Signal transduction in the plant immune response

Complementary biochemical and genetic approaches are being used to dissect the signaling network that regulates the innate immune response in plants. Receptor-mediated recognition of invading pathogens triggers a signal amplification loop that is based on synergistic interactions between nitric oxide, reactive oxygen intermediates and salicylic acid. Alternative resistance mechanisms in Arabidopsis are deployed against different types of pathogens; these mechanisms are mediated by either salicylic acid or the growth regulators jasmonic acid and ethylene.     

Root Defense Responses to Fungal Pathogens: A Molecular Perspective

This review will focus on the molecular and genetic mechanisms underlying defense responses of roots to fungal pathogens. Soil-borne pathogens, including Phytophthora, Pythium, Fusarium, and Bipolaris, represent major sources of biotic stress in the rhizosphere and roots of plants. Molecular recognition and signaling leading to effective resistance has been demonstrated to occur between host and Phytophthora, or Pythium. The hypersensitive response and apoptotic cell death, two oxidative processes that limit biotrophic pathogens, generally act to exacerbate disease symptoms induced by necrotrophic organisms. Although pathogenesis-related proteins can be expressed in roots during pathogen challenge, salicylic acid has not been implicated in root-mediated interactions. Jasmonic acid and ethylene have been found to mediate parallel as well as synergistic pathways that confer partial tolerance to necrotrophic pathogens, as well as induced systemic resistance to root and foliar pathogens. Genomics approaches are revealing new networks of defense-signaling pathways, and have the potential of elucidating those pathways that are important in root-defense responses.     

GDSL lipase-like 1 regulates systemic resistance associated with ethylene signaling in Arabidopsis

Systemic resistance is induced by necrotizing pathogenic microbes and non-pathogenic rhizobacteria and confers protection against a broad range of pathogens. Here we show that Arabidopsis GDSL LIPASE-LIKE 1 (GLIP1) plays an important role in plant immunity, eliciting both local and systemic resistance in plants. GLIP1 functions independently of salicylic acid but requires ethylene signaling. Enhancement of GLIP1 expression in plants increases resistance to pathogens including Alternaria brassicicola , Erwinia carotovora and Pseudomonas syringae , and limits their growth at the infection site. Furthermore, local treatment with GLIP1 proteins is sufficient for the activation of systemic resistance, inducing both resistance gene expression and pathogen resistance in systemic leaves. The PDF1.2 -inducing activity accumulates in petiole exudates in a GLIP1-dependent manner and is fractionated in the size range of less than 10 kDa as determined by size exclusion chromatography. Our results demonstrate that GLIP1-elicited systemic resistance is dependent on ethylene signaling and provide evidence that GLIP1 may mediate the production of a systemic signaling molecule(s).     

Induced systemic resistance (ISR) in plants: mechanism of action

Plants possess a range of active defense apparatuses that can be actively expressed in response to biotic stresses (pathogens and parasites) of various scales (ranging from microscopic viruses to phytophagous insect). The timing of this defense response is critical and reflects on the difference between coping and succumbing to such biotic challenge of necrotizing pathogens/parasites. If defense mechanisms are triggered by a stimulus prior to infection by a plant pathogen, disease can be reduced. Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are two forms of induced resistance wherein plant defenses are preconditioned by prior infection or treatment that results in resistance against subsequent challenge by a pathogen or parasite. Selected strains of plant growth-promoting rhizobacteria (PGPR) suppress diseases by antagonism between the bacteria and soil-borne pathogens as well as by inducing a systemic resistance in plant against both root and foliar pathogens. Rhizobacteria mediated ISR resembles that of pathogen induced SAR in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Several rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface whereas other rhizobacteria trigger different signaling pathway independent of SA. The existence of SA-independent ISR pathway has been studied in Arabidopsis thaliana, which is dependent on jasmonic acid (JA) and ethylene signaling. Specific Pseudomonas strains induce systemic resistance in viz., carnation, cucumber, radish, tobacco, and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways besides extended protection to a broader spectrum of pathogens than ISR/SAR alone. Beside Pseudomonas strains, ISR is conducted by Bacillus spp. wherein published results show that several specific strains of species B. amyloliquifaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B.sphaericus elicit significant reduction in the incidence or severity of various diseases on a diversity of hosts.     

GDSL lipase 1 regulates ethylene signaling and ethylene-associated systemic immunity in Arabidopsis

Arabidopsis GDSL lipase 1 (GLIP1) has been shown to modulate systemic immunity through the regulation of ethylene signaling components. Here we demonstrate that the constitutive triple response mutant ctr1-1 requires GLIP1 for the ethylene response, gene expression, and pathogen resistance. The glip1-1 mutant was defective in induced resistance following primary inoculation of necrotrophic pathogens, whereas GLIP1-overexpressing plants showed resistance to multiple pathogens. Necrotrophic infection triggered the downregulation of EIN3 and the activation of ERF1 and SID2 in a GLIP1-dependent manner. These results suggest that GLIP1 positively and negatively regulates ethylene signaling, resulting in an ethylene-associated, necrotroph-induced immune response.     

