Jasmonates meet fatty acids: functional analysis of a new acyl-coenzyme A synthetase family from Arabidopsis thaliana

Arabidopsis thaliana contains a large number of genes encoding carboxylic acid-activating enzymes, including long-chain fatty acyl-CoA synthetase (LACS), 4-coumarate:CoA ligases (4CL), and proteins closely related to 4CLs with unknown activities. The function of these 4CL-like proteins was systematically explored by applying an extensive substrate screen, and it was uncovered that activation of fatty acids is the common feature of all active members of this protein family, thereby defining a new group of fatty acyl-CoA synthetase, which is distinct from the known LACS family. Significantly, four family members also displayed activity towards different biosynthetic precursors of jasmonic acid (JA), including 12-oxo-phytodienoic acid (OPDA), dinor-OPDA, 3-oxo-2(2'-[Z]-pentenyl)cyclopentane-1-octanoic acid (OPC-8), and OPC-6. Detailed analysis of in vitro properties uncovered significant differences in substrate specificity for individual enzymes, but only one protein (At1g20510) showed OPC-8:CoA ligase activity. Its in vivo function was analysed by transcript and jasmonate profiling of Arabidopsis insertion mutants for the gene. OPC-8:CoA ligase expression was activated in response to wounding or infection in the wild type but was undetectable in the mutants, which also exhibited OPC-8 accumulation and reduced levels of JA. In addition, the developmental, tissue- and cell-type specific expression pattern of the gene, and regulatory properties of its promoter were monitored by analysing promoter::GUS reporter lines. Collectively, the results demonstrate that OPC-8:CoA ligase catalyses an essential step in JA biosynthesis by initiating the beta-oxidative chain shortening of the carboxylic acid side chain of its precursors, and, in accordance with this function, the protein is localized in peroxisomes.

Chemical Interference of Pathogen-associated Molecular Pattern-triggered Immune Responses in Arabidopsis Reveals a Potential Role for Fatty-acid Synthase Type II Complex-derived Lipid Signals

We describe an experimental setup using submerged cultures of Arabidopsis seedlings in 96-well microtiter plates that permits chemical intervention of rapid elicitor-mediated immune responses. Screening of a chemical library comprising 120 small molecules with known biological activities revealed four compounds reducing cellulysin- or flg22-activated gene expression of the early pathogen-associated molecular patterns (PAMP)-responsive ATL2 gene. One chemical, oxytriazine, was found to induce ATL2 gene expression in the absence of PAMP. By monitoring additional flg22-triggered immediate early plant responses, we present evidence that two compounds, triclosan and fluazinam, interfere with the accumulation of reactive oxygen species and internalization of the activated plasma membrane resident FLS2 immune receptor. Using triclosan structure types and enzyme activity inhibition assays, Arabidopsis MOD1 enoyl-acyl carrier protein reductase, a subunit of the fatty-acid synthase type II (FAS II) complex, was identified as a likely cellular target of triclosan. Inhibition of all tested elicitor-triggered early immune responses by triclosan indicates a potential role for signaling lipids in flg22-triggered immunity. Chemical profiling of eca mutants, each showing deregulated ATL2 gene expression, with the identified compounds revealed mutantspecific response patterns and allowed us to deduce tentative action sites of ECA genes relative to the compound targets.

A new type of peroxisomal acyl-coenzyme A synthetase from Arabidopsis thaliana has the catalytic capacity to activate biosynthetic precursors of jasmonic acid

Arabidopsis thaliana contains a large number of genes that encode carboxylic acid-activating enzymes, including nine long-chain fatty acyl-CoA synthetases, four 4-coumarate:CoA ligases (4CL), and 25 4CL-like proteins of unknown biochemical function. Because of their high structural and sequence similarity with bona fide 4CLs and their highly hydrophobic putative substrate-binding pockets, the 4CL-like proteins At4g05160 and At5g63380 were selected for detailed analysis. Following heterologous expression, the purified proteins were subjected to a large scale screen to identify their preferred in vitro substrates. This study uncovered a significant activity of At4g05160 with medium-chain fatty acids, medium-chain fatty acids carrying a phenyl substitution, long-chain fatty acids, as well as the jasmonic acid precursors 12-oxo-phytodienoic acid and 3-oxo-2-(2'-pentenyl)-cyclopentane-1-hexanoic acid. The closest homolog of At4g05160, namely At5g63380, showed high activity with long-chain fatty acids and 12-oxo-phytodienoic acid, the latter representing the most efficiently converted substrate. By using fluorescent-tagged variants, we demonstrated that both 4CL-like proteins are targeted to leaf peroxisomes. Collectively, these data demonstrate that At4g05160 and At5g63380 have the capacity to contribute to jasmonic acid biosynthesis by initiating the beta-oxidative chain shortening of its precursors.

