Molecular Analysis of Plant Defense Responses to Plant Pathogens

A number of inducible plant responses are believed to contribute to disease resistance. These responses include the hypersensitive reaction, phytoalexin synthesis, and the production of chitinase, glucanase, and hydroxyproline-rich glycoproteins. Because of the coordinate induction of these responses, it has been difficult to determine whether they are functional defense responses, and if they are, how they specifically contribute to disease resistance. Recent developments in molecular biology have provided experimental techniques that will reveal the specific contribution of each response to disease resistance. In this paper, we describe a strategy to determine if the hypersensitive reaction is a functional plant defense mechanism.     

Signal Molecules for Plant Defense Responses to Biotic Stress

Plants are capable of recognizing the penetrating pathogens and of responding to their attack by the activation of the defense systems. Signal transduction from the receptor to the cell genome is required for this activation. Recently, signal molecules have been found, which are involved in the signal transduction triggered in response to biotic stress. The data accumulated imply the presence of a complex and well-coordinated signal network in plant cells. This net controls plant defense responses to pathogen attacks.     

Terpene Down-Regulation Triggers Defense Responses in Transgenic Orange Leading to Resistance against Fungal Pathogens

Terpenoid volatiles are isoprene compounds that are emitted by plants to communicate with the environment. In addition to their function in repelling herbivores and attracting carnivorous predators in green tissues, the presumed primary function of terpenoid volatiles released from mature fruits is the attraction of seed-dispersing animals. Mature oranges (Citrus sinensis) primarily accumulate terpenes in peel oil glands, with d-limonene accounting for approximately 97% of the total volatile terpenes. In a previous report, we showed that down-regulation of a d-limonene synthase gene alters monoterpene levels in orange antisense (AS) fruits, leading to resistance against Penicillium digitatum infection. A global gene expression analysis of AS versus empty vector (EV) transgenic fruits revealed that the down-regulation of d-limonene up-regulated genes involved in the innate immune response. Basal levels of jasmonic acid were substantially higher in the EV compared with AS oranges. Upon fungal challenge, salicylic acid levels were triggered in EV samples, while jasmonic acid metabolism and signaling were drastically increased in AS orange peels. In nature, d-limonene levels increase in orange fruit once the seeds are fully viable. The inverse correlation between the increase in d-limonene content and the decrease in the defense response suggests that d-limonene promotes infection by microorganisms that are likely involved in facilitating access to the pulp for seed-dispersing frugivores.Plants are sessile organisms that produce and emit a vast array of volatile organic compounds (VOCs) to communicate between parts of the same plant and with other plants. It is generally accepted that the original role of these compounds in nature is related to defense functions (Degenhardt et al., 2003). Most VOCs are terpenoids, fatty acid degradation compounds, phenylpropanoids, and amino acid-derived products. Among these, terpenoids are likely to be the most abundant and expensive to produce (Gershenzon, 1994). Terpenoids are isoprenoid-derived compounds synthesized through the condensation of C5 isoprene units, a process that is catalyzed by a wide diversity of terpene synthases using geranyl diphosphate (GDP), farnesyl diphosphate (FDP), and geranylgeranyl diphosphate (GGDP) as substrates. These reactions give rise to the C5 hemiterpenes, the C10 monoterpenes, the C15 sesquiterpenes, and the C20 diterpenes (Dudareva et al., 2006).In green tissues, volatile terpenoid synthesis is either induced upon wounding or occurs constitutively; terpenes can be then stored in specific organs or tissues where they would be most effective in defense responses, such as leaf trichomes, resin ducts and lacticifers, pockets near the epidermis, or secretory