The role of ethylene and wound signaling in resistance of tomato to Botrytis cinerea

Ethylene, jasmonate, and salicylate play important roles in plant defense responses to pathogens. To investigate the contributions of these compounds in resistance of tomato (Lycopersicon esculentum) to the fungal pathogen Botrytis cinerea, three types of experiments were conducted: (a) quantitative disease assays with plants pretreated with ethylene, inhibitors of ethylene perception, or salicylate; (b) quantitative disease assays with mutants or transgenes affected in the production of or the response to either ethylene or jasmonate; and (c) expression analysis of defense-related genes before and after inoculation of plants with B. cinerea. Plants pretreated with ethylene showed a decreased susceptibility toward B. cinerea, whereas pretreatment with 1-methylcyclopropene, an inhibitor of ethylene perception, resulted in increased susceptibility. Ethylene pretreatment induced expression of several pathogenesis-related protein genes before B. cinerea infection. Proteinase inhibitor I expression was repressed by ethylene and induced by 1-methylcyclopropene. Ethylene also induced resistance in the mutant Never ripe. RNA analysis showed that Never ripe retained some ethylene sensitivity. The mutant Epinastic, constitutively activated in a subset of ethylene responses, and a transgenic line producing negligible ethylene were also tested. The results confirmed that ethylene responses are important for resistance of tomato to B. cinerea. The mutant Defenseless, impaired in jasmonate biosynthesis, showed increased susceptibility to B. cinerea. A transgenic line with reduced prosystemin expression showed similar susceptibility as Defenseless, whereas a prosystemin-overexpressing transgene was highly resistant. Ethylene and wound signaling acted independently on resistance. Salicylate and ethylene acted synergistically on defense gene expression, but antagonistically on resistance.     

Differential effectiveness of salicylate-dependent and jasmonate/ethylene-dependent induced resistance in Arabidopsis

Salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are each involved in the regulation of basal resistance against different pathogens. These three signals play important roles in induced resistance as well. SA is a key regulator of pathogen-induced systemic acquired resistance (SAR), whereas JA and ET are required for rhizobacteria-mediated induced systemic resistance (ISR). Both types of induced resistance are effective against a broad spectrum of pathogens. In this study, we compared the spectrum of effectiveness of SAR and ISR using an oomycete, a fungal, a bacterial, and a viral pathogen. In noninduced Arabidopsis plants, these pathogens are primarily resisted through either SA-dependent basal resistance (Peronospora parasitica and Turnip crinkle virus [TCV]), JA/ET-dependent basal resistance responses (Alternaria brassicicola), or a combination of SA-, JA-, and ET-dependent defenses (Xanthomonas campestris pv. armoraciae). Activation of ISR resulted in a significant level of protection against A. brassicicola, whereas SAR was ineffective against this pathogen. Conversely, activation of SAR resulted in a high level of protection against P. parasitica and TCV, whereas ISR conferred only weak and no protection against P. parasitica and TCV, respectively. Induction of SAR and ISR was equally effective against X. campestris pv. armoraciae. These results indicate that SAR is effective against pathogens that in noninduced plants are resisted through SA-dependent defenses, whereas ISR is effective against pathogens that in noninduced plants are resisted through JA/ET-dependent defenses. This suggests that SAR and ISR constitute a reinforcement of extant SA- or JA/ET-dependent basal defense responses, respectively.     

Warriors at the gate that never sleep: non-host resistance in plants

The native resistance of most plant species against a wide variety of pathogens is known as non-host resistance (NHR), which confers durable protection to plant species. Only a few pathogens or parasites can successfully cause diseases. NHR is polygenic and appears to be linked with basal plant resistance, a form of elicited protection. Sensing of pathogens by plants is brought about through the recognition of invariant pathogen-associated molecular patterns (PAMPs) that trigger downstream defense signaling pathways. Race-specific resistance, (R)-gene mediated resistance, has been extensively studied and reviewed, while our knowledge of NHR has advanced only recently due to the improved access to excellent model systems. The continuum of the cell wall (CW) and the CW-plasma membrane (PM)-cytoskeleton plays a crucial role in perceiving external cues and activating defense signaling cascades during NHR. Based on the type of hypersensitive reaction (HR) triggered, NHR was classified into two types, namely type-I and type-II. Genetic analysis of Arabidopsis mutants has revealed important roles for a number of specific molecules in NHR, including the role of SNARE-complex mediated exocytosis, lipid rafts and vesicle trafficking. As might be expected, R-gene mediated resistance is found to overlap with NHR, but the extent to which the genes/pathways are common between these two forms of disease resistance is unknown. The present review focuses on the various components involved in the known mechanisms of NHR in plants with special reference to the role of CW-PM components.     

