Plant Responses to Many biotic and abiotic stresses are orchestrated locally and systemically by signaling molecules Known As The Jasmonates (JaS). JaS also Regulate Such diverse processes As pollen maturation and Wound Responses in Arabidopsis. Here We review recent advances in our Understanding of How JA. biosynthesis is regulated, The signaling functions of Different JaS, and How The JA Signal May be transduced via an E3 ubiquitin ligase. We also examine How outputs from The JA, salicylic acid (SA), and ethylene Signal pathways are Integrated in The Regulation of. Stress Response and Plant Development. We Use The term jasmonate to include The biologically Active intermediates in jasmonic acid biosynthesis ,. The Pathway for As Well As The biologically Active Derivatives of jasmonic acid. These compounds are widely Distributed in Plants and affect a Variety of processes (Creelman and Mullet, 1,997), including Fruit ripening, production of viable pollen, root growth, tendril coiling, Plant Response to. wounding and abiotic stress, and defenses against insects and pathogens. . The function of JaS in defense was proposed by Farmer and Ryan (Farmer and Ryan, One Thousand Nine Hundred Ninety-Two), Who Provided Evidence for a causal Link between wounding (As caused by Insect herbivores), The Formation of JaS, and. The Induction of genes for Proteinase. inhibitors that Deter Insect feeding. In particular, they proposed that wounding caused release of linolenic acid (LA), The presumed precursor of JaS, from Membrane lipids. New Evidence Indicates that JA signaling in Plants is generally As proposed by Farmer and Ryan, but. more Complex than they envisaged. This New Evidence Indicates that intermediates in JA biosynthesis Have Distinctive Biological Activity, an E3 ubiquitin ligase that Regulates Most Probably JA Responses in Arabidopsis, and that The JA signaling pathways Signal Pathway interacts with Other defense. A Great Deal of What We currently know About JA. Studies on signaling Comes from Arabidopsis and Tomato. However, there are discrepancies between The proposed JA signaling pathways Several of these Species, and IT is Not yet clear Reflect Whether these fundamental gaps in Knowledge or Reveal Differences in mechanism. For example, Arabidopsis mutants defective in JA biosynthesis or perception are deficient in defense responses and are male sterile (Feys et al., 1994; McConn and Browse, 1996; Vijayan et al., 1998), whereas tomato mutants apparently defective in JA biosynthesis. or perception Have deficient defenses but are male FERTILE (Howe et AL., the 1996th; Li et AL., 2,001th). Similarly, The systemic Induction of JA Responses in Tomato is Through The Well-characterized Systemin Signal Pathway (Constabel et AL.,. 1995; Ryan, 2000; Ryan et al., 2002), but in Arabidopsis there is no evidence for an equivalent pathway. , Even though systemic signaling Can be demonstrated (Kubigsteltig et AL., 1 999). The JA Signal Pathway Signal transduction involves Several events: The Primary Wound Stress The perception of stimulus and transduction of The Signal or locally and systemically; The perception of this Signal and Induction of JA biosynthesis; The perception of JA and Induction of Responses; and finally, integration of JA signaling with outputs from The SA, ethylene, and Other signaling pathways. Perception of The Signal That The Stimulus and Initiates Production of JA Biosynthesis. JA signaling Can be induced by a Range of abiotic stresses, including osmotic Stress (Kramell. et al., 1995), wounding, drought, and exposure to "elicitors," which include chitins, oligosaccharides, oligogalaturonides (Doares et al., 1995. ), and extracts from Yeast (Parchmann et AL., one thousand nine hundred and ninety-seven; Leon et AL., 2001). JA biosynthesis in Arabidopsis is also regulated by cues in The Developing Stamen, Where jasmonic acid is Required for Development pollen. However, We do Not yet know How these stresses or Developmental cues are perceived. One Approach has been to search for The Earliest Response to Stress, which would Therefore be a candidate for a Component of The Stress perception / Signal transduction Pathway. A mitogen-Activated protein kinase named WIPK. is transcribed minutes After Tobacco is wounded (Seo et AL. ., The 1,995th), and The WIPK protein product is Activated (Seo et AL., 1999). jasmonic acid and ITS methyl Ester accumulate in wounded Tobacco Plants, but do Not accumulate in wounded transgenic Plants, in which Expression of WIPK is genetically suppressed. . This Indicates that Expression of WIPK is Required for Wound-induced JA biosynthesis. However, The wounded transgenic Plants accumulated SA and transcripts of The Gene pathogenesis Related protein 1 (PR1), Indicating that Suppression of The JA Pathway permits Wound Induction of The SA. Pathway (Seo et AL., the 1,995th). More significantly, transgenic Tobacco Plants overexpressing WIPK accumulate JA and Proteinase inhibitor 2 (PIN2) transcripts (Seo et AL., 1,999). Apparently Therefore, The Wound-induced Transcription of WIPK and Activation of. The