Recent advances in plant immunity: recognition, signaling, response, and evolution

Innate immune system is employed by plants to defend against phytopathogenic microbes through specific perception of non-self molecules and subsequent initiation of resistance responses. Current researches elucidate that plants mostly rely on cell surface-located pattern recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat proteins (NB-LRRs) to recognize pathogen-associated molecular patterns (PAMPs) and effector proteins from microbial pathogens, initiating PAMP- and effector-triggered immunity (PTI and ETI), respectively. Some pathogenic bacterial effector proteins are usually secreted into plant cells and play a virulence function by suppressing plant PTI, implying an evolutionary process of plant immunity from PTI to ETI. In the past several years, a great progress has been achieved to reveal fascinating molecular mechanisms underlying the pathogenic recognition, resistance signaling transduction, and plant immunity evolution. Here, we summarized the latest breakthroughs about these topics, and offered an integral understanding of plant molecular immunity.     

类受体激酶FER调节植物与病原菌相互作用的分子机制

植物细胞依赖细胞质膜上的受体感知并传递环境信号,而受体通过与配体特异结合启动一系列下游信号转导途径,维持植物正常的生命活动及其对外界环境变化的适应。类受体激酶是其中一类重要受体,通常由胞外结合结构域、跨膜结构域和胞内激酶结构域3部分组成,是植物适应外界环境变化的重要调节枢纽。FER属于CrRLK1L类受体蛋白激酶家族,...     

Plant programmed cell death: can't live with it; can't live without it

The decision of whether a cell should live or die is fundamental for the wellbeing of all organisms. Despite intense investigation into cell growth and proliferation, only recently has the essential and equally important idea that cells control/programme their own demise for proper maintenance of cellular homeostasis gained recognition. Furthermore, even though research into programmed cell death (PCD) has been an extremely active area of research there are significant gaps in our understanding of the process in plants. In this review, we discuss PCD during plant development and pathogenesis, and compare/contrast this with mammalian apoptosis.     

MAMPs and MIMPs: proposed classifications for inducers of innate immunity

Plants encode a sophisticated innate immune system. Resistance against potential pathogens often relies on active responses. Prerequisite to the induction of defences is recognition of the pathogenic threat. Significant advances have been made in our understanding of the non-self molecules that are recognized by plants and the means by which plants perceive them. Established terms describing these recognition events, including microbe-associated molecular pattern (MAMP), MAMP-receptor, effector, and resistance (R) protein, need clarification to represent our current knowledge adequately. In this review we propose criteria to classify inducers of plant defence as either MAMPs or microbe-induced molecular patterns (MIMPs). We refine the definition of MAMP to mean a molecular sequence or structure in ANY pathogen-derived molecule that is perceived via direct interaction with a host defence receptor. MIMPs are modifications of host-derived molecules that are induced by an intrinsic activity of a pathogen-derived effector and are perceived by a host defence receptor. MAMP-receptors have previously been classified separately from R-proteins as a discrete class of surveillance molecules. However, MAMP-receptors and R-proteins cannot be distinguished on the basis of their protein structures or their induced responses. We propose that MAMP-receptors and MIMP-receptors are each a subset of R-proteins. Although our review is based on examples from plant pathogens and plants, the principles discussed might prove applicable to other organisms.     

Plant invaders and their novel natural enemies: who is naïve?

Introduced exotic species encounter a wide range of non‐coevolved enemies and competitors in their new range. Evolutionary novelty is a key aspect of these interactions, but who benefits from novelty: the exotic species or their new antagonists? Paradoxically, the novelty argument has been used to explain both the release from and the suppression by natural enemies. We argue that this paradox can be solved by considering underlying interaction mechanisms. Using plant defenses as a model, we argue that mismatches between plant and enemy interaction traits can enhance plant invasiveness in the case of toxin‐based defenses, whereas invasiveness is counteracted by mismatches in recognition‐based defenses and selective foraging of generalist herbivores on plants with rare toxins. We propose that a mechanistic understanding of ecological mismatches can help to explain and predict when evolutionary novelty will enhance or suppress exotic plant invasiveness. This knowledge may also enhance our understanding of plant abundance following range expansion, or during species replacements along successional stages.     