SNARE-protein-mediated disease resistance at the plant cell wall

Failure of pathogenic fungi to breach the plant cell wall constitutes a major component of immunity of non-host plant species--species outside the pathogen host range--and accounts for a proportion of aborted infection attempts on 'susceptible' host plants (basal resistance). Neither form of penetration resistance is understood at the molecular level. We developed a screen for penetration (pen) mutants of Arabidopsis, which are disabled in non-host penetration resistance against barley powdery mildew, Blumeria graminis f. sp. hordei, and we isolated the PEN1 gene. We also isolated barley ROR2 (ref. 2), which is required for basal penetration resistance against B. g. hordei. The genes encode functionally homologous syntaxins, demonstrating a mechanistic link between non-host resistance and basal penetration resistance in monocotyledons and dicotyledons. We show that resistance in barley requires a SNAP-25 (synaptosome-associated protein, molecular mass 25 kDa) homologue capable of forming a binary SNAP receptor (SNARE) complex with ROR2. Genetic control of vesicle behaviour at penetration sites, and plasma membrane location of PEN1/ROR2, is consistent with a proposed involvement of SNARE-complex-mediated exocytosis and/or homotypic vesicle fusion events in resistance. Functions associated with SNARE-dependent penetration resistance are dispensable for immunity mediated by race-specific resistance (R) genes, highlighting fundamental differences between these two resistance forms.

SNARE-protein-mediated disease resistance at the plant cell wall

Failure of pathogenic fungi to breach the plant cell wall constitutes a major component of immunity of non-host plant species--species outside the pathogen host range--and accounts for a proportion of aborted infection attempts on 'susceptible' host plants (basal resistance). Neither form of penetration resistance is understood at the molecular level. We developed a screen for penetration (pen) mutants of Arabidopsis, which are disabled in non-host penetration resistance against barley powdery mildew, Blumeria graminis f. sp. hordei, and we isolated the PEN1 gene. We also isolated barley ROR2 (ref. 2), which is required for basal penetration resistance against B. g. hordei. The genes encode functionally homologous syntaxins, demonstrating a mechanistic link between non-host resistance and basal penetration resistance in monocotyledons and dicotyledons. We show that resistance in barley requires a SNAP-25 (synaptosome-associated protein, molecular mass 25 kDa) homologue capable of forming a binary SNAP receptor (SNARE) complex with ROR2. Genetic control of vesicle behaviour at penetration sites, and plasma membrane location of PEN1/ROR2, is consistent with a proposed involvement of SNARE-complex-mediated exocytosis and/or homotypic vesicle fusion events in resistance. Functions associated with SNARE-dependent penetration resistance are dispensable for immunity mediated by race-specific resistance (R) genes, highlighting fundamental differences between these two resistance forms.

The promoter of the potato chitinase C gene directs expression to epidermal cells

Chitinases are ubiquitous proteins that occur in all plants in multiple isoforms. We have isolated the ChtC2 gene encoding an unusual, basic (class I) chitinase from potato ( Solanum tuberosum L.). In contrast to other chitinase genes, ChtC2 is not activated by infection, but rather constitutively expressed in leaves and stems where it is restricted to epidermal cells. Sequence analysis revealed a number of potential regulatory elements in the promoter, but most striking was the presence of a 319-bp direct repeat located between -333 and -968 upstream of the transcription start site. For a functional analysis, a 1,322-bp promoter fragment and two 5' deletions of 782 bp and 162 bp in length were translationally fused to the beta-glucuronidase (GUS) reporter gene and used for transient expression studies by particle bombardment. All promoter constructs conferred expression of GUS activity in different epidermal cell types of potato leaves. Expression in parenchyma cells of the leaf mesophyll was not detectable with any of the ChtC2 gene promoter constructs, in contrast to the pattern observed with the 35S promoter from cauliflower mosaic virus. The epidermis-specific expression of the reporter gene was confirmed using transgenic potato plants containing the fusion of the entire ChtC2 promoter with the GUS reporter. Histochemical analysis indicated that the promoter was only active in epidermal cells of leaves.

