The endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA), whose levels increase on pathogen infection, activate separate sets of genes encoding antimicrobial proteins in Arabidopsis thaliana. The pathogeninducible genes PR-1, PR-2, and PR-5 require SA signaling for activation, whereas the plant defensin gene PDF1.2, along with a PR-3 and PR-4 gene, are induced by pathogens via an SA-independent and JA-dependent pathway. An Arabidopsis mutant, coi1, that is affected in the JA-response pathway shows enhanced susceptibility to infection by the fungal pathogens Alternaria brassicicola and Botrytis cinerea but not to Peronospora parasitica, and vice versa for two Arabidopsis genotypes (npr1 and NahG) with a defect in their SA response. Resistance to P. parasitica was boosted by external application of the SA-mimicking compound 2,6-dichloroisonicotinic acid [Delaney, T., et al. (1994) Science 266, 1247-1250] but not by methyl jasmonate (MeJA), whereas treatment with MeJA but not 2,6-dichloroisonicotinic acid elevated resistance to Alternaria brassicicola. The protective effect of MeJA against A. brassicicola was the result of an endogenous defense response activated in planta and not a direct effect of MeJA on the pathogen, as no protection to A. brassicicola was observed in the coi1 mutant treated with MeJA. These data point to the existence of at least two separate hormone-dependent defense pathways in Arabidopsis that contribute to resistance against distinct microbial pathogens.

A 5-kD plant defensin was purified from Arabidopsis leaves challenged with the fungus Alternaria brassicicola and shown to possess antifungal properties i n vitro. The corresponding plant defensin gene was induced after treatment of leaves with methyl jasmonate or ethylene but not with salicylic acid or 2,6-dichloroisonicotinic acid. When challenged with A. brassicicola, the levels of the plant defensin protein and mRNA rose both in inoculated leaves and in nontreated leaves of inoculated plants (systemic leaves). These events coincided with an increase i n the endogenous jasmonic acid content of both types of leaves. Systemic pathogen-induced expression of the plant defensin gene was unaffected in Arabidopsis transformants (nahG) or mutants (nprl and c p r l ) affected in the salicylic acid response but was strongly reduced in the Arabidopsis mutants ein2 and Coil that are blocked in their response to ethylene and methyl jasmonate, respectively.Our results indicate that systemic pathogen-induced expression of the plant defensin gene in Arabidopsis is independent of salicylic acid but requires components of the ethylene and jasmonic acid response.

Activation of the plant defensin gene PDF1.2 in Arabidopsis by pathogens has been shown previously to be blocked in the ethylene response mutant ein2-1 and the jasmonate response mutant coi1-1. In this work, we have further investigated the interactions between the ethylene and jasmonate signal pathways for the induction of this defense response. Inoculation of wild-type Arabidopsis plants with the fungus Alternaria brassicicola led to a marked increase in production of jasmonic acid, and this response was not blocked in the ein2-1 mutant. Likewise, A. brassicicola infection caused stimulated emission of ethylene both in wild-type plants and in coi1-1 mutants. However, treatment of either ein2-1 or coi1-1 mutants with methyl jasmonate or ethylene did not induce PDF1.2 , as it did in wild-type plants. We conclude from these experiments that both the ethylene and jasmonate signaling pathways need to be triggered concomitantly, and not sequentially, to activate PDF1.2 upon pathogen infection. In support of this idea, we observed a marked synergy between ethylene and methyl jasmonate for the induction of PDF1.2 in plants grown under sterile conditions. In contrast to the clear interdependence of the ethylene and jasmonate pathways for pathogen-induced activation of PDF1.2 , functional ethylene and jasmonate signaling pathways are not required for growth responses induced by jasmonate and ethylene, respectively. INTRODUCTIONHigher plants induce various defense responses when they are attacked by microbial pathogens, such as fungi, bacteria, or viruses. These defense responses include suicide of the attacked host cell (the so-called hypersensitive response); the production of antimicrobial secondary metabolites (called phytoalexins); the production of pathogenesisrelated (PR) proteins, of which many exert antimicrobial properties; and the production and oxidative cross-linking of cell wall polymers. The efficacy of these defense responses often determines whether plants are susceptible to infection by a pathogen.Elicitors secreted by or released from microbial invaders are the primary signal for induction of plant defense responses (Ebel and Cosio, 1994). Each pathogen produces a particular mixture of elicitors, which are sometimes accompanied by suppressors, and these molecules interact with receptors on the host cells that further translate the primary signal into particular events in the plasma membrane, the cytosol, and/or the nucleus (Shirasu et al., 1996). Induction of some defense genes requires the generation of secondary endogenous signal molecules (stress hormones) by the challenged cells in the infection site. The secondary signal molecules in turn set in motion signal transduction cascades in receiving cells, eventually leading to activation of pathogen-responsive genes. Secondary signal molecules thus serve to amplify and spread the response of the host after initial recognition of the pathogen. Several secondary signal molecules whose synthesis is increased in response to elicitor recognition and that ar...

