Showing papers on "Phytoalexin published in 1996"

Tansley Review No. 86 Accumulation of phytoalexins: defence mechanism and stimulus response system.

Abstract: SUMMARY Phytoalexin synthesis is a defence-response- that is characterized by a requirement for a number of distinct elements, all of which must be present for the response to be expressed fully. These same elements: a signal, a cellular receptor, a signal transduction system and a responsive metabolic system, are also used to describe a stimulus-response system. A number of molecular species can function as signal molecules or elicitors of phytoalexin synthesis, including poly- and oligosaccharides, proteins and polypeptides, and fatty acids. Few receptors for elicitors have been identified but those that have been are proteins located on the plasma membrane of the plant. Induction of phytoalexin synthesis involves selective and co-ordinated activation of specific defence response genes, including those encoding the enzymes of phytoalexin synthesis, and these genes constitute the responsive metabolic system. The separate, and distant, locations of the receptor and the responsive genes means that the event in which the signal is perceived by the receptor must be relayed to the genes by means of a second messenger system. Several second messengers are candidates for such a coupling- or signal transduction-system, including udenosine-3′,5′-cyclic monophosphate, Ca2+, diacylglycerol and inositol 1,4,5-trisphosphate, active oxygen species and jasmonic acid. Each has been examined as a possible component of the signal transduction system mediating between the elicitor receptor interaction and the phytoalexin synthesis it induces. Analysis of the signalling events is made complex by the simultaneous solicitation by the invading micro-organism of several defence responses, each of which might involve elements of a different signal system. The same complexity is evident which the role of phytoalexin accumulation in resistance is analysed. Evaluation of the contribution made by phytoalexin accumulation towards resistance has been attempted by the use of various inhibitors and enhancers of the process. Transgenic and mutant plants with specific alterations in one or more ot those elements necessary for the plant to respond to the signals for phytoalexin synthesis and other defence responses, are beginning to aid resolution of the complex pattern ot signalling events and the respective roles of the inducible defence mechanisms in resistance.

Involvement of Jasmonic Acid in Elicitor-Induced Phytoalexin Production in Suspension-Cultured Rice Cells

Abstract: It has been suggested that jasmonic acid (JA) could be an integral part of a general signal transduction system regulating inducible defense genes in plants. It was reported that treatment with an elicitor (N-acetylchitoheptaose) induced production of phytoalexin in suspension-cultured rice (Oryza sativa L.) cells. In this study, the role of JA in the induction of phytoalexin production by N-acetylchitoheptaose was investigated. Exogenously applied ([plus or minus])-JA (10–4 M) clearly induced the production of momilactone A, a major phytoalexin, in suspension-cultured rice cells. On the other hand, in rice cells treated with N-acetylchitoheptaose, endogenous JA was rapidly and transiently accumulated prior to accumulation of momilactone A. Treatment with ibuprofen, an inhibitor of JA biosynthesis, reduced production of momilactone A in the cells treated with N-acetylchitoheptaose, but the addition of ([plus or minus])-JA increased production of momilactone A to levels higher than those in the elicited rice cells. These results strongly suggest that JA functions as a signal transducer in the induction of biosynthesis of momilactone A by N-acetylchitoheptaose in suspension-cultured rice cells.

Nitric Oxide Induces Phytoalexin Accumulation in Potato Tuber Tissues

Abstract: We investigated whether nitric oxide (NO) radical could induce phytoalexin production. Treatment of potato tuber tissues with 1-hydroxy-2-oxo-3,3-bis(2-aminoethyl)1-triazene (NOC-18), an NO-releasing compound, induced the accumulation of the potato phytoalexin rishitin. This induction was inhibited by carboxy-2-phenyl-4,4,5,5-tetramethylimidazoline 1-oxyl-3-oxide (carboxy-PTI0), an NO-specific scavenger, or by Tiron, a radical scavenger, suggesting a phytoalexin inducing activity for NO.

