Healthy and unhealthy plants: The effect of stress on the metabolism of Brassicaceae

Brassicaceae plants are one of the most popular vegetables consumed all over the world and considered to be a good source of bioactive phytochemicals. Additionally, Brassica species and varieties are increasingly becoming a research model in plant science, as a consequence of the importance of their primary and secondary metabolites. Plant interaction with environmental stress factors including animals and insects herbivory, pathogens, metal ions, light, among others, is known to lead to the activation of various defense mechanisms resulting in a qualitative and/or quantitative change in plant metabolite production. Pre-harvest and/or post-harvest conditions are also known to affect this, since plants produce signaling molecules (e.g. salicylic acid, jasmonic acid, etc.) that cause a direct or indirect activation of metabolic pathways. That ultimately affects the production of phytochemicals, such as carbohydrates (sucrose and glucose), amino acids, phenolics (phenylpropanoids and flavonoids) and glucosinolates. These phytochemicals have diverse applications due to their antimicrobial, antioxidant and anti-carcinogenic properties, but on the other hand these compounds or their breakdown products can act as anti-nutritional factors in diet. In this review we report a wide range of the stress-induced metabolic responses in the Brassica plants commonly used for human consumption.     

Pathogens pulling the strings: Effectors manipulating salicylic acid and phenylpropanoid biosynthesis in plants

During evolution, plants have developed sophisticated ways to cope with different biotic and abiotic stresses. Phytohormones and secondary metabolites are known to play pivotal roles in defence responses against invading pathogens. One of the key hormones involved in plant immunity is salicylic acid (SA), of which the role in plant defence is well established and documented. Plants produce an array of secondary metabolites categorized in different classes, with the phenylpropanoids as major players in plant immunity. Both SA and phenylpropanoids are needed for an effective immune response by the plant. To successfully infect the host, pathogens secrete proteins, called effectors, into the plant tissue to lower defence. Secreted effectors can interfere with several metabolic or signalling pathways in the host to facilitate infection. In this review, we will focus on the different strategies pathogens have developed to affect the levels of SA and phenylpropanoids to increase plant susceptibility.     

Nutritional antioxidants and adaptive cell responses: an update

Many plant antioxidants, intaken through the daily diet or plant-derived dietary supplements, have been shown able to prevent free radical-related diseases by counteracting cell oxidative stress. However, it is now considered that the in vivo beneficial effects of these phytochemicals are unlikely to be explained just by their antioxidant capability. Several plant antioxidants exhibit hormetic properties, by acting as 'low-dose stressors' that may prepare cells to resist more severe stress. In fact, low doses of these phytochemicals activate cell signaling pathways (being the most prominent examples the modulation of the Nrf2/Keap1 pathway, the NF-κB pathway and the Sirtuin-FOXO pathway) but high doses are cytotoxic. Herein we review the adaptive responses induced by the most known plant hormetic antioxidants, which are sulforaphane, resveratrol, curcumin, flavonoids, green tea catechins and diallylsulphides, as well as the molecular mechanisms involved in such responses. Furthermore, this review outlines that the hormetic properties of these bioactive plant antioxidants might be successfully employed for realizing health-promoting dietary interventions especially in the field of neurodegenerative diseases and cancer.     

Changing trends in biotechnology of secondary metabolism in medicinal and aromatic plants

Main conclusion

Medicinal and aromatic plants are known to produce secondary metabolites that find uses as flavoring agents, fragrances, insecticides, dyes and drugs. Biotechnology offers several choices through which secondary metabolism in medicinal plants can be altered in innovative ways, to overproduce phytochemicals of interest, to reduce the content of toxic compounds or even to produce novel chemicals. Detailed investigation of chromatin organization and microRNAs affecting biosynthesis of secondary metabolites as well as exploring cryptic biosynthetic clusters and synthetic biology options, may provide additional ways to harness this resource. Plant secondary metabolites are a fascinating class of phytochemicals exhibiting immense chemical diversity. Considerable enigma regarding their natural biological functions and the vast array of pharmacological activities, amongst other uses, make secondary metabolites interesting and important candidates for research. Here, we present an update on changing trends in the biotechnological approaches that are used to understand and exploit the secondary metabolism in medicinal and aromatic plants. Bioprocessing in the form of suspension culture, organ culture or transformed hairy roots has been successful in scaling up secondary metabolite production in many cases. Pathway elucidation and metabolic engineering have been useful to get enhanced yield of the metabolite of interest; or, for producing novel metabolites. Heterologous expression of putative plant secondary metabolite biosynthesis genes in a microbe is useful to validate their functions, and in some cases, also, to produce plant metabolites in microbes. Endophytes, the microbes that normally colonize plant tissues, may also produce the phytochemicals produced by the host plant. The review also provides perspectives on future research in the field.