The substrate specificity-determining amino acid code of 4-coumarate:CoA ligase

To reveal the structural principles determining substrate specificity of 4-coumarate:CoA ligase (4CL), the crystal structure of the phenylalanine activation domain of gramicidin S synthetase was used as a template for homology modeling. According to our model, 12 amino acid residues lining the Arabidopsis 4CL isoform 2 (At4CL2) substrate binding pocket (SBP) function as a signature motif generally determining 4CL substrate specificity. We used this substrate specificity code to create At4CL2 gain-of-function mutants. By increasing the space within the SBP we generated ferulic- and sinapic acid-activating At4CL2 variants. Increasing the hydrophobicity of the SBP resulted in At4CL2 variants with strongly enhanced conversion of cinnamic acid. These enzyme variants are suitable tools for investigating and influencing metabolic channeling mediated by 4CL. Knowledge of the 4CL specificity code will facilitate the prediction of substrate preference of numerous, still uncharacterized 4CL-like proteins.

4-Coumarate:coenzyme A ligase has the catalytic capacity to synthesize and reuse various (di)adenosine polyphosphates

4-Coumarate:coenzyme A ligase (4CL) is known to activate cinnamic acid derivatives to their corresponding coenzyme A esters. As a new type of 4CL-catalyzed reaction, we observed the synthesis of various mono- and diadenosine polyphosphates. Both the native 4CL2 isoform from Arabidopsis (At4CL2 wild type) and the At4CL2 gain of function mutant M293P/K320L, which exhibits the capacity to use a broader range of phenolic substrates, catalyzed the synthesis of adenosine 5'-tetraphosphate (p(4)A) and adenosine 5'-pentaphosphate when incubated with MgATP(-2) and tripolyphosphate or tetrapolyphosphate (P(4)), respectively. Diadenosine 5',5''',-P(1),P(4)-tetraphosphate represented the main product when the enzymes were supplied with only MgATP(2-). The At4CL2 mutant M293P/K320L was studied in more detail and was also found to catalyze the synthesis of additional dinucleoside polyphosphates such as diadenosine 5',5'''-P(1),P(5)-pentaphosphate and dAp(4)dA from the appropriate substrates, p(4)A and dATP, respectively. Formation of Ap(3)A from ATP and ADP was not observed with either At4CL2 variant. In all cases analyzed, (di)adenosine polyphosphate synthesis was either strictly dependent on or strongly stimulated by the presence of a cognate cinnamic acid derivative. The At4CL2 mutant enzyme K540L carrying a point mutation in the catalytic center that is critical for adenylate intermediate formation was inactive in both p(4)A and diadenosine 5',5''',-P(1),P(4)-tetraphosphate synthesis. These results indicate that the cinnamoyl-adenylate intermediate synthesized by At4CL2 not only functions as an intermediate in coenzyme A ester formation but can also act as a cocatalytic AMP-donor in (di)adenosine polyphosphate synthesis.

Primary structure and tissue-specific expression of the pathogenesis-related protein PR-1b in potatodagger

Summary The infection of potato (Solanum tuberosum) leaves with the late blight pathogen Phytophthora infestans, or treatment with fungal elicitor, leads to the massive accumulation of pathogenesis-related (PR) proteins in the extracellular leaf space. The most abundant of these proteins was purified to apparent homogeneity and identified as a new, basic member of the PR-1 family of defence proteins, designated PR-1b. Antibodies raised against the protein and a cDNA isolated by differential screening were used to study the temporal and spatial patterns of PR-1b protein and mRNA distribution in healthy and infected potato tissues. PR-1b was present in old leaves and at low levels also in the carpels of flowers. In leaves, strong accumulation of PR-1b mRNA and protein occurred in response to infection by the oomycete pathogen Phytophthora infestans or the bacterial pathogen Pseudomonas syringae pv. maculicola. PR-1b mRNA and protein accumulation was clearly initiated at the infection site, but a delayed and sustained accumulation was also observed in neighbouring, uninfected leaves of potato plants. Tissue- and cell type-specific expression of PR-1b was analysed by immunohistochemical and in situ RNA hybridization techniques. Appreciable amounts of PR-1b protein and mRNA were localized in epidermal cells, guard cells of the stomata, glandular trichomes, crystal idioblasts, and cells of the vascular system of infected leaves. However, no significant differences in the amounts and distribution patterns of PR-1b could be observed between compatible and incompatible interactions of potato and Phytophthora infestans, indicating that PR-1b expression is not involved in determining cultivar/race-specific resistance in potato.