cavities in Citrus spp. (Langenheim, 1994; Turner et al., 2000; Trapp and Croteau, 2001; Voo et al., 2012). Genetic engineering experiments have demonstrated that specific terpenoid compounds emitted by leaves can intoxicate, repel, or deter herbivores (Aharoni et al., 2003; Wu et al., 2006), or they may attract the natural predators and parasitoids of damaging herbivores to protect plants from further damage (Kappers et al., 2005; Schnee et al., 2006). These terpenoids are naturally found in complex mixtures, and it has been proposed that they can act synergistically, as in conifer resin, for simultaneous protection against pests and pathogens (Phillips and Croteau, 1999). Although fatty acid degradation products (such as jasmonates) and phenylpropanoids (such as salicylates) as well as their volatile and nonvolatile precursors are clearly involved in many induced defense responses against pests and pathogens (Glazebrook, 2005), much less is known regarding the participation of terpenoid volatiles in the defense against microorganisms in plants and about the possible interactions of these terpenoids with phytohormones.In contrast to their function in leaves, when released from flowers and mature fruits, the main function of terpenoid volatiles is in the attraction of pollinators (Pichersky and Gershenzon, 2002; Kessler et al., 2008; Junker and Blüthgen, 2010; Schiestl, 2010) and seed-dispersing animals (Lomáscolo et al., 2010; Rodríguez et al., 2011b), respectively. Fruit maturation and ripening are usually associated with large increases in the synthesis and accumulation of specific flavored volatiles, which are proposed to function as signals for seed dispersal (Auldridge et al., 2006; Goff and Klee, 2006; Rodríguez et al., 2013).Upon wounding, plant responses to biotic stresses are orchestrated locally and systemically by signaling molecules. Among these molecules, the jasmonates regulate defenses against arthropod herbivores and necrotroph fungal pathogens as well as biotrophic pathogens, such as some mildews (Ellis and Turner, 2001; Stintzi et al., 2001; Kessler et al., 2004; Li et al., 2005; Wasternack, 2007; Browse and Howe, 2008). In addition to jasmonates, molecules such as salicylic acid (SA) and ethylene appear to regulate distinct defense pathways and are major synergistic (Mur et al., 2006) or antagonistic (De Vos et al., 2005) regulators of plant innate immunity. Plants produce a specific blend of these alarm signals after pathogen or pest attacks, and the production of these molecules varies greatly in quantity, composition, and timing. These signals activate differential sets of defense-related genes that eventually determine the nature of the defense response against the attacker (Reymond and Farmer, 1998; Rojo et al., 2003; De Vos et al., 2005). All genes that encode enzymes involved in the biosynthesis of jasmonates are jasmonic acid (JA) inducible (Wasternack, 2006), indicating that JA biosynthesis is regulated by positive feedback. The precursor for the biosynthesis of JA is α-linolenic acid. The activity of the 13-lipoxygenase (LOX), allene oxide synthase (AOS), and allene oxide cyclase (AOC) enzymes converts α-linolenic acid to cis-(+)-12-oxophytodienoic acid (OPDA). OPDA REDUCTASE3 catalyzes the reduction of OPDA (and dinor-OPDA) to oxo-pentenyl-cycloheptane-octanoic acid, which, in turn, undergoes three rounds of β-oxidation leading to jasmonyl-CoA formation. Jasmonyl-CoA is then cleaved by a putative thioesterase yielding (+)-7-iso-JA, which equilibrates to the more stable (−)-JA (Wasternack and Kombrink, 2010).The exogenous application of jasmonates on plants and the existence of mutant and/or transgenic plants altered in JA biosynthesis or signaling have led to altered susceptibility or resistance to pathogens. Impaired JA biosynthesis or signaling is generally associated with decreased levels of defensive compounds, including VOCs, and reduced plant biomass and/or fitness under insect attack (Howe et al., 1996; Halitschke and Baldwin, 2004). For example, Arabidopsis (Arabidopsis thaliana) mutants defective in JA perception (e.g. coronatine-insensitive1 [coi1]) or biosynthesis (e.g. aos and defective in anther dehiscence1) are susceptible to pathogen infections (Feys et al., 1994; Xie et al., 1998; Park