The role of abscisic acid in plant-pathogen interactions

The effect of the abiotic stress hormone abscisic acid on plant disease resistance is a neglected field of research. With few exceptions, abscisic acid has been considered a negative regulator of disease resistance. This negative effect appears to be due to the interference of abscisic acid with biotic stress signaling that is regulated by salicylic acid, jasmonic acid and ethylene, and to an additional effect of ABA on shared components of stress signaling. However, recent research shows that abscisic acid can also be implicated in increasing the resistance of plants towards pathogens via its positive effect on callose deposition.     

Requirement of functional ethylene-insensitive 2 gene for efficient resistance of Arabidopsis to infection by Botrytis cinerea

Inoculation of wild-type Arabidopsis plants with the fungus Alternaria brassicicola results in systemic induction of genes encoding a plant defensin (PDF1.2), a basic chitinase (PR-3), and an acidic hevein-like protein (PR-4). Pathogen-induced induction of these three genes is almost completely abolished in the ethylene-insensitive Arabidopsis mutant ein2-1. This indicates that a functional ethylene signal transduction component (EIN2) is required in this response. The ein2-1 mutants were found to be markedly more susceptible than wild-type plants to infection by two different strains of the gray mold fungus Botrytis cinerea. In contrast, no increased fungal colonization of ein2-1 mutants was observed after challenge with avirulent strains of either Peronospora parasitica or A. brassicicola. Our data support the conclusion that ethylene-controlled responses play a role in resistance of Arabidopsis to some but not all types of pathogens.     

The Chitinase A from the baculovirus AcMNPV enhances resistance to both fungi and herbivorous pests in tobacco

Biotechnology has allowed the development of novel strategies to obtain plants that are more resistant to pests, fungal pathogens and other agents of biotic stress. The obvious advantages of having genotypes with multiple beneficial traits have recently fostered the development of gene pyramiding strategies, but less attention has been given to the study of genes that can increase resistance to different types of harmful organisms. Here we report that a recombinant Chitinase A protein of the Autographa californica nuclear polyhedrosis virus (AcMNPV) has both antifungal and insecticide properties in vitro. Transgenic tobacco plants expressing an active ChiA protein showed reduced damages caused by fungal pathogens and lepidopteran larvae, while did not have an effect on aphid populations. To our knowledge, this is the first report on the characterisation and expression in plants of a single gene that increases resistance against herbivorous pests and fungal pathogens and not affecting non-target insects. The implications and the potential of the ChiA gene for plant molecular breeding and biotechnology are discussed.     

Leaf stage-associated resistance is correlated with phytohormones in a pathosystem-dependen manner

It has been reported in several pathosystems that disease resistance can vary in leaves at different stages. However, how general this leaf stage-associated resistance is, and the molecular mechanism(s) underlying it, remain largely unknown. Here, we investigated the effect of leaf stage on basal resistance, effectortriggered immunity(ETI) and nonhost resistance, using eight pathosystems involving the hosts Arabidopsis thaliana, Nicotiana tabacum, and N. benthamiana and the pathogens Sclerotinia sclerotiorum, Pseudomonas syringae pv. tabaci, P. syringae pv. tomato DC3000, and Xanthomonas oryzae pv. oryzae(Xoo). We show evidence that leaf stage-associated resistance exists ubiquitously in plants, but with varying intensity at different stages in diverse pathosystems. Microarray expression profiling assays demonstrated that hundreds of genes involved in defense responses, phytohormone biosynthesis and signaling, and calcium signaling, were differentially expressed between leaves at different stages. The Arabidopsis mutants sid1, sid2-3, ein2, jar1-1, aba1 and aao3 lost leaf stage-associated resistance to S. sclerotiorum, and the mutants aba1 and sid2-3 were affected in leaf stage-associated RPS2/Avr Rpt2t-conferred ETI, whereas only the mutant sid2-3 influenced leaf stage-associated nonhost resistance to Xoo. Our results reveal that the phytohormones salicylic acid,ethylene, jasmonic acid and abscisic acid likely play an essential, but pathosystem-dependent, role in leaf stageassociated resistance.     