Occurrence of phytoalexins and phenolic compounds in endomycorrhizal interactions, and their potential role in biological control

Abstract: This paper will review work mainly done during the last twenty years on the involvement of phytoalexin and phenolic compounds in mycorrhizal interactions. It has been observed that phytoalexins and associated molecules accumulate in roots after mycorrhizal infection, but less intensively and more slowly than in pathogenic interactions. Following mycorrhizal infection, enzymes of phenylpropanoid metabolism have been shown to be activated differentially. Some flavonoids and isoflavonoids have been reported to stimulate in vitro germination of mycorrhizal fungi or in vitro mycorrhizal infection, but their biological significance in signalling between the two symbiotic partners, and in biocontrol of plant disease by arbuscular mycorrhizal fungi, have not yet been elucidated.

Mode of action of the Arabidopsis thaliana phytoalexin camalexin and its role in Arabidopsis-pathogen interactions.

Abstract: The virulent Arabidopsis thaliana pathogen Pseudomonas syringae pv. maculicola strain ES4326 (Psm ES4326) and other gram-negative bacteria are sensitive to camalexin (3-thiazol-2'-yl-indole), the Arabidopsis phytoalexin. Furthermore, Psm ES4326 is unable to degrade camalexin or to become tolerant to it. Apparently, Psm ES4326 is a successful pathogen even though it elicits synthesis of a host phytoalexin to which it is sensitive. Assays of membrane integrity revealed that, like other phytoalexins, camalexin disrupts bacterial membranes, suggesting that camalexin toxicity is a consequence of membrane disruption. A screen for camalexin-resistant mutants of Psm ES4326 yielded only partially resistant mutants, which displayed partial resistance in both killing and membrane integrity assays. These mutants were also resistant to low concentrations of tetracycline and nalidixic acid, suggesting that they were affected in components of the outer membrane. The mutants were not distinguishable from Psm ES4326 in virulence assays. Camalexin was toxic to Arabidopsis cells growing in tissue culture. However, comparison of the extent of cell death associated with disease symptoms in infected leaves of wild-type Arabidopsis and a camalexin-deficient mutant suggested that camalexin does not contribute significantly to cell death in infected tissue.

Relationship between Active Oxygen Species, Lipid Peroxidation, Necrosis, and Phytoalexin Production Induced by Elicitins in Nicotiana.

Abstract: Excised leaves of Nicotiana tabacum var Xanthi and Nicotiana rustica were treated with cryptogein and capsicein, basic and acidic elicitins, respectively. Both compounds induced leaf necrosis, the intensity of which depended on concentration and duration of treatment. N. tabacum var Xanthi was the most sensitive species and cryptogein was the most active elicitin. Lipid peroxidation in elicitin-treated Nicotiana leaves was closely correlated with the appearance of necrosis. Elicitin treatments induced a rapid and transient burst of active oxygen species (AOS) in cell cultures of both Nicotiana species, with the production by Xanthi cells being 6-fold greater than that by N. rustica. Similar maximum AOS production levels were observed with both elicitins, but capsicein required 10-fold higher concentrations than those of cryptogein. Phytoalexin production was lower in response to both elicitins in N. tabacum var Xanthi cells than in N. rustica cells, and capsicein was the most efficient elicitor of this response. In cryptogein-treated cell suspensions, phytoalexin synthesis was unaffected by diphenyleneiodonium, which inhibited AOS generation, nor was it affected by tiron or catalase, which suppressed AOS accumulation in the extracellular medium. These results suggest that AOS production, lipid peroxidation, and necrosis are directly related, whereas phytoalexin production depends on neither the presence nor the intensity of these responses.

Degradation of stilbene‐type phytoalexins in relation to the pathogenicity of Botrytis cinerea to grapevines

Abstract: The ability of eight isolates of Botrytis cinerea to degrade the stilbene phytoalexins, resveratrol and pterostilbene, was compared with their pathogenicity to grapevines. All strains which degraded resveratrol and pterostilbene were highly or moderately pathogenic to in vitro cultures of grapevines (Vitis rupestris) after inoculation with agar disks containing mycelium, while those which were unable to degrade phytoalexins were non-pathogenic. In all cases, the hydroxystilbene-degrading activity was related to the presence of laccase activity in the culture filtrates, as shown by using syringaldazine as substrate. The role of laccase-mediated degradation of phytoalexins in relation to pathogenicity of B. cinerea to grapevines in discussed.