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Various mechanisms to fend off microbial invaders have been devised by a11 living organisms, including microorganisms themselves. The most sophisticated of these mechanisms relies on the synthesis of immunoglobulins directed against specific microbial targets. However, immunoglobulin-based immunity operates only in a relatively minor subset of living species, namely the higher vertebrates. A much more ancient and widespread defense strategy involves the production of small peptides that exert antimicrobial properties. As products of single genes, antimicrobial peptides can be synthesized in a swift and flexible way, and because of their small size they can be produced by the host with a minimal input of energy and biomass. Wellknown examples of antimicrobial peptides are the cecropins that accumulate in the hemolymph of many invertebrates in response to injury or infection (reviewed by Boman and Hultmark, 1987) and the magainins that are secreted by glands in the skin of amphibians (reviewed by Bevins and Zasloff, 1990). Cecropins and magainins are small (2040 residues) basic peptides displaying an amphipathic a-helical structure that can integrate in microbial membranes to form ion channels (Duclohier, 1994).Another class of antimicrobial peptides is formed by the Cys-rich peptides, which in contrast to cecropins and magainins, have a complex cystine-stabilized three-dimensional folding pattern often involving antiparallel P-sheets. Defensins are one class among the numerous types of Cys-rich antimicrobial peptides, which differ in length, number of cystine, bonds, or folding pattern (reviewed by Boman, 1995). Insect defensins (3443 residues, three disulfide bridges) are, like cecropins, produced in a pathogeninducible manner by the insect fat body and secreted in the hemolymph (reviewed by Hoffmann and Hétru, 1992).

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 ethyleneinsensitive 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 ethylenecontrolled responses play a role in resistance of Arabidopsis to some but not all types of pathogens.

Summary The phytoalexin‐deficient Arabidopsis mutant pad3‐1, which is affected in the production of the indole‐type phytoalexin camalexin, has previously been shown not to display altered susceptibility to either the bacterium Pseudomonas syringae (Glazebrook & Ausubel 1994; Proc. Natl. Acad. Sci. USA, 91: 8955–8959) or the biotrophic fungi Peronospora parasitica (Glazebrook et al. 1997; Genetics, 146: 381–392) and Erysiphe orontii (Reuber et al. 1998; Plant J. 16: 473–485). We now show that this mutant is markedly more susceptible than its wild‐type parental line to infection by the necrotrophic fungus Alternaria brassicicola, but not to Botrytis cinerea. A strong camalexin response was elicited in wild‐type plants inoculated with either Alternaria brassicicola or Botrytis cinerea, whereas no camalexin could be detected in pad3‐1 challenged with these fungi. Hence, PAD3 appears to be a key determinant in resistance to at least A. brassicicola. The induction of salicylate‐dependent and jasmonate/ethylene‐dependent defense genes was not reduced in Alternaria‐challenged pad3‐1 plants compared to similarly treated wild‐type plants. Camalexin production could not be triggered by exogenous application of either salicylate, ethylene or jasmonate and was not, or not strongly, reduced in mutants with defects in perception of these defense‐related signal molecules. Camalexin‐production appears to be controlled by a pathway that exhibits little cross‐talk with salicylate‐, ethylene‐ and jasmonate‐dependent signalling events.

The phytohormone ethylene is a principal modulator in many aspects of plant life, including various mechanisms by which plants react to pathogen attack. Induced ethylene biosynthesis and subsequent intracellular signaling through a single conserved pathway have been well characterized. This leads to a cascade of transcription factors consisting of primary EIN3-like regulators and downstream ERF-like transcription factors. The latter control the expression of various effector genes involved in various aspects of systemic induced defense responses. Moreover, at this level significant cross-talk occurs with other defense response pathways controlled by salicylic acid and jasmonate, eventually resulting in a differentiated disease response.