et al., 2002; Turner et al., 2002). In contrast, mutants (e.g. constitutive expression of vegetative storage protein1 and Arabidopsis Ser/Thr phosphatase of type 2C1) with constitutive or wound-induced activation of the JA pathway exhibit enhanced resistance to fungal pathogens and pests and phenotypes characteristic of JA-treated plants (Ellis and Turner, 2001; Ellis et al., 2002; Schweighofer et al., 2007).Sweet orange (Citrus sinensis) is a perennial tree species that is exposed to recurrent biotic and abiotic challenges during its decades of growth in orchards. Orange fruits undergo a nonclimacteric maturation process in which the biochemistry, physiology, and structure of the organ are altered to complete the release of mature seeds. These changes typically include fruit growth and texture modification; color change through the degradation of chlorophylls and a parallel induction of carotenogenesis in the peel (flavedo) and pulp; flavonoid accumulation in the pulp; increases and decreases in the sugar and acid contents, respectively; and global accumulation and selective emission of volatile terpenoids (Spiegel-Roy and Goldschmidt, 1996). In nature, d-limonene accumulates gradually in the oil glands of the peel during fruit development and reaches its maximum level shortly before the breaker stage, followed by a steady decline during maturation (Attaway et al., 1967; Kekelidze et al., 1989; Rodríguez et al., 2011b). The high amount of d-limonene that accumulates in orange peels has a tremendous metabolic cost, suggesting an important biological role for this terpene and other related compounds in the interactions between fruits and the biotic environment.Previously, we examined the biological role of d-limonene by manipulating oil gland chemistry via the antisense (AS) overexpression of a d-limonene synthase gene from Satsuma mandarin (Citrus unshiu) in orange fruits. Compared with empty vector (EV) controls, fruit peels from AS transformants showed a dramatic reduction in d-limonene accumulation; decreased levels of other monoterpenes, sesquiterpenes, and monoterpene aldehydes; and increased levels of monoterpene alcohols. When challenged with the necrotroph fungus Penicillium digitatum, the causal agent of green mold rot, AS-transformed fruits were highly resistant to fungal infection. Full susceptibility to P. digitatum infection was restored when AS fruits were supplemented with d-limonene but not other monoterpene alcohols, indicating that d-limonene accumulation in the orange peel was required for the successful progress of this plant-pathogen interaction (Rodríguez et al., 2011a, 2011b). Green mold rot is the most important postharvest disease of citrus fruit worldwide, accounting for up to 60% to 80% of total losses during postharvest life of the fruit. P. digitatum is considered to be a specialist pathogen of citrus fruits that efficiently infects the peel through injuries in which ubiquitous fungal spores germinate and rapidly colonize the surrounding areas (Droby et al., 2008). The control of this pathogen relies heavily on the use of synthetic chemicals, but concerns regarding their potential negative effects on human health and also the generation of fungicide-resistant strains have encouraged finding alternatives, such as the generation of citrus trees with fruits that are genetically resistant to the pathogen.In this work, to better understand the mechanism underlying the constitutive resistance to P. digitatum conferred by the reduction of limonene in AS orange fruits, we analyzed the pattern of fruit growth and the morphological and biochemical developmental characteristics and performed a global analysis of gene expression using a 20K citrus microarray. The study is supplemented by examining the possible involvement of key hormone signals and isoprenoid precursors in the fruit peel. We report here that the reduced level of d-limonene in AS fruits is tightly associated with the constitutive activation of defense response signaling cascades. Our results establish, to our knowledge for the first time, a correlation between increased volatile terpene content and the decline of defense responses in a fleshy fruit during maturation, which would facilitate necrotroph fungal infections in citrus fruits.     

