Cross talk between signaling pathways in pathogen defense

Plant responses to herbivores are complex. Genes activated on herbivore attack are strongly correlated with the mode of herbivore feeding and the degree of tissue damage at the feeding site. Phloem-feeding whiteflies and aphids that produce little injury to plant foliage are perceived as pathogens and activate the salicylic acid (SA)-dependent and jasmonic acid (JA)/ethylene-dependent signaling pathways. Differential expression of plant genes in response to closely related insect species suggest that some elicitors generated by phloem-feeding insects are species-specific and are dependent on the herbivore's developmental stage. Other elicitors for defense-gene activation are likely to be more ubiquitous. Analogies to the pathogen-incompatible reactions are found. Chewing insects such as caterpillars and beetles and cell-content feeders such as mites and thrips cause more extensive tissue damage and activate wound-signaling pathways. Herbivore feeding is not equivalent to mechanical wounding. Wound responses are a part of the induced responses that accompany herbivore feeding. Herbivores induce direct defenses that interfere with herbivore feeding, growth and development, fecundity, and fertility. In addition, herbivores induce an array of volatiles that creates an indirect mechanism of defense. Volatile blends provide specific cues to attract herbivore parasites and predators to infested plants. The nature of the elicitors for volatile production is discussed.

The Myriad Plant Responses to Herbivores.

Cross-talk between ethylene and jasmonate signaling pathways determines the activation of a set of defense responses against pathogens and herbivores. However, the molecular mechanisms that underlie this cross-talk are poorly understood. Here, we show that ethylene and jasmonate pathways converge in the transcriptional activation of ETHYLENE RESPONSE FACTOR1 (ERF1), which encodes a transcription factor that regulates the expression of pathogen response genes that prevent disease progression. The expression of ERF1 can be activated rapidly by ethylene or jasmonate and can be activated synergistically by both hormones. In addition, both signaling pathways are required simultaneously to activate ERF1, because mutations that block any of them prevent ERF1 induction by any of these hormones either alone or in combination. Furthermore, 35S:ERF1 expression can rescue the defense response defects of coi1 (coronative insensitive1) and ein2 (ethylene insensitive2); therefore, it is a likely downstream component of both ethylene and jasmonate signaling pathways. Transcriptome analysis in Col;35S:ERF1 transgenic plants and ethylene/jasmonate-treated wild-type plants further supports the notion that ERF1 regulates in vivo the expression of a large number of genes responsive to both ethylene and jasmonate. These results suggest that ERF1 acts downstream of the intersection between ethylene and jasmonate pathways and suggest that this transcription factor is a key element in the integration of both signals for the regulation of defense response genes.

ETHYLENE RESPONSE FACTOR1 Integrates Signals from Ethylene and Jasmonate Pathways in Plant Defense

In spite of the importance of jasmonates (JAs) as plant growth and stress regulators, the molecular components of their signaling pathway remain largely unknown. By means of a genetic screen that exploits the cross talk between ethylene (ET) and JAs, we describe the identification of several new loci involved in JA signaling and the characterization and positional cloning of one of them, JASMONATE-INSENSITIVE1 (JAI1/JIN1). JIN1 encodes AtMYC2, a nuclear-localized basic helix-loop-helix-leucine zipper transcription factor, whose expression is rapidly upregulated by JA, in a CORONATINE INSENSITIVE1–dependent manner. Gain-of-function experiments confirmed the relevance of AtMYC2 in the activation of JA signaling. AtMYC2 differentially regulates the expression of two groups of JA-induced genes. The first group includes genes involved in defense responses against pathogens and is repressed by AtMYC2. Consistently, jin1 mutants show increased resistance to necrotrophic pathogens. The second group, integrated by genes involved in JA-mediated systemic responses to wounding, is activated by AtMYC2. Conversely, Ethylene-Response-Factor1 (ERF1) positively regulates the expression of the first group of genes and represses the second. These results highlight the existence of two branches in the JA signaling pathway, antagonistically regulated by AtMYC2 and ERF1, that are coincident with the alternative responses activated by JA and ET to two different sets of stresses, namely pathogen attack and wounding.

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JASMONATE-INSENSITIVE1 Encodes a MYC Transcription Factor Essential to Discriminate between Different Jasmonate-Regulated Defense Responses in Arabidopsis

Despite its importance in a variety of plant defense responses, our understanding of how jasmonic acid (JA) functions at the biochemical level is limited. Several amino acid conjugates of JA were tested for their ability to complement the JA-insensitive Arabidopsis thaliana mutant jar1-1. Unlike free JA, JA-Ile inhibited root growth in jar1-1 to the same extent as in the wild type, whereas JA-Val, JA-Leu, and JA-Phe were ineffective inhibitors in both genotypes. Thin-layer chromatography and gas chromatography–mass spectrometry (GC-MS) analysis of products produced in vitro by recombinant JAR1 demonstrated that this enzyme forms JA-amido conjugates with several amino acids, including JA-Ile. JA-Val, -Leu, -Ile, and -Phe were each quantified in Arabidopsis seedlings by GC-MS. JA-Ile was found at 29.6 pmole g−1 fresh weight (FW) in the wild type but was more than sevenfold lower in two jar1 alleles. JA-Leu, -Val, and -Phe were present at only low levels in both genotypes. Expression of wild-type JAR1 in transgenic jar1-1 plants restored sensitivity to JA and elevated JA-Ile to the same level as in the wild type. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) conjugated to JA was also found in plant tissue at 18.4 pmole g−1 FW. JA-ACC was determined not be an effective jasmonate root inhibitor, and surprisingly, was twofold higher in the mutants than in the wild type. This suggests that another JA-conjugating enzyme(s) is present in Arabidopsis. Synthesis of JA-ACC might provide a mechanism to coregulate the availability of JA and ACC for conversion to the active hormones JA-Ile and ethylene, respectively. We conclude that JAR1 is a JA-amino synthetase that is required to activate JA for optimal signaling in Arabidopsis. Plant hormone activation by conjugation to amino acids and the enzymes involved in their formation were previously unknown.

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The oxylipin signal jasmonic acid is activated by an enzyme that conjugates it to isoleucine in Arabidopsis.

Jasmonic acid has properties of a plant hormone, including the induction of specific genes associated with plant defense. We previously described jar1-1, an Arabidopsis jasmonate response mutant that exhibits reduced sensitivity to methyl jasmonate. We have further characterized this mutant and two new alleles; jar1-2 from a gamma irradiated population, and jar1-4 from a T-DNA mutant population. Seedling root growth in jar1-1 was equally insensitive to methyl jasmonate and jasmonic acid, indicating that the defect was not in the conversion of methyl jasmonate to the acid. None of the jar1 mutants showed an altered sensitivity to auxin, cytokinin, or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, indicating that the lesion does not affect the general uptake or transport of hormones. A soil fungus, Pythium irregulare, was found to blight jar1-1. Cultures of this organism caused the symptoms in all three jar1 mutants but not in wild type, indicating that increased susceptibility was due to the lesion in the JAR1 locus. A fatty acid desaturase triple mutant that is defective in the biosynthesis of jasmonic acid (J. Browse, Washington State University) was also susceptible, confirming that jasmonate is involved in resistance. The jar1-1 locus was mapped to the lower end of chromosome 2, about 11.4 cM from as1 and 1.6 cM from cer8. These results establish that jasmonate signaling plays an important role in resistance to soil micro-organisms in plants.

Casey C. Lehman

Papers

Jasmonate signaling mutants of Arabidopsis are susceptible to the soil fungus Pythium irregulare