Identification of the substrate specificity-conferring amino acid residues of 4-coumarate:coenzyme A ligase allows the rational design of mutant enzymes with new catalytic properties

4-Coumarate:coenzyme A ligases (4CLs) generally use, in addition to coumarate, caffeate and ferulate as their main substrates. However, the recently cloned Arabidopsis thaliana isoform At4CL2 is exceptional because it has no appreciable activity with ferulate. On the basis of information obtained from the crystal structure of the phenylalanine-activating domain of gramicidin S-synthetase, 10 amino acid residues were identified that may form the substrate binding pocket of 4CL. Among these amino acids, representing the putative "substrate specificity motif," only one residue, Met(293), was not conserved in At4CL2, compared with At4CL1 and At4CL3, two isoforms using ferulate. Substitution of Met(293) or Lys(320), another residue of the putative substrate specificity motif, which in the predicted three-dimensional structure is located in close proximity to Met(293), by smaller amino acids converted At4CL2 to an enzyme capable of using ferulate. The activity with caffeate was not or only moderately affected. Conversely, substitution of Met(293) by bulky aromatic amino acids increased the apparent affinity (K(m)) for caffeate up to 10-fold, whereas single substitutions of Val(294) did not affect substrate use. The results support our structural assumptions and suggest that the amino acid residues 293 and 320 of At4CL2 directly interact with the 3-methoxy group of the phenolic substrate and therefore allow a first insight into the structural principles determining substrate specificity of 4CL.

Two differentially regulated class II chitinases from parsley

Two distinct cDNA clones, PcCHI1 and PcCHI2, with high sequence similarity to plant chitinases were isolated from parsley (Petroselinum crispum), expressed in Escherichia coli, and the encoded proteins functionally identified as endochitinases. Different expression patterns of the corresponding mRNAs and proteins in infected and uninfected parsley plants indicated distinct roles of the two isoforms in both pathogen defense and plant development. Infection of parsley leaf buds with Phytophthora sojae resulted in the rapid, transient and highly localized accumulation of PcCHI1 mRNA and protein around infection sites, whereas PcCHI2 mRNA and protein were systemically induced at later infection stages. Similar differences in the timing of induction were observed in elicitor-treated, suspension-cultured parsley cells. In uninfected plants, PcCHI1 mRNA was particularly abundant in the transmitting tract of healthy flowers, suggesting a role in the constitutive protection of susceptible transmitting tissue of the style against pathogen ingress and/or in the fertilization process, possibly by affecting pollen tube growth. Localization of PcCHI2 mRNA and protein in the parenchymatic collenchyme of young pedicels may indicate a function in the constitutive protection of this tissue. In addition to such distinct roles of PcCHI1 and PcCHI2 in preformed and induced pathogen defense, both chitinases may have endogenous regulatory functions in plant development.

Mutational analysis of 4-coumarate:CoA ligase identifies functionally important amino acids and verifies its close relationship to other adenylate-forming enzymes

4-Coumarate:coenzyme A ligase (4CL) is a key enzyme of general phenylpropanoid metabolism which provides the precursors for a large variety of important plant secondary products, such as lignin, flavonoids, or phytoalexins. To identify amino acids important for 4CL activity, eight mutations were introduced into Arabidopsis thaliana At4CL2. Determination of specific activities and K(m) values for ATP and caffeate of the heterologously expressed and purified proteins identified four distinct classes of mutants: enzymes with little or no catalytic activity; enzymes with greatly reduced activity but wild-type K(m) values; enzymes with drastically altered K(m) values; and enzymes with almost wild-type properties. The latter class includes replacement of a cysteine residue which is strictly conserved in 4CLs and had previously been assumed to be directly involved in catalysis. These results substantiate the close relationship between 4CL and other adenylate-forming enzymes such as luciferases, peptide synthetases, and fatty acyl-CoA synthetases.