Antimicrobial Phytoprotectants and Fungal Pathogens: A Commentary

Many plants produce antifungal secondary metabolites. These may be preformed compounds which are found in healthy plants and which may represent in-built chemical barriers to infection by potential pathogens (preformed antimicrobial compounds or phytoanticipins). Alternatively they may be synthesized in response to pathogen attack as part of the plant defence response (phytoalexins). If these molecules do play a role in protecting plants against pathogen attack, then successful pathogens are presumably able to circumvent or tolerate these defences. Strategies may include avoidance, enzymatic degradation, and/or nondegradative mechanisms. This review outlines the different ways in which fungal pathogens may counter the antifungal compounds produced by their host plants and summarizes the evidence for and against these compounds as antimicrobial phytoprotectants.     

转基因改良植物抗真菌病害的策略及其进展

随着近几年来生物技术的迅猛发展,一系列新的广谱持久性抗病基因已经被鉴定、克隆并应用于转基因改良抗病性的实践。概述目前植物抗真菌病害的基因工程策略及其研究进展：（1）基于寄主－病原菌相互识别和信号传导体系的基因工程策略;（2）基于抗真菌蛋白的基因工程策略。     

Resistance and Susceptibility of Plants to Fungal Pathogens

Plants are under continuous threat of infection by pathogens endowed with diverse strategies to colonize their host. Comprehensive biochemical and genetic approaches are now starting to reveal the complex signaling pathways that mediate plant disease resistance. Initiation of defense signaling often involves specific recognition of invading pathogens by the products of specialized host resistance (R) genes. Potential resistance signaling components have been identified by mutational analyses to be required for specific resistance in the model Arabidopsis and some crop species. Strikingly, many of the components share similarity to that of innate immune systems in animals. Evidence is also accumulating that plant pathogens have a number of ways to evade host defenses during the early stages of infection, similar to animal pathogens. These strategies are becoming much better understood in a number of plant–pathogen interactions. In this review, we focus on the current knowledge of host factors that control plant resistance and susceptibility to fungal pathogens. The knowledge accumulated in these studies will serve a fundamental basis for combating diseases in strategic molecular agriculture.     

Fungal Pathogens: The Battle for Plant Infection

The attempted infection of a plant by a pathogen, such as a fungus or an Oomycete, may be regarded as a battle whose major weapons are proteins and smaller chemical compounds produced by both organisms. Indeed, plants produce an astonishing plethora of defense compounds that are still being discovered at a rapid pace. This pattern arose from a multi-million year, ping-pong?type co-evolution, in which plant and pathogen successively added new chemical weapons in this perpetual battle. As each defensive innovation was established in the host, new ways to circumvent it evolved in the pathogen. This complex co-evolution process probably explains not only the exquisite specificity observed between many pathogens and their hosts, but also the ineffectiveness or redundancy of some defensive genes which often encode enzymes with overlapping activities. Plants evolved a complex, multi-level series of structural and chemical barriers that are both constitutive or preformed and inducible. These defenses may involve strengthening of the cell wall, hypersensitive response (HR), oxidative burst, phytoalexins and pathogenesis-related (PR) proteins. The pathogen must successfully overcome these obstacles before it succeeds in causing disease. In some cases, it needs to modulate or modify plant cell metabolism to its own benefit and/or to abolish defense reactions. Central to the activation of plant responses is timely perception of the pathogen by the plant. A crucial role is played by elicitors which, depending on their mode of action, are broadly classified into nonspecific elicitors and highly specific elicitors or virulence effector/avirulence factors. A protein battle for penetration is then initiated, marking the pathogen attempted transition from extracellular to invasive growth before parasitism and disease can be established. Three major types of defense responses may be observed in plants: non-host resistance, host resistance, and host pathogenesis. Plant innate immunity may comprise a continuum from non-host resistance involving the detection of general elicitors to host-specific resistance involving detection of specific elicitors by R proteins. It was generally assumed that non-host resistance was based on passive mechanisms and that nonspecific rejection usually arose as a consequence of the non-host pathogen failure to breach the first lines of plant defense. However, recent evidence has blurred the clear-cut distinction among non-host resistance, host-specific resistance and disease. The same obstacles are also serious challenges for host pathogens, reducing their success rate significantly in causing disease. Indeed, even susceptible plants mount a (insufficient) defense response upon recognition of pathogen elicited molecular signals. Recent evidence suggests the occurrence of significant overlaps between the protein components and signalling pathways of these types of resistance, suggesting the existence of both shared and unique features for the three branches of plant innate immunity.     