Programming good relations--development of the arbuscular mycorrhizal symbiosis

The majority of plants live in symbiotic associations with fungi or bacteria that improve their nutrition. Critical steps in a symbiosis are mutual recognition and subsequently the establishment of an intimate association, which involves the penetration of plant tissues and, in many cases, the invasion of individual host cells by the microbial symbiont. Recent advances revealed that in the arbuscular mycorrhizal symbiosis with soil fungi of the order Glomeromycota, plant-derived signals attract fungal hyphae and stimulate their growth. Upon physical attachment of the fungal symbiont to the root surface, an active plant developmental program prepares the epidermal cells for penetration by the fungus. Thus, plants actively help symbiotic fungi to colonize their roots rather than just tolerating them.     

Plant immunity: a lesson from pathogenic bacterial effector proteins

Phytopathogenic bacteria inject an array of effector proteins into host cells to alter host physiology and assist the infection process. Some of these effectors can also trigger disease resistance as a result of recognition in the plant cell by cytoplasmic immune receptors. In addition to effector-triggered immunity, plants immunity can be triggered upon the detection of Pathogen/Microbe-Associated Molecular Patterns by surface-localized immune receptors. Recent progress indicates that many bacterial effector proteins use a variety of biochemical properties to directly attack key components of PAMP-triggered immunity and effector-triggered immunity, providing new insights into the molecular basis of plant innate immunity. Emerging evidence indicate that the evolution of disease resistance in plants is intimately linked to the mechanism by which bacterial effectors promote parasitism. This review focuses on how these studies have conceptually advanced our understanding of plant–pathogen interactions.     

Bacterial lipopolysaccharides induce defense responses associated with programmed cell death in rice cells

PAMP (pathogen-associated molecular pattern) recognition plays an important role during the innate immune response in both plants and animals. Lipopolysaccharides (LPS) derived from Gram-negative bacteria are representative of typical PAMP molecules and have been reported to induce defense-related responses, including the suppression of the hypersensitive response, the expression of defense genes and systemic resistance in plants. However, the details regarding the precise molecular mechanisms underlying these cellular responses, such as the molecular machinery involved in the perception and transduction of LPS molecules, remain largely unknown. Furthermore, the biological activities of LPS on plants have so far been reported only in dicots and no information is thus available regarding their functions in monocots. In our current study, we report that LPS preparations for various becteria, including plant pathogens and non-pathogens, can induce defense responses in rice cells, including reactive oxygen generation and defense gene expression. In addition, global analysis of gene expression induced by two PAMPs, LPS and chitin oligosaccharide, also reveals a close correlation between the gene responses induced by these factors. This indicates that there is a convergence of signaling cascades downstream of their corresponding receptors. Furthermore, we show that the defense responses induced by LPS in the rice cells are associated with programmed cell death (PCD), which is a finding that has not been previously reported for the functional role of these molecules in plant cells. Interestingly, PCD induction by the LPS was not detected in cultured Arabidopsis thaliana cells.     

Plant ecoacoustics: a sensory ecology approach

Many interactions of plants with the environment have an acoustic component, including the actions of herbivores and pollinators, wind and rain. Although plants have long been tested for their response to single tones or music, their response to naturally occurring sources of sound and vibration is barely explored. We argue that progress in understanding the ecology and evolution of plant acoustic sensing requires testing how plants respond to acoustic features of their natural environments, using methods that precisely measure and reproduce the stimulus experienced by the plant.     

植物化学通讯研究进展

 生物的信息传递是生命科学中引人入胜的研究领域之一,生物种间种内和个体内都存在着物理和化学等各种信息交流方式。植物种间种内是否通过物理信号进行通讯交流还是一个未知数,但邻近的同种或异种植物通过化学物质为媒介的通讯关系确是客观存在的。最近,愈来愈多的研究证明：许多陆生植物种可以合成并释放特定的次生物质,这些次生物质可以通过空气和土壤两种载体进行信息传递,尤其是在植物受到侵袭和寄生条件下。茉莉酮酸甲酯、水杨酸甲酯和乙烯等挥发性次生物质被确证为以空气为媒介进行植物种间和种内通讯的化学信号分子。植物根分泌的黄酮和氢醌等分子也可以经土壤媒介传递信息。由于在自然条件下植物根系分泌物的收集和活性信号分子的俘获及鉴定技术还未能突破,这增加了以土壤为媒介的植物种间和种内化学通讯关系研究的难度。但不论如何,植物的化学通讯是植物种间和种内交流的主要方式,植物间的化学通讯关系的研究还处于突破的前夜,这方面的任一研究成果都会引起世界性的关注。因此,破译植物种间和种内化学通讯密码具有重要的学术价值。     