     

Autophagy–virus interplay in plants: from antiviral recognition to proviral manipulation

Autophagy is a conserved self-cleaning and renewal system required for cellular homeostasis and stress tolerance. Autophagic processes are also implicated in the response to ‘non-self’ such as viral pathogens, yet the functions and mechanisms of autophagy during plant virus infection have only recently started to be revealed. Compelling evidence now indicates that autophagy is an integral part of antiviral immunity in plants. It can promote the hypersensitive cell death response upon incompatible viral infections or mediate the selective elimination of entire particles and individual proteins from compatible viruses in a pathway similar to xenophagy in animals. Several viruses, however, have evolved measures to antagonize xenophagic degradation or utilize autophagy to suppress disease-associated cell death and other defence pathways like RNA silencing. Here, we highlight the current advances and gaps in our understanding of the complex autophagy–virus interplay and its consequences for host immunity and viral pathogenesis in plants.     

In the trenches of plant pathogen recognition: Role of NB-LRR proteins

As in nearly every discipline of plant biology, new insights are constantly changing our understanding of plant immunity. It is now clear that plant immunity is controlled by two layers of inducible responses: basal responses triggered by conserved microbial features and specific responses triggered by gene-for-gene recognition of pathogen effector proteins by host resistance (R) proteins. The nucleotide-binding domain leucine-rich repeat (NB-LRR) class of R proteins plays a major role in the combat against a wide range of plant pathogens. The variation that has been generated and is maintained within these conserved proteins has diversified their specificity, subcellular localisations, activation and recognition mechanisms, allowing them to specifically adapt to different plant–pathogen interaction systems. This review addresses recent advances in the molecular role of NB-LRR proteins in pathogen recognition and activation of plant defence responses.     

Autophagy in plants

Autophagy is a highly conserved processing mechanism in eukaryotes whereby cytoplasmic components are engulfed in double-membrane vesicles called autophagosomes and are delivered into organelles such as lysosomes (mammal) or vacuoles (yeast/plant) for degradation and recycling of the resulting molecules. Isolation of yeastAUTOPHAGY (ATG) genes has facilitated the identification of correspondingArabidopsis ATG genes based on sequence similarity. Genetic and molecular analyses using knockout and/or knockdown mutants of those genes have unraveled the biological functions of autophagy during plant development, nutrient recycling, and environmental stress responses. Additional roles for autophagy have been suggested in the degradation of oxidized proteins during oxidative stress and the regulation of hypersensitive response (HR)-programmed cell death (PCD) during innate immunity. Our review summarizes knowledge about the structure and function of autophagic pathways andATG components, and the biological roles of autophagy in plants.     

植物基质金属蛋白酶的研究进展

基质金属蛋白酶(MMPs)是高度保守的锌依赖型内肽酶家族.医学研究表明,人体MMPs不仅在一系列生理过程中发挥关键作用,而且与很多重大疾病关联.例如,MMPs在恶性肿瘤组织中的表达量大幅度上升,和肿瘤的侵袭转移密切相关.MMPs也广泛存在于高等植物,它可能参与植物发育调控、免疫应答及非生物逆境胁迫响应等多个方面.该文对近年来国内外有关植物MMPs的分布、结构特点、活性调节以及生物学功能等方面的研究进展进行综述,并对该领域的研究趋势和重点问题进行了讨论.     

