance, a component physiological trait to drought tolerance, it is now timely that this pathway be reviewed in the context of drought stress signalling. In this review, the significance of tetrapyrrole biosynthesis under drought stress is addressed, with particular emphasis on the inter‐relationships with major stress signalling cascades driven by reactive oxygen species (ROS) and organellar retrograde signalling. We propose that unlike the chlorophyll branch, the heme branch of the pathway plays a key role in mediating intracellular drought stress signalling and stimulating ROS detoxification under drought stress. Determining how the tetrapyrrole biosynthetic pathway is involved in stress signalling provides an opportunity to identify gene targets for engineering drought‐tolerant crops.     

The influence of the light environment and photosynthesis on oxidative signalling responses in plant–biotrophic pathogen interactions

Plants grow in a constantly fluctuating environment, which has driven the evolution of a highly flexible metabolism and development necessary for their sessile lifestyle. In contrast to the situation in the natural world, the detailed dissection of the regulatory networks that govern plants' responses to abiotic insults and their interaction with pathogens have been studied almost exclusively in controlled environments where a single challenge has been applied. However, the question arises of how such pathways operate when the plant is subjected to multiple stresses, especially where the expression of overlapping gene sets and common signalling molecules, such as reactive oxygen species (ROS), are implicated. This review will focus on the responsiveness of leaves to their light environment and how this might influence both basal and induced resistance to infection by biotrophic pathogens. While several signalling pathways operate in a complex network of defence responses, the functioning of the salicylic acid (SA) signalling pathway will receive specific consideration. This is because information is becoming available of its role in abiotic stress responses and it dependency on light. This article covers several topics, some of which formerly have received scant attention. These include the effects of infection on photosynthetic performance and carbohydrate metabolism, the parallels between the induction of acclimation to high light and immunity to pathogens, the role of light in the functioning of the SA signalling pathway and the light sensitivity of lesion formation and the use of lesion mimic mutants and transgenic plants. Finally, a model is proposed that attempts to extrapolate these controlled environment-based studies to the functioning of defences against pathogens in a field-grown crop.     

Asperpyrone A attenuates RANKL‐induced osteoclast formation through inhibiting NFATc1, Ca2+ signalling and oxidative stress

Imbalance of osteoblast and osteoclast in adult leads to a variety of bone‐related diseases, including osteoporosis. Thus, suppressing the activity of osteoclastic bone resorption becomes the main therapeutic strategy for osteoporosis. Asperpyrone A is a natural compound isolated from Aspergillus niger with various biological activities of antitumour, antimicrobial and antioxidant. The present study was designed to investigate the effects of Asperpyrone A on osteoclastogenesis and to explore its underlining mechanism. We found that Asperpyrone A inhibited RANKL‐induced osteoclastogenesis in a dose‐dependent manner when the concentration reached 1 µm, and with no cytotoxicity until the concentration reached to 10 µm. In addition, Asperpyrone A down‐regulated the mRNA and protein expression of NFATc1, c‐fos and V‐ATPase‐d2, as well as the mRNA expression of TRAcP and Ctsk. Furthermore, Asperpyrone A strongly attenuated the RNAKL‐induced intracellular Ca²⁺ oscillations and ROS (reactive oxygen species) production in the process of osteoclastogenesis and suppressed the activation of MAPK and NF‐κB signalling pathways. Collectively, Asperpyrone A attenuates RANKL‐induced osteoclast formation via suppressing NFATc1, Ca²⁺ signalling and oxidative stress, as well as MAPK and NF‐κB signalling pathways, indicating that this compound may become a potential candidate drug for the prevention or treatment of osteoporosis.     

Salicylic acid antagonism of EDS1‐driven cell death is important for immune and oxidative stress responses in Arabidopsis

Reactive oxygen species (ROS) have emerged as signals in the responses of plants to stress. Arabidopsis Enhanced Disease Susceptibility1 (EDS1) regulates defense and cell death against biotrophic pathogens and controls cell death propagation in response to chloroplast‐derived ROS. Arabidopsis Nudix hydrolase7 (nudt7) mutants are sensitized to photo‐oxidative stress and display EDS1‐dependent enhanced resistance, salicylic acid (SA) accumulation and initiation of cell death. Here we explored the relationship between EDS1, EDS1‐regulated SA and ROS by examining gene expression profiles, photo‐oxidative stress and resistance phenotypes of nudt7 mutants in combination with eds1 and the SA‐biosynthetic mutant, sid2. We establish that EDS1 controls steps downstream of chloroplast‐derived O₂^?? that lead to SA‐assisted H₂O₂ accumulation as part of a mechanism limiting cell death. A combination of EDS1‐regulated SA‐antagonized and SA‐promoted processes is necessary for resistance to host‐adapted pathogens and for a balanced response to photo‐oxidative stress. In contrast to SA, the apoplastic ROS‐producing enzyme NADPH oxidase RbohD promotes initiation of cell death during photo‐oxidative stress. Thus, chloroplastic O₂^?? signals are processed by EDS1 to produce counter‐balancing activities of SA and RbohD in the control of cell death. Our data strengthen the idea that EDS1 responds to the status of O₂^?? or O₂^??‐generated molecules to coordinate cell death and defense outputs. This activity may enable the plant to respond flexibly to different biotic and abiotic stresses in the environment.     