Functional aspects of cell patterning in aerial epidermis

Plants have evolved epidermal cells that have specialized functions as adaptations to life on land. Many of the functions of these specialized cells are dependent, to a significant extent, on their arrangement within the aerial epidermis. Considerable progress has been made over the past two years in understanding the patterning mechanisms of trichomes and stomata in Arabidopsis leaves at the molecular level. How universal are these patterning programmes, and how are they adjusted to meet the changing functions of specialized epidermal cells in different plant organs? In this review, we compare the patterning of stomata and trichomes in different plant species, describe environmental and developmental factors that alter cell patterning, and discuss how changes in patterning might relate to cell function. Patterning is an important aspect to the functioning of aerial epidermal cells, and a greater understanding of the processes that are involved will significantly enhance our understanding of how cellular activities are integrated in multicellular plants.     

Plant systems for recognition of pathogen-associated molecular patterns

Research of the last decade has revealed that plant immunity consists of different layers of defense that have evolved by the co-evolutional battle of plants with its pathogens. Particular light has been shed on PAMP- (pathogen-associated molecular pattern) triggered immunity (PTI) mediated by pattern recognition receptors. Striking similarities exist between the plant and animal innate immune system that point for a common optimized mechanism that has evolved independently in both kingdoms. Pattern recognition receptors (PRRs) from both kingdoms consist of leucine-rich repeat receptor complexes that allow recognition of invading pathogens at the cell surface. In plants, PRRs like FLS2 and EFR are controlled by a co-receptor SERK3/BAK1, also a leucine-rich repeat receptor that dimerizes with the PRRs to support their function. Pathogens can inject effector proteins into the plant cells to suppress the immune responses initiated after perception of PAMPs by PRRs via inhibition or degradation of the receptors. Plants have acquired the ability to recognize the presence of some of these effector proteins which leads to a quick and hypersensitive response to arrest and terminate pathogen growth.     

Gene-for-gene interactions: bacterial avirulence proteins specify plant disease resistance

Resistance of plants to bacterial pathogens is often controlled by corresponding genes for resistance and avirulence in host and pathogen, respectively. Fifty years after discovery of the genetic basis of gene-for-gene interactions, several avirulence and plant resistance genes have been isolated and are being studied on the molecular level. Tremendous progress has been made due to a better understanding of type III secretion systems that are required for bacterial pathogenicity. We are beginning to grasp how the plant actually recognizes bacterial avirulence determinants. The current view is that the bacterium translocates avirulence proteins into the host cell by the Hrp type III secretion system and that recognition occurs in the plant cell.     

Avirulence and resistance genes in the Cladosporium fulvum-tomato interaction.

The fungus Cladosporium fulvum infects tomato and secretes various proteins that are recognized by resistant plants that respond with a hypersensitive response. Strains of the fungus that escape recognition by tomato are virulent. Resistance genes in tomato, either directly or indirectly involved in recognition of the fungal proteins, encode extracellular membrane-anchored, leucine-rich repeat proteins, which occur in gene clusters. Much progress has been made in our understanding of the evolution of recognitional specificities in the host plant.     

New lights in early steps of in vitro fertilization in plants

Studies using in vitro fertilization systems in animals and lower plants have led to a better understanding of the initial steps of fertilization and their underlying mechanisms. These mechanisms remain to be elucidated in flowering plants. Recent progress related to the development of in vitro fertilization systems using maize as a plant model is presented in this review. Their potential for leading to a better understanding of the process of gametic recognition and fusion and of the early events triggering egg activation and zygote formation are also discussed.     