protein product activates JA biosynthesis and suppresses SA-dependent signaling (Figure 1). Figure 1. View larger Version: In this page In a New Window. Download As PowerPoint Slide Figure 1. Mutants Reveals Gene Expression in JA Signal Transduction Pathways Interaction between. defense. Two G of total RNA from Each Sample was analyzed on gel blots on nylon Filters. Filters were probed with radiolabeled, polymerase chain Reaction-Generated DNA fragments from PR1, PDF1.2, Thi2.1, and 18S rRNA genes. (A ) Seedlings Grown for 10 days were on Murashige and Skoog (MS) agar, then Transferred to. Fresh MS agar (-) or MS agar supplemented with 50 uM SA for 2 Further days (+). (B) Seedlings were Grown for 12 days on MS agar. (C) Model for positive (arrows) and negative (Bars) Interactions. . between The JA, ethylene, and SA Signal pathways during Response to pathogens, and pests or wounding. Gene symbols (in italics) are defined in The text; Proteins are Upper Case, Not Italic. Similarly, in Arabidopsis, a mitogen-Activated protein. kinase MPK4 is named Activated 2 to 5 min After wounding (Ichimura et AL., The 2 Thousandth). The Mpk4 Mutant is Dwarfed, has elevated levels of SA, and has constitutive Expression of systemic acquired resistance (SAR) and The defense-Related. Gene PR1 (Petersen et AL., 2000). dwarfing is reduced and PR1 is expressed in Mpk4 Not The NahG Plants containing salicylic acid TRANSGENE encoding a hydroxylase, which Reduces Level salicylic acid. Significantly, these transgenic Plants Fail to Express The JA- also. Plant genes regulated defensin 1.2 (PDF1.2) and Thionin 2.1 (Thi2.1) After Treatment with JA. Assuming that did Not The Plants contain a Low Level of SA sufficient to antagonize JA Responses (Niki et AL., in 1998), The. result indicates that the MPK4 cascade may simultaneously suppress SA biosynthesis and promote JA perception / response required for induction of PDF1. 2 and Thi2.1. Therefore, MPK4 appears to Regulate JA perception / Response Rather than JA biosynthesis, and would Therefore Act at a Different Point in The JA Pathway than does WIPK (Figure 1). Assuming that The antibody that detects MPK4 Identifies The. Same protein As that defined by. Mpk4, these results also indicate that The Wound-induced Activation of MPK4 is Probably Too Rapid for The Activating Signal to be Newly Biosynthesised JA. It is Therefore more likely that MPK4 is Activated by The Primary Stress perception / transduction Signal, or possibly by The. Rapid release of JA from endogenous Stores (Stelmach et AL., the 2,001th). A Critical question, Therefore, is Whether MPK4 is Activated by a JA Signal alone. The Arabidopsis Mutant constitutive Expression of vegetative Storage protein (Cev1) was Isolated on The BASIS. of constitutive Expression of a luciferase reporter for The vegetative Storage protein (VSP) promoter. . It is Dwarfed, has constitutive production of JA and ethylene, Expression of constitutive PDF1.2, Thi2.1, and The chitinase CHI, and has defenses against fungal pathogens Enhanced (Ellis and Turner, 2001, Figures 1A and 1B) and an. insect. Pest. The phenotype is partially suppressed in Mutant Cev1 The Coronatine insensitive 1 (Coi1) and in The ethylene resistant 1 (Etr1) Mutant Backgrounds, and The Mutant triple, Cev1; coi1; Etr1 is Wild type except for slightly SHORTER Roots (Ellis et AL., 2002nd). This Indicates that Cev1 induces biosynthesis of JA and ethylene, and ITS Mutant phenotype is largely Determined by Responses to these signaling molecules. Cev1, Therefore, Acts at an. Early Step in The Stress perception / transduction Pathway, Before JA and ethylene biosynthesis (Figure 1C). Map-based cloning of CEV1 identified IT As The Cellulose synthetase Gene CESA3. Accordingly, Cev1 had reduced Cellulose content, and Wild-type Plants Treated with. Cellulose synthetase inhibitors Have a Near- Phenocopy Exhibit Enhanced and JA Responses of The Mutant Cev1. Apparently, in The Cell Wall Alterations Can initiate JA signaling (Ellis et AL., 2002nd). When Tomato leaves are damaged by herbivores or by Simple mechanical wounding. , JA signaling and defense are systemically Activated Gene Expression Within hours. Requires Prosystemin The systemic Signal, a 200-amino-acid precursor that gives Rise to 18-amino-acid polypeptide The Systemin by proteolytic processing (Ryan and Pearce, 1998th; Ryan et AL., Two thousand and two). Systemin The production of H2O2 induces biosynthesis of jasmonic acid and The Subsequent Gene Expression and Induction of defense (Orozco-Cardenas et AL., 2 001). Regulation of JaS The Biosynthesis of JA biosynthesis involves Coincident The apparently Induction of at least Five genes for biosynthetic. enzymes, products of which are The Targeted to The chloroplast. Gene products for beta-oxidation are Targeted to The peroxisome, and jasmonic acid Modify Gene products that are presumably cytoplasmic. The genes for JA biosynthesis are induced at The Site of JA Formation. Growing evidence indicates that developmentally regulated JA biosynthesis in Ar.
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