Reactive oxygen species stimulate homologous recombination in plants

Coping with the continuous production of free radicals is a daily routine of the cell. Despite their toxicity, the reactive oxygen species (ROS) are involved in dual physiological action – signal transduction and immune response. We analysed the influence of oxidative stress‐generating compounds, rose Bengal (RB), paraquat (PQ) and amino‐triazole (ATZ) on the genome stability of Arabidopsis using transgenic recombination‐monitoring plants. Homologous recombination frequencies in plants were increased upon the treatment with RB and PQ but not ATZ. Application of the N‐acetyl‐L ‐cysteine (NAC), radicals scavenging compound, decreased the DNA damage caused by RB. Interestingly, the incubation of plants with very low concentration of RB (less than 0.2 µM ) led to the subsequent increase in plant tolerance to methyl methane sulfonate (MMS): stronger plants with a lower increase of homologous recombination frequency. In contrast, the incubation of plants with 0.5 µM of RB resulted in the potentiation of the MMS effect: the weaker plants with higher frequency of recombination. The data of the present study suggest the existence of a dual concentration‐dependent role of ROS in plants.     

Physiological roles of nonselective cation channels in plants: from salt stress to signalling and development

Nonselective cation channels (NSCCs) catalyse passive fluxes of cations through plant membranes. NSCCs do not, or only to a small extent, select between monovalent cations, and several are also permeable to divalent cations. Although a number of NSCC genes has been identified in plant genomes, a direct correlation between gene products and in vivo observed currents is still largely absent for most NSCCs. In this review, physiological functions and molecular properties of NSCCs are critically discussed. Recent studies have demonstrated that NSCCs are directly involved in a multitude of stress responses, growth and development, uptake of nutrients and calcium signalling. NSCCs can also function in the perception of external stimuli and as signal transducers for reactive oxygen species, pathogen elicitors, cyclic nucleotides, membrane stretch, amino acids and purines.     

强光下硫化氢通过促进光系统II的活性来缓解水稻的光抑制

以水稻品种‘II优084’为材料,测定了强光胁迫下,水稻光合速率、叶绿素荧光快速诱导曲线（OJIP）以及O2ˉ·和H2O2在水稻叶片中积累的影响。结果表明强光胁迫下,水稻的净光合速率及气孔导度下降;光系统II（PSII）反应中心关闭的比例以及电子传递链中光系统II受体侧原初醌受体（QA）的还原程度增加;PSII反应中心电子传递的量子产额、能量以及传递到下游电子链的比率下降;光抑制下PSII的过剩能量向PSI的状态装换减少;自由基的产生增加。而施加作为硫化氢（H2S）供体的外源硫氢化钠（NaHS）后,上述影响PSII活性的指标的负变化被缓解,捕光天线复合体LHC通过在两个光系统之间的移动,来调节两个光系统的能量分配。强光下H2S处理能促进LHC离开PSII,与PSI结合,从而减少PSII分配的激发能,增加PSI分配的激发能,缓解了PSII的过度还原。以上结果表明外源H2S通过促进PSII的光合活性来缓解水稻光抑制伤害。     

基于光子学技术研究弱光胁迫早期拟南芥光合损伤机理

弱光限制植物的光合作用,降低了光合作用效率,造成农业产量下降.本文主要研究了弱光处理早期,拟南芥光合作用相关指标的变化.研究中发现在弱光处理的早期,植株生长表型和最大光化学效率（Fv/Fm）没有明显变化,实际光化学效率Y（Ⅱ）以及光系统电子传递效率（ETR）下降较明显.此外,弱光处理原生质体,利用2 ＇,7＇-二氯二氢荧光素二乙酯（dichlorofluorescin diacetate,H2DCF-DA）染色,共聚焦显微镜观察,发现细胞中有较明显的活性氧（ROS）合成,且定位于叶绿体.该研究结果为植物弱光耐受性的研究提供理论依据.