Conservation and divergence in plant microRNAs

MicroRNAs (miRNAs) are a class of small, non-coding RNAs that regulate gene expression in eukaryotic cells. The past decade has seen an explosion in our understanding of the sets of miRNA genes encoded in the genomes in different species of plants and the mechanisms by which miRNAs interact with target RNAs. A subset of miRNA families (and their binding sites in target RNAs) are conserved between angiosperms and basal plants, suggesting they predate the divergence of existing lineages of plants. However, the majority of miRNA families expressed by any given plant species have a narrow phylogenetic distribution. As a group, these "young" miRNAs genes appear to be evolutionarily fluid and lack clearly understood biological function. The goal of this review is to summarize our understanding of the sets of miRNA genes and miRNA targets that exist in various plant species and to discuss hypotheses that explain the patterns of conservation and divergence observed among microRNAs in plants.     

Endophyte bioprospecting in South Asian medicinal plants: an attractive resource for biopharmaceuticals

From the last several years, there has been an increasing interest in plant-associated bacteria commonly referred to as endophytes that reside asymptomatically in the internal plant tissues. This interest peaked since the last two decades due to the recognition that endophytes within medicinal plants have the capability to mimic and produce the bioactive metabolites of the host plant. A number of medicinal plants have been used for centuries by the people of South Asia to cure various diseases and this has led to abundant usage experience. Relating to prior ethanopharmacological experiences, scientists have searched for medicinal plants that could be valued sources for endophytes yielding novel metabolites of pharmaceutical importance. This review is therefore an effort to encompass the most recent efforts in the exploration of medicinal plants of South Asia and their endophytes.

     

Molecular and Functional Diversity of RNA Editing in Plant Mitochondria

RNA editing is a fundamental biochemical process relating to the modification of nucleotides in messenger RNAs of functional genes in cells. RNA editing leads to re-establishment of conserved amino acid residues for functional proteins in nuclei, chloroplasts, and mitochondria. Identification of RNA editing factors that contributes to target site recognition increases our understanding of RNA editing mechanisms. Significant progress has been made in recent years in RNA editing studies for both animal and plant cells. RNA editing in nuclei and mitochondria of animal cells and in chloroplast of plant cells has been extensively documented and reviewed. RNA editing has been also extensively documented on plant mitochondria. However, functional diversity of RNA editing factors in plant mitochondria is not overviewed. Here, we review the biological significance of RNA editing, recent progress on the molecular mechanisms of RNA editing process, and function diversity of editing factors in plant mitochondrial research. We will focus on: (1) pentatricopeptide repeat proteins in Arabidopsis and in crop plants; (2) the progress of RNA editing process in plant mitochondria; (3) RNA editing-related RNA splicing; (4) RNA editing associated flower development; (5) RNA editing modulated male sterile; (6) RNA editing-regulated cell signaling; and (7) RNA editing involving abiotic stress. Advances described in this review will be valuable in expanding our understanding in RNA editing. The diverse functions of RNA editing in plant mitochondria will shed light on the investigation of molecular mechanisms that underlies plant development and abiotic stress tolerance.     

Ergosterol,an orphan fungal microbe‐associated molecular pattern (MAMP)

Fungal pathogens continue to pose a significant threat to crop production and food supply. The early stages of plant–fungus interactions are mostly mediated by microbe‐associated molecular pattern (MAMP) molecules, perceived by plant pattern recognition receptors (PRRs). Currently, the identified fungal MAMP molecules include chitin, chitosan, β‐glucans, elicitins and ergosterol. Although the molecular battles between host plants and infecting fungal phytopathogens have been studied extensively, many aspects still need to be investigated to obtain a holistic understanding of the intrinsic mechanisms, which is paramount in combating fungal plant diseases. Here, an overview is given of the most recent findings concerning an ‘orphan’ fungal MAMP molecule, ergosterol, and we present what is currently known from a synopsis of different genes, proteins and metabolites found to play key roles in induced immune responses in plant–fungus interactions. Clearly, integrative investigations are still needed to provide a comprehensive systems‐based understanding of the dynamics associated with molecular mechanisms in plant–ergosterol interactions and associated host responses.