Detection and cellular localization of elemental sulphur in disease-resistant genotypes of Theobroma cacao

Abstract: DISEASE-RESISTANT genotypes are the basis for controlling many major microbial pathogens of economic plants. Resistance is often linked with organic, antimicrobial phytoalexins, produced de novo in cells surrounding apoptotic or 'hypersensitive' cells that die rapidly after contact with incompatible pathogens1–3. Here we report the production by resistant genotypes of Theobroma cacaoof four phytoalexins in response to a xylem-invading fungal pathogen; these comprised two phenolics, a triterpenoid and, highly unusually in a higher eukaryote, elemental sulphur as cyclooctasulphur S8. Energy-dispersive X-ray microanalysis revealed a high accumulation of sulphur only in cells and structures in potential contact with the vascular pathogen; that is, xylem parenchyma, xylem vessel cell walls and gels occluding vessels. Our data provide a rare example of cellular localization of an antimicrobial substance and evidence for the first time for accumulation of elemental sulphur in a plant linked with a resistance response; this discovery comes centuries after man first used elemental sulphur as a potent fungicide4,5.

Role of Jasmonic Acid as a Signaling Molecule in Copper Chloride-elicited Rice Phytoalexin Production

Abstract: Phytoalexins are antimicrobial secondary metabolites which accumulate in plants against fungal invasion. Their production is triggered not only by fungal invasion, but also by a variety of elicitors. In rice plants, we have shown that CuCl2 is a potent abiotic elicitor. Jasmonic acid has recently become known to play an important role in secondary metabolite production in plants at the cellular level. This led us to speculate, in CuCl2-elicited rice leaves, that JA might also play an important role as a signal transducer for phytoalexin production. The endogenous level of JA increased rapidly in CuCl2-elicited rice leaves, and exogenously applied JA caused a large amount of phytoalexin production in rice leaves. This phytoalexin production by CuCl2 decreased when rice leaves were treated with JA biosynthesis inhibitors, but that by JA did not. JA is thus suggested to play an important role in the elicitation process leading to phytoalexin production in rice leaves.

Two novel genes rapidly and transiently activated in suspension-cultured rice cells by treatment with N-acetylchitoheptaose, a biotic elicitor for phytoalexin production.

Abstract: By using subtracted probes, two cDNA clones of rice, EL2 and EL3, were isolated as genes responsive within 6 min to jV-acetylchit oheptaose, a potent biotic elicitor for phytoalexin biosynthesis. Analyses of the sequence of the cDNAs showed that both of EL2 and EL3 encoded basic proteins with no significant similarities to those of known genes.

High-affinity binding of fungal β-glucan elicitors to cell membranes of species of the plant family Fabaceae

Abstract: Microsomal preparations of six species of the plant family Fabaceae were screened for high-affinity binding of branched (1 → 3), (1 → 6)-β-glucans. Oligoglucosides of this type are specific elicitors of phytoalexin accumulation in soybean (Glycine max L.), a member of this family. The species studied were alfalfa (Medicago sativa L.), broadbean (Vicia faba L.), chickpea (Cicer arietinum L.), french bean (Phaseolus vulgaris L.), pea (Pisum sativum L.), and white lupin (Lupinus albus L.). A 125I-labeled 4-(2-aminophenyl)ethylamine conjugate of a (1 → 3), (1 → 6)-β-glucan fraction with an average degree of polymerization (DP) of 18, obtained from mycelial walls of Phytophthora sojae, was used as radioligand for initial screening. The structural complexity of this fraction allowed the identification of binding sites with affinities for isomeric structures other than the (1 → 3), (1 → 6) hepta-β-glucoside for which soybean binding sites display highest affinity. Radioligand competition experiments against unlabeled fungal β-glucan resulted in the identification of high-affinity binding in alfalfa, bean, lupin, and pea. Half-maximal competition concentrations (IC50) for fungal β-glucan in these species were between 5 and 30 nM. Pseudoheterologous radioligand competition by unlabeled hepta-β-glucoside showed that for alfalfa, lupin and pea the IC50 values for this structure (4 to 16 nM) were similar to those of soybean (7.7 nM). Bean microsomes, however, displayed an IC50 significantly higher than soybean (68 nM) suggesting that the structural motif recognized by its binding sites is not identical to that of soybean or the other three species. Radioligand saturation assays with alfalfa, lupin and pea microsomes using an 125I-labeled aminophenylethylamine hepta-β-glucoside conjugate gave dissociation constants (Kd) of 5.3, 3.7, and 1.8 nM, respectively. The affinity of these sites for hepta-β glucoside was in the same range as that of soybean (Kd 1–3 nM), whereas the affinity of the binding sites of bean for the same ligand was significantly lower (Kd = 33 nM). Good correlation was found between the presence of high-affinity binding and the accumulation of isoflavonoid phytoalexins in roots of alfalfa, bean, chickpea and pea seedlings after exposure to fungal β-glucan. Lupin displayed a strong wound-induced accumulation of prenylated isoflavones which was independent of the presence of β-glucan, making it impossible to determine phytoalexin induction in response to elicitor. No specific binding or phytoalexin accumulation in response to β glucans was observed in broadbean. This is the first report on the existence of possibly homologous elicitor-binding sites within a plant taxonomic family and may provide preliminary evidence for putative evolutionary relationships in pathogen perception mechanisms in plants.