The food safety perspective of antibiotic resistance

Bacterial antimicrobial resistance in both the medical and agricultural fields has become a serious problem worldwide. Antibiotic resistant strains of bacteria are an increasing threat to animal and human health, with resistance mechanisms having been identified and described for all known antimicrobials currently available for clinical use. There is currently increased public and scientific interest regarding the administration of therapeutic and sub-therapeutic antimicrobials to animals, due primarily to the emergence and dissemination of multiple antibiotic resistant zoonotic bacterial pathogens. This issue has been the subject of heated debates for many years, however, there is still no complete consensus on the significance of antimicrobial use in animals, or resistance in bacterial isolates from animals, on the development and dissemination of antibiotic resistance among human bacterial pathogens. In fact, the debate regarding antimicrobial use in animals and subsequent human health implications has been going on for over 30 years, beginning with the release of the Swann report in the United Kingdom. The latest report released by the National Research Council (1998) confirmed that there were substantial information gaps that contribute to the difficulty of assessing potential detrimental effects of antimicrobials in food animals on human health. Regardless of the controversy, bacterial pathogens of animal and human origin are becoming increasingly resistant to most frontline antimicrobials, including expanded-spectrum cephalosporins, aminoglycosides, and even fluoroquinolones. The lion's share of these antimicrobial resistant phenotypes is gained from extra-chromosomal genes that may impart resistance to an entire antimicrobial class. In recent years, a number of these resistance genes have been associated with large, transferable, extra-chromosomal DNA elements, called plasmids, on which may be other DNA mobile elements, such as transposons and integrons. These DNA mobile elements have been shown to transmit genetic determinants for several different antimicrobial resistance mechanisms and may account for the rapid dissemination of resistance genes among different bacteria. The increasing incidence of antimicrobial resistant bacterial pathogens has severe implications for the future treatment and prevention of infectious diseases in both animals and humans. Although much scientific information is available on this subject, many aspects of the development of antimicrobial resistance still remain uncertain. The emergence and dissemination of bacterial antimicrobial resistance is the result of numerous complex interactions among antimicrobials, microorganisms, and the surrounding environments. Although research has linked the use of antibiotics in agriculture to the emergence of antibiotic-resistant foodborne pathogens, debate still continues whether this role is significant enough to merit further regulation or restriction.     

Arabidopsis thaliana plants differentially modulate auxin biosynthesis and transport during defense responses to the necrotrophic pathogen Alternaria brassicicola

? Although the role of auxin in biotrophic pathogenesis has been extensively studied, relatively little is known about its role in plant resistance to necrotrophs. ? Arabidopsis thaliana mutants defective in different aspects of the auxin pathway are generally more susceptible than wild-type plants to the necrotrophic pathogen Alternaria brassicicola. We show that A.?brassicicola infection up-regulates auxin biosynthesis and down-regulates the auxin transport capacities of infected plants, these effects being partially dependent on JA signaling. We also show that these effects of A.?brassicicola infection together lead to an enhanced auxin response in host plants. ? Application of IAA and MeJA together synergistically induces the expression of defense marker genes PDF1.2 (PLANT DEFENSIN 1.2) and HEL (HEVEIN-LIKE), suggesting that enhancement of JA-dependent defense signaling may be part of the auxin-mediated defense mechanism involved in resistance to necrotrophic pathogens. ? Our results provide molecular evidence supporting the hypothesis that JA and auxin interact positively in regulating plant resistance to necrotrophic pathogens and that activation of auxin signaling by JA may contribute to plant resistance to necrotrophic pathogens.     

The role of strigolactones and ethylene in disease caused by Pythium irregulare

Plant hormones play key roles in defence against pathogen attack. Recent work has begun to extend this role to encompass not just the traditional disease/stress hormones, such as ethylene, but also growth‐promoting hormones. Strigolactones (SLs) are the most recently defined group of plant hormones with important roles in plant–microbe interactions, as well as aspects of plant growth and development, although the knowledge of their role in plant–pathogen interactions is extremely limited. The oomycete Pythium irregulare is a poorly controlled pathogen of many crops. Previous work has indicated an important role for ethylene in defence against this oomycete. We examined the role of ethylene and SLs in response to this pathogen in pea (Pisum sativum L.) at the molecular and whole‐plant levels using a set of well‐characterized hormone mutants, including an ethylene‐insensitive ein2 mutant and SL‐deficient and insensitive mutants. We identified a key role for ethylene signalling in specific cell types that reduces pathogen invasion, extending the work carried out in other species. However, we found no evidence that SL biosynthesis or response influences the interaction of pea with P. irregulare or that synthetic SL influences the growth or hyphal branching of the oomycete in vitro. Future work should seek to extend our understanding of the role of SLs in other plant interactions, including with other fungal, bacterial and viral pathogens, nematodes and insect pests.