Ascochyta rabiei: A threat to global chickpea production

The necrotrophic fungus Ascochyta rabiei causes Ascochyta blight (AB) disease in chickpea. A. rabiei infects all aerial parts of the plant, which results in severe yield loss. At present, AB disease occurs in most chickpea‐growing countries. Globally increased incidences of A. rabiei infection and the emergence of new aggressive isolates directed the interest of researchers toward understanding the evolution of pathogenic determinants in this fungus. In this review, we summarize the molecular and genetic studies of the pathogen along with approaches that are helping in combating the disease. Possible areas of future research are also suggested.Taxonomykingdom Mycota, phylum Ascomycota, class Dothideomycetes, subclass Coelomycetes, order Pleosporales, family Didymellaceae, genus Ascochyta, species rabiei. Primary host A. rabiei survives primarily on Cicer species.Disease symptoms A. rabiei infects aboveground parts of the plant including leaves, petioles, stems, pods, and seeds. The disease symptoms first appear as watersoaked lesions on the leaves and stems, which turn brown or dark brown. Early symptoms include small circular necrotic lesions visible on the leaves and oval brown lesions on the stem. At later stages of infection, the lesions may girdle the stem and the region above the girdle falls off. The disease severity increases at the reproductive stage and rounded lesions with concentric rings, due to asexual structures called pycnidia, appear on leaves, stems, and pods. The infected pod becomes blighted and often results in shrivelled and infected seeds.Disease management strategiesCrop failures may be avoided by judicious practices of integrated disease management based on the use of resistant or tolerant cultivars and growing chickpea in areas where conditions are least favourable for AB disease development. Use of healthy seeds free of A. rabiei, seed treatments with fungicides, and proper destruction of diseased stubbles can also reduce the fungal inoculum load. Crop rotation with nonhost crops is critical for controlling the disease. Planting moderately resistant cultivars and prudent application of fungicides is also a way to combat AB disease. However, the scarcity of AB‐resistant accessions and the continuous evolution of the pathogen challenges the disease management process.Useful websites https://www.ndsu.edu/pubweb/pulse‐info/resourcespdf/Ascochyta%20blight%20of%20chickpea.pdf https://saskpulse.com/files/newsletters/180531_ascochyta_in_chickpeas‐compressed.pdf http://www.pulseaus.com.au/growing‐pulses/bmp/chickpea/ascochyta‐blight http://agriculture.vic.gov.au/agriculture/pests‐diseases‐and‐weeds/plant‐diseases/grains‐pulses‐and‐cereals/ascochyta‐blight‐of‐chickpea http://www.croppro.com.au/crop_disease_manual/ch05s02.php https://www.northernpulse.com/uploads/resources/722/handout‐chickpeaascochyta‐nov13‐2011.pdf http://oar.icrisat.org/184/1/24_2010_IB_no_82_Host_Plant https://www.crop.bayer.com.au/find‐crop‐solutions/by‐pest/diseases/ascochyta‐blight     

Systemic Root-Shoot Signaling Drives Jasmonate-Based Root Defense against Nematodes

1. Download : Download high-res image (212KB)
2. Download : Download full-size image

     

OsJAR1 Contributes Mainly to Biosynthesis of the Stress-Induced Jasmonoyl-Isoleucine Involved in Defense Responses in Rice

Jasmonate plays key roles in plant growth and stress responses, as in defense against pathogen attack. Jasmonoyl-isoleucine (JA-Ile), a major active form of jasmonates, is thought to play a pivotal role in plant defense responses, but the involvement of JA-Ile in rice defense responses, including phytoalexin production, remains largely unknown. Here we found that OsJAR1 contributes mainly to stress-induced JA-Ile production by the use of an osjar1 Tos17 mutant. The osjar1 mutant was impaired in JA-induced expression of JA-responsive genes and phytoalexin production, and these defects were restored genetically. Endogenous JA-Ile was indispensable to the production of a flavonoid phytoalexin, sakuranetin, but not to that of diterpenoid phytoalexins in response to heavy metal stress and the rice blast fungus. The osjar1 mutant was also found to be more susceptible to the blast fungus than the parental wild type. These results suggest that JA-Ile production makes a contribution to rice defense responses with a great impact on stress-induced sakuranetin production.     

Macromolecules Released by a Plant Growth-promoting Rhizobacterium as Elicitors of Systemic Resistance in Tomato to Bacterial and Fungal Pathogens

One of 500 rhizobacteria isolated from soil, rhizosphere and rhizoplane of healthy tomato plants was previously selected in laboratory, greenhouse and field tests as a good inducer of systemic resistance. This plant growth‐promoting rhizobacterium (PGPR) was identified as Bacillus cereus by fatty‐acid analysis. Bacillus cereus bacterial cells were removed from liquid culture by centrifugation and the supernatant repeatedly dialyzed (cut‐off = 12 000 daltons) against distilled water. Dialysates applied to roots protected tomato plants against leaf fungal and bacterial pathogens, evidence that macromolecules synthesized by the PGPR and released into the environment act as elicitors of systemic resistance.     