Suppression of fungal beta-glucan-induced plant defence in soybean (Glycine max L.) by cyclic 1,3-1,6-beta-glucans from the symbiont Bradyrhizobium japonicum

Abstract: The production of antimicrobial phytoalexins is one of the best-known inducible defence responses following microbial infection of plants or treatment with elicitors. In the legume soybean (Glycine max L.), 1,3-1,6-β-glucans derived from the fungal pathogen Phytophthora sojae have been identified as potent elicitors of the synthesis of the phytoalexin, glyceollin. Recently it has been reported that during symbiotic interaction between soybean and the nitrogen-fixing bacterium Bradyrhizobium japonicum USDA 110 the bacteria synthesize cyclic 1,3-1,6-β-glucans. Here we demonstrate that both the fungal and the bacterial β-glucans are ligands of β-glucan-binding sites which are putative receptors for the elicitor signal compounds in soybean roots. Whereas the fungal β-glucans stimulate phytoalexin synthesis at low concentrations, the bacterial cyclic 1,3-1,6-β-glucans appear to be inactive even at relatively high concentrations. Competition studies indicate that increasing concentrations of the bacterial 1,3-1,6-β-glucans progressively inhibit stimulation of phytoalexin synthesis in a bioassay induced by the fungal 1,3-1,6-β-glucans. Another type of cyclic β-glucan, a 1,2-β-glucan from Rhizobium meliloti, that does not nodulate on soybean, seems to be inactive as elicitor and as ligand of the β-glucan-binding sites. These results may indicate a novel mechanism for a successful plant-symbiont interaction by suppressing the plant's defence response.

Synthesis of 7-Methoxyapigeninidin and Its Fungicidal Activity against Gloeocercospora sorghi

Abstract: In the structure of sakuranetin, which was isolated as a phytoalexin from the rice plant, the methoxy group at C-7 has been shown to be important for its high activity. Apigeninidin was isolated as a phytoalexin from sorghum, but it had no methoxy group at C-7. We prepared 7-methoxyapigeninidin and compared its fungicidal activity with that of apigeninidin. The 7-methoxyapigeninidin showed higher activity against sorghum fungi than apigeninidin, suggesting that the methoxy group at C-7 was important for the high fungicidal activity.

The Induction of Phenolic Compounds in Rice After Infection By the Stem Nematode Ditylenchus Angustus