Three 4-coumarate:coenzyme A ligases in Arabidopsis thaliana represent two evolutionarily divergent classes in angiosperms

The enzyme 4-coumarate:CoA ligase (4CL) plays a key role in channelling carbon flow into diverse branch pathways of phenylpropanoid metabolism which serve important functions in plant growth and adaptation to environmental perturbations. Here we report on the cloning of the 4CL gene family from Arabidopsis thaliana and demonstrate that its three members, At4CL1, At4CL2 and At4CL3, encode isozymes with distinct substrate preference and specificities. Expression studies revealed a differential behaviour of the three genes in various plant organs and upon external stimuli such as wounding and UV irradiation or upon challenge with the fungus, Peronospora parasitica. Phylogenetic comparisons indicate that, in angiosperms, 4CL can be classified into two major clusters, class I and class II, with the At4CL1 and At4CL2 isoforms belonging to class I and At4CL3 to class II. Based on their enzymatic properties, expression characteristics and evolutionary relationships, At4CL3 is likely to participate in the biosynthetic pathway leading to flavonoids whereas At4CL1 and At4CL2 are probably involved in lignin formation and in the production of additional phenolic compounds other than flavonoids.

A distinct member of the basic (class I) chitinase gene family in potato is specifically expressed in epidermal cells

We have isolated cDNA clones encoding class I chitinase (ChtC) from potato leaves which share a high degree of nucleotide and amino acid sequence similarity to other, previously described basic (class I) chitinases (ChtB) from potato. Despite this similarity, characteristic features distinguish ChtC from ChtB, including an extended proline-rich linker region between the hevein and catalytic domains and presence of a potential glycosylation site (NDT) in the deduced protein. These differences are in accordance with the properties of purified chitinase C which is glycosylated and hence has a higher molecular mass in comparison to chitinase B. In contrast to the coding sequences, the 3'-untranslated regions of ChtC and ChtB exhibited a low degree of similarity, which allowed us to generate gene-specific probes to study the genomic organization and expression of both types of gene. Genomic DNA blots suggest that ChtC and ChtB are each encoded by one or two genes per haploid genome. RNA blot analysis showed that in healthy potato plants ChtC mRNA is most abundant in young leaves, the organs which also contain high levels of chitinase C. By contrast, ChtB mRNA abundance is highest in old leaves, which accumulate chitinase B. By in situ RNA hybridization with gene-specific probes we could demonstrate that ChtC mRNA in leaves is restricted to epidermal cells, whereas ChtB mRNA showed no distinct pattern of cell-type-specific localization. Infection of potato leaves with Phytophthora infestans, or treatment with fungal elicitor, ethylene, or wounding resulted in accumulation of both ChtC and ChtB mRNAs; however, for ChtC, in contrast to ChtB, no corresponding accumulation of the encoded protein could be detected, suggesting a post-transcriptional mechanism of regulation. Salicylic acid treatment did not induce accumulation of either mRNA. The possible functional implications of these findings for pathogen defence and developmental processes are discussed.

Primary structure and expression of acidic (class II) chitinase in potato

Infection of potato (Solanum tuberosum) leaves by the late blight fungus Phytophthora infestans or treatment with fungal elicitor leads to a strong increase in chitinase activity. We isolated cDNAs encoding acidic (class II) chitinases (ChtA) from potato leaves and determined their structures and expression patterns in healthy and stressed plants. From the total number of cDNAs and the complexity of genomic DNA blots we conclude that acidic chitinase in potato is encoded by a gene family which is considerably smaller than that encoding basic (class I) chitinase (ChtB). The deduced amino acid sequences show 78 to 96% identity to class II chitinases from related plant species tomato, tobacco) whereas the identity to basic chitinases of potato is in the range of 60%. RNA blot analysis revealed that both acidic and basic chitinases were strongly induced by infection or elicitor treatment and that the induction occurred both locally at the site of infection and systemically in upper uninfected leaves. In contrast, a differential response to other types of stress was observed. Acidic chitinase mRNA was strongly induced by salicylic acid, whereas basic chitinase mRNA was induced by ethylene or wounding. In healthy, untreated plants, acidic chitinase mRNA accumulated also in an organ-, cell-type- and development-specific manner as revealed by RNA blot analysis and in situ RNA hybridization. Relatively high transcript levels were observed in old leaves and young internodes as well as in vascular tissue and cells constituting the stomatal complex in leaves and petioles. Lower, but appreciable mRNA levels were also detectable in roots and various flower organs, particularly in sepals and stamens. The possible implications of these findings in pathogen defense, development and growth processes are discussed.