Responses of Aquatic Fungal Communities on Leaf Litter to Temperature‐Change Events

Increases of extreme weather events are predicted to occur with ongoing climate change, but impacts to freshwaters have rarely been examined. We assessed the effects of temperature on leaf‐litter associated fungi by exposing leaves colonized in a stream to 18 °C (control), 25 °C, or 18 °C after freezing. Treatments altered fungal dominance on leaves; Lunulospora curvula sporulation was stimulated by increased temperature and stopped by the freeze‐thaw treatment. Fungal biomass and diversity decreased at 18 °C after freezing, but not at 25 °C. Leaf decomposition was retarded by the freeze‐thaw treatment (k = –0.024 day^–1) and stimulated at 25 °C (k = –0.069 day^–1). Results suggest that occasional freezing may constrain fungal diversity and their ecological functions, while warming appears to accelerate plant‐litter decomposition in streams. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Prospects for the Management of Invasive Alien Weeds Using Co-Evolved Fungal Pathogens: A Latin American Perspective

Invasive alien weeds pose a serious threat to the biodiversity of natural ecosystems and a significant constraint to agricultural production worldwide. The use of co-evolved natural enemies, a strategy referred to as classical biological control (CBC), has proven to be a potentially efficacious, cost-effective, and safe option for the management of alien weeds. An analysis of CBC of invasive weeds in Latin America is presented, which shows that only 5% of the worldwide releases of agents, overwhelmingly arthropod, have been in this region. Fungal pathogens are increasingly being considered in CBC programmes, and there are now 11 examples of Latin American fungi having been released as biocontrol agents in other regions of the world. In contrast, only three weed pathogens have been deliberately released in the region. Possible reasons for the paucity of CBC programmes in Latin America are presented, despite the presence of a significant number of alien weed species (60 are listed). An analysis of these weeds reveals that many of them could be amenable to control using natural enemies, including nine weed species for which CBC programmes have been successfully implemented elsewhere in the world. In addition, for many of these 60 species, a co-evolved and damaging mycobiota has already been recorded. The prospects for management of invasive alien weeds in Latin America, using co-evolved fungal pathogens, are assessed with particular reference to selected species from the genera Ambrosia, Broussonetia, Calotropis, Commelina, Cyperus, Dichrostachys, Echinochloa, Pittosporum, Rottboellia, Rubus, Sonchus and Ulex. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correction to: A New Combination with D-Cateslytin to Eradicate Root Canal Pathogens

International Journal of Peptide Research and Therapeutics - The article A New Combination with D?Cateslytin to Eradicate Root Canal Pathogens, written by Claire Ehlinger, Pauline Dartevelle,...     

The Cotton Wall-Associated Kinase GhWAK7A Mediates Responses to Fungal Wilt Pathogens by Complexing with the Chitin Sensory Receptors

Plant receptor-like kinases (RLKs) are important players in response to pathogen infections. Verticillium and Fusarium wilts, caused by Verticillium dahliae (Vd) and Fusarium oxysporum f. sp vasinfectum (Fov), respectively, are among the most devastating diseases in cotton (Gossypium spp). To understand the cotton response to these soil-borne fungal pathogens, we performed a genome-wide in silico characterization and functional screen of diverse RLKs for their involvement in cotton wilt diseases. We identified Gossypium hirsutum GhWAK7A, a wall-associated kinase, that positively regulates cotton response to both Vd and Fov infections. Chitin, the major constituent of the fungal cell wall, is perceived by lysin-motif-containing RLKs (LYKs/CERK1), leading to the activation of plant defense against fungal pathogens. A conserved chitin sensing and signaling system is present in cotton, including chitin-induced GhLYK5-GhCERK1 dimerization and phosphorylation, and contributes to cotton defense against Vd and Fov. Importantly, GhWAK7A directly interacts with both GhLYK5 and GhCERK1 and promotes chitin-induced GhLYK5-GhCERK1 dimerization. GhWAK7A phosphorylates GhLYK5, which itself does not have kinase activity, but requires phosphorylation for its function. Consequently, GhWAK7A plays a crucial role in chitin-induced responses. Thus, our data reveal GhWAK7A as an important component in cotton response to fungal wilt pathogens by complexing with the chitin receptors.