Abstract: Changes in the phenolic profiles of five resistant and two suspectible varieties of deep water rice were examined following infection by the rice stem nematode Ditylenchus angustus. Within resistant cultivars, inoculated plants were either symptomless or exhibited symptoms of incompatible (resistant), compatible (suspectible) or partially compatible responses. Extracts of leaf tissues from inoculated and uninoculated plants were made in methanol (80% v/v) and analysed by reverse phase HPLC. Tissues from inoculated plants with and without symptoms were examined separately. Traces revealed the presence of 13 to 18 UV absorbing constituents of rice leaf extracts, depending on the variety. Chlorogenic acid occurred in small amounts and only in resistant plants, but there was no correlation between constitutive compounds and resistance or susceptibility. The secondary chemistry of tissues exhibiting symptoms changed in all resistant varieties. Levels of chlorogenic acid increased in response to infection. Furthermore in two resistant selections of rice variety Rayada 16-06, a newly formed compound, the rice phytoalexin sakuranetin was also isolated. There was correlative evidence that both of these compounds could have a functional role in resistance. At 5 days after inoculation the concentrations of sakuranetin in Rayada 16-06 were 8 & 13 μg/g leaf in plants with 10 and 200 D. angustus/plant, respectively. No change in secondary chemistry occurred in susceptible varieties.

Comparative Studies of Elicitors That Induce Phytoalexin in Oats

Abstract: The activity of the four elicitors, a N-acetylchitopentaose [(GLNAc) 5 ], a chitopentaose [(GLN) 5 ], Ag + ion and victorin, which induce phytoalexins in oats were compared using the following assays: i) electrolyte leakage, ii) effects of Ca 2+ on the induction of phytoalexin and iii) difference in inducedmetabolite profiles. The results suggest that at least two distinct mechanisms are involved in phytoalexin induction in oats. Among the tested elicitors, (GLNAC) 5 appears to induce phytoalexin directly by binding to its putative receptor, while induction by other elicitors may be a secondary effect, presumably due to the damage of the plasma membrane caused by the elicitors.

Phytotoxin production by Alternaria linicola and phytoalexin production by the linseed host

Abstract: Summary. Alternaria linicola produced a wide range of secondary metabolites when grown in a defined culture medium. Reverse phase chromatography fractions produced disease-like symptoms on linseed cultivars and a range of non-host species indicating the presence of phytotoxic components. Characterised via thin layer chromatography, these included the non-host specific phytotoxins tenuazonic acid, alternariol monomethyl ether, tentoxin and two destruxin-type compounds (which closely resembled destruxin A and destruxin B). The identity of four of the compounds was confirmed by two dimensional thin layer chromatography and proton nuclear magnetic resonance spectroscopy. In a second experiment, Linum leaf material infected with conidia of A. linicola and blastospores of Melampsora lini was extracted using a facilitated diffusion extraction technique. The resultant extracts contained a number of compounds which were fungitoxic to Cladosporium cladospiroides and, to a lesser extent, Alternaria brassicicola. One such compound corresponded to the phytoalexin coniferyl alcohol. Quantitative differences in the amount of the fungitoxic compounds produced between the inoculated and uninoculated resistant and susceptible host genotype combinations suggested that the production of fungitoxic compounds was greater in response to attempted colonisation. On this basis it is proposed that phytoalexin production is a component of the resistance reaction. The results from these investigations are discussed in relation to recent research on the ecology of the pathogen and the possible roles of phytotoxin production by the pathogen and phytoalexin production by the host on disease development.

The role of Ca(2+) in elicitation of phytoalexin synthesis in cell culture of onion (Allium cepa L.).

Abstract: Treatment of Allium cepa L. cellsuspension cultures with a biotic elicitor derived from the fungus Botrytis cinerea, resulted in phytoalexin synthesis. Two phytoalexins, 5-octylcyclopenta-1,3-dione and 5-hexyl-cyclopenta-1,3-dione, were accumulated in cultured onion cells. Removal of extracellular Ca(2+) by the calcium chelator ethylene glycol bis(b-aminoethyl ether) N,N'-tetraacetic acid abolished the elicitor-mediated phytoalexin synthesis. The calcium channel blockers, verapamil and 8-N,N-(dimethylamino)octyl-3,4,5-trimethoxybenzoate caused similar effects, whereas the addition of the Ca(2+) ionophore A23187 enhanced the accumulation of phytoalexins in the absence of the elicitor. Increase in the cytoplasmic Ca(2+) concentration in elicitor-treated onion cells was observed as monitored by the fluorescent calcium indicator indo-1. These observations suggest that Ca(2+) acts as a second messenger in the regulation of phytoalexin synthesis in cultured onion cells.