Roles of small RNAs in plant disease resistance

The interaction between plants and pathogens represents a dynamic competition between a robust immune system and efficient infectious strategies. Plant innate immunity is composed of complex and highly regulated molecular networks, which can be triggered by the perception of either conserved or race‐specific pathogenic molecular signatures. Small RNAs are emerging as versatile regulators of plant development, growth and response to biotic and abiotic stresses. They act in different tiers of plant immunity, including the pathogen‐associated molecular pattern‐triggered and the effector‐triggered immunity. On the other hand, pathogens have evolved effector molecules to suppress or hijack the host small RNA pathways. This leads to an arms race between plants and pathogens at the level of small RNA‐mediated defense.Here, we review recent advances in small RNA‐mediated defense responses and discuss the challenging questions in this area.     

Unravelling how plants benefit from ROS and NO reactions,while resisting oxidative stress

Background and Aims Reactive oxygen species (ROS) and reactive nitrogen species (RNS), such as nitric oxide (NO), play crucial roles in the signal transduction pathways that regulate plant growth, development and defence responses, providing a nexus of reduction/oxidation (redox) control that impacts on nearly every aspect of plant biology. Here we summarize current knowledge and concepts that lay the foundations of a new vision for ROS/RNS functions – particularly through signalling hubs – for the next decade.Scope Plants have mastered the art of redox control using ROS and RNS as secondary messengers to regulate a diverse range of protein functions through redox-based, post-translational modifications that act as regulators of molecular master-switches. Much current focus concerns the impact of this regulation on local and systemic signalling pathways, as well as understanding how such reactive molecules can be effectively used in the control of plant growth and stress responses.Conclusions The spectre of oxidative stress still overshadows much of our current philosophy and understanding of ROS and RNS functions. While many questions remain to be addressed – for example regarding inter-organellar regulation and communication, the control of hypoxia and how ROS/RNS signalling is used in plant cells, not only to trigger acclimation responses but also to create molecular memories of stress – it is clear that ROS and RNS function as vital signals of living cells.     

植物中逆境反应相关的WRKY转录因子研究进展

WRKY转录因子是植物体内一类比较大的转录因子家族,它在植物的生长发育以及抗逆境反应中起着非常重要的作用。本文综述了WRKY转录因子在植物应对冻害、干旱、盐害等非生物胁迫与病原菌、虫害等生物胁迫反应中的重要调控功能,并概括了WRKY转录因子在调控这些逆境反应中的机制。     

Autophagic degradation of leaf starch in plants

Autophagy is an evolutionarily conserved process in eukaryotic cells that functions to degrade cytoplasmic components in the vacuole or lysosome. Previous research indicates that the core molecular machinery of autophagosome formation works well in plants, and plant autophagy plays roles in diverse biological processes such as nutrient recycling, development, immunity and responses to a variety of abiotic stresses. Recently, we reported that autophagy contributed to leaf starch degradation, which had been thought to be a process confined to chloroplasts. This finding demonstrated a previously unidentified pathway of leaf starch depletion and a new role of basal autophagy in plants.     

Plant immune system: Basal immunity

Plants have an efficient system of innate immunity that is based on the effective detection of potentially harmful microorganisms and rapid induction of defense responses. The first level of plant immunity is basal immunity, which is induced by the conserved molecular structures of microbes, such as bacterial flagellins or fungal chitin, or molecules that result from the interaction of plants with pathogens, for example oligosaccharides and peptides (“danger signals”). Plants recognize these inducers through receptors localized to the plasma membrane, represented mainly by receptor-like protein kinases or receptor-like proteins. Activation of the receptor by a ligand triggers a complex network of signaling events, which eventually cause an array of plant defense responses to prevent further spread of the pathogen.     

The Salmonella Type III Effector SspH2 Specifically Exploits the NLR Co-chaperone Activity of SGT1 to Subvert Immunity