Primary structure and expression of mRNAs encoding basic chitinase and 1,3-beta-glucanase in potato

Infection of potato leaves (Solanum tuberosum L. cv. Datura) by the late blight fungus Phytophthora infestans, or treatment with fungal elicitor leads to a strong increase in chitinase and 1,3-beta-glucanase activities. Both enzymes have been implicated in the plant's defence against potential pathogens. In an effort to characterize the corresponding genes, we isolated complementary DNAs encoding the basic forms (class I) of both chitinase and 1,3-beta-glucanase, which are the most abundant isoforms in infected leaves. Sequence analysis revealed that at least four genes each are expressed in elicitor-treated leaves. The structural features of the potato chitinases include a hydrophobic signal peptide at the N-terminus, a hevein domain which is characteristic of class I chitinases, a proline- and glycine-rich linker region which varies among all potato chitinases, a catalytic domain, and a C-terminal extension. The potato 1,3-beta-glucanases also contain a N-terminal hydrophobic signal peptide and a C-terminal extension, the latter comprising a potential glycosylation site. RNA blot hybridization experiments showed that basic chitinase and 1,3-beta-glucanase are strongly and coordinately induced in leaves in response to infection, elicitor treatment, ethylene treatment, or wounding. In addition to their activation by stress, both types of genes are regulated by endogenous factors in a developmental and organ-specific manner. Appreciable amounts of chitinase and 1,3-beta-glucanase mRNAs were found in old leaves, stems, and roots, as well as in sepals of healthy, untreated plants, whereas tubers, root tips, and all other flower organs (petals, stamen, carpels) contained very low levels of both mRNAs. In young leaves and stems, chitinase and 1,3-beta-glucanase were differentially expressed. While chitinase mRNA was abundant in these parts of the plant, 1,3-beta-glucanase mRNA was absent. DNA blot analysis indicated that in potato, chitinase and 1,3-beta-glucanase are encoded by gene families of considerable complexity.

Purification and characterization of extracellular, acidic chitinase isoenzymes from elicitor-stimulated parsley cells

Treatment of cultured parsley cells (Petroselinum crispum) with fungal elicitor caused large increases in the activities of chitinase and 1,3-beta-glucanase. Chitinase activity accumulated predominantly in the culture medium, whereas 1,3-beta-glucanase activity was located almost exclusively intracellularly. Extracellular chitinase activity was resolved into six different isoenzymes, all of which were purified and characterized. All six isoforms were acidic proteins (pI 3.8-5.3), with molecular mass 30-38 kDa. Four were exochitinases and two were endochitinases. The most abundant isoform also showed lysozyme activity. Three of the exochitinases were glycoproteins and two of these were reactive with an antiserum specific for xylose in complex glycosidic structures. The exochitinases constituted relatively small proportions of the total chitinase activity and may serve a different function in cellular metabolism compared to the more abundant endochitinases.

Rapid, systemic repression of the synthesis of ribulose 1,5-bisphosphate carboxylase small-subunit mRNA in fungus-infected or elicitor-treated potato leaves

The levels of ribulose 1,5-bisphosphate carboxylase small-subunit (SSU) mRNA and protein decreased considerably in potato (Solanum tuberosum L.) leaves upon infection with the pathogenic fungus,Phytophthora infestans, or upon treatment with an elicitor preparation from the fungal culture fluid. This effect occurred systemically throughout the affected leaf, regardless of whether the interaction withP. infestans was compatible or incompatible. Using the comparatively drastic and synchronous response to fungal elicitor, we demonstrated that the repression of SSU synthesis was caused by rapid gene inactivation. The timing of repression was similar to that observed previously for the transcriptional activation of various pathogen defense reactions. This supports the hypothesis that induction of the extensive, multi-component defense response requires repression of other cellular functions to ensure metabolic balance.

Differential early activation of defense-related genes in elicitor-treated parsley cells

A cDNA library from cultured parsley (Petroselinum crispum) cells was differentially screened using labeled run-off transcripts derived from nucleic of elicitor-treated and untreated cells. This resulted in the isolation of 18 independent cDNA families representing putative defense-related genes. All genes are rapidly and transiently activated after elicitor application, but the time courses of transcriptional activity exhibit considerable variations, indicating differences in the mechanisms of gene regulation.