To further its pathogenesis, S. Typhimurium delivers effector proteins into host cells, including the novel E3 ubiquitin ligase (NEL) effector SspH2. Using model systems in a cross-kingdom approach we gained further insight into the molecular function of this effector. Here, we show that SspH2 modulates innate immunity in both mammalian and plant cells. In mammalian cell culture, SspH2 significantly enhanced Nod1-mediated IL-8 secretion when transiently expressed or bacterially delivered. In addition, SspH2 also enhanced an Rx-dependent hypersensitive response in planta. In both of these nucleotide-binding leucine rich repeat receptor (NLR) model systems, SspH2-mediated phenotypes required its catalytic E3 ubiquitin ligase activity and interaction with the conserved host protein SGT1. SGT1 has an essential cell cycle function and an additional function as an NLR co-chaperone in animal and plant cells. Interaction between SspH2 and SGT1 was restricted to SGT1 proteins that have NLR co-chaperone function and accordingly, SspH2 did not affect SGT1 cell cycle functions. Mechanistic studies revealed that SspH2 interacted with, and ubiquitinated Nod1 and could induce Nod1 activity in an agonist-independent manner if catalytically active. Interestingly, SspH2 in vitro ubiquitination activity and protein stability were enhanced by SGT1. Overall, this work adds to our understanding of the sophisticated mechanisms used by bacterial effectors to co-opt host pathways by demonstrating that SspH2 can subvert immune responses by selectively exploiting the functions of a conserved host co-chaperone.     

All hands on deck—the role of chloroplasts, endoplasmic reticulum, and the nucleus in driving plant innate immunity

Plant innate immunity is mediated by cell membrane and intracellular immune receptors that function in distinct and overlapping cell-signaling pathways to activate defense responses. It is becoming increasingly evident that immune receptors rely on components from multiple organelles for the generation of appropriate defense responses. This review analyzes the defense-related functions of the chloroplast, nucleus, and endoplasmic reticulum (ER) during plant innate immunity. It details the role of the chloroplasts in synthesizing defense-specific second messengers and discusses the retrograde signal transduction pathways that exist between the chloroplast and nucleus. Because the activities of immune modulators are regulated, in part, by their subcellular localization, the review places special emphasis on the dynamics and nuclear–cytoplasmic transport of immune receptors and regulators and highlights the importance of this process in generating orderly events during an innate immune response. The review also covers the recently discovered contributions of the ER quality-control pathways in ensuring the signaling competency of cell surface immune receptors or immune regulators.     

Fungal Pls1 tetraspanins as key factors of penetration into host plants: a role in re-establishing polarized growth in the appressorium?

The ability of plant pathogenic fungi to infect their host depends on successful penetration into plant tissues. This process often involves the differentiation of a specialized cell, the appressorium. Signalling pathways required for appressorium formation are conserved among fungi. However, the functions involved in appressorium maturation and penetration peg formation are still poorly understood. Recent studies have shown that Pls1 tetraspanins control an appressorial function required for penetration into host plants and are likely conserved among plant pathogenic fungi. Tetraspanins are small membrane proteins widely distributed among ascomycetes and basidiomycetes defining two distinct families; Pls1 tetraspanins are found in both ascomycetes and basidiomycetes and Tsp2 tetraspanins are specific to basidiomycetes. Both fungal tetraspanins families have similar secondary structures shared with animal tetraspanins. Pls1 tetraspanins are present as single genes in genomes of ascomycetes, allowing a unique opportunity to study their function in appressorium mediated penetration. Experimental evidence suggests that Pls1 tetraspanins are required for the formation of the penetration peg at the base of the appressorium, probably through re-establishing cell polarity.     

Elicitation and suppression of microbe-associated molecular pattern-triggered immunity in plant-microbe interactions

Recent studies have uncovered fascinating molecular mechanisms underlying plant-microbe interactions that coevolved dynamically. As in animals, the primary plant innate immunity is immediately triggered by the detection of common pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs). Different MAMPs are often perceived by distinct cell-surface pattern-recognition receptors (PRRs) and activate convergent intracellular signalling pathways in plant cells for broad-spectrum immunity. Successful pathogens, however, have evolved multiple virulence factors to suppress MAMP-triggered immunity. Specifically, diverse pathogenic bacteria have employed the type III secretion system to deliver a repertoire of virulence effector proteins to interfere with host immunity and promote pathogenesis. Plants challenged by pathogens have evolved the secondary plant innate immunity. In particular, some plants possess the specific intracellular disease resistance (R) proteins to effectively counteract virulence effectors of pathogens for effector-triggered immunity. This potent but cultivar-specific effector-triggered immunity occurs rapidly with localized programmed cell death/hypersensitive response to limit pathogen proliferation and disease development. Remarkably, bacteria have further acquired virulence effectors to block effector-triggered immunity. This review covers the latest findings in the dynamics of MAMP-triggered immunity and its interception by virulence factors of pathogenic bacteria.