Transient Induction of Phenylalanine Ammonia-Lyase and 4-Coumarate: CoA Ligase mRNAs in Potato Leaves Infected with Virulent or Avirulent Races of Phytophthora infestans

Infection of potato leaves with the fungal pathogen Phytophthora infestans (Pi) resulted in the rapid stimulation of phenylpropanoid metabolism. Increases in the activities of several mRNAs, including those encoding phenylalanine ammonia-lyase (PAL) and 4-coumarate:CoA ligase (4CL), were detectable within a few hours postinoculation, as demonstrated by two-dimensional gel electrophoresis of proteins synthesized in vitro. This effect was closely mimicked by application of Pi culture filtrate through cut leaf stems. PAL and 4CL mRNA activities were also rapidly and transiently induced in potato cell suspension cultures by treatments with Pi culture filtrate or arachidonic acid. This induction was exploited to generate cDNA probes complementary to PAL and 4CL mRNAs. Blot hybridizations using these probes revealed almost immediate, transient and coordinate increases in the transcription rates and subsequent changes in the amounts of PAL and 4CL mRNAs in leaves treated with Pi culture filtrate. Similar changes in the mRNA amounts were found in infected leaves of potato cultivars carrying resistance genes R1 (cv Datura) or R4 (cv Isola), independent of whether a virulent or an avirulent Pi pathotype was used for inoculation. These results are discussed in relation to recent cytological observations with the same potato cultivars and Pi pathotypes.

Occurrence of phytoalexins and other putative defense-related substances in uninfected parsley plants

Considerable amounts of the following substances were found in uninfected parsley (Petroselinum crispum) cotyledons: furanocoumarins, the putative phytoalexins of this and some related plant species, two enzymes of the furanocoumarin pathway (S-adenosyl-L-methionine: xanthotoxol and S-adenosyl-L-methionine: bergaptol O-methyltransferases), two hydrolytic enzymes (1,3-β-glucanase, EC 3.2.1.39, and chitinase, EC 3.2.1.14), and 'pathogenesis-related' proteins. The furanocoumarins and the methyltransferase activities reached their highest levels at the onset of cotyledon senescence as the hydrolytic enzymes increased from low to relatively high activity values. The relative amounts of pathogenesis-related proteins 1 and 2, as well as the corresponding mRNAs, also increased markedly. Two enzymes of general phenylpropanoid metabolism, L-phenylalanine ammonia-lyase and 4-coumarate: CoA ligase, decreased in activity in a biphasic fashion during cotyledon development. At all developmental stages, the levels of these putative defense-related agents in total cotyledon extracts were too high to enable detection of, possibly, additional changes upon infection with zoospores of Phytophthora megasperma f. sp. glycinea, a fungal pathogen to which parsley shows a non-host, hypersensitive resistance response.

Responses of cultured parsley cells to elicitors from phytopathogenic fungi : timing and dose dependency of elicitor-induced reactions

Cultured parsley cells (Petroselinum crispum) responded to treatment with heat-released soluble cell-wall fragments (elicitors) from several different phytopathogenic fungi by forming coumarin derivatives (phytoalexins). This response was preceded in all cases by large but transient increases in the activities of two enzymes of general phenylpropanoid metabolism, phenylalanine ammonia-lyase (PAL) and 4-coumarate:CoA ligase (4CL). The activities of two hydrolytic enzymes, chitinase and 1,3-beta-glucanase, also increased strongly in elicitor-treated cells, whereas the activities of three enzymes participating in primary metabolism were affected differently by the elicitor treatment. Glucose-6-phosphate dehydrogenase increased, phosphofructokinase remained almost constant, and pyrophosphate:fructose-6-phosphate phosphotransferase declined sharply in activity. Different amounts of cell-wall preparations from various phytopathogenic fungi were required for maximum elicitor activity. While three oomycetes (Phytophthora spp.) yielded the most active elicitors studied (maximum coumarin accumulation at concentrations of about 10 microgram per milliliter), cell-wall preparations from an ascomycete and three deuteromycetes gave comparable results only at 10 to 100 times higher concentrations. Optimal induction of PAL, 4CL, and chitinase with Phytophthora elicitor required only about 1 microgram per milliliter, whereas 1,3-beta-glucanase induction showed a dose dependence similar to that observed for coumarins. The elicitor concentration had pronounced effects not only on the extent, but also on the timing of all induced reactions.