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This review examines the main features of natural phytoalexines of flavonoid and stilbenoid natures, which are secondary metabolism products in numerous plants widely used as biologically active substances in the medicine, pharmacology, and agricultural plants protection. We considered the role of flavonoids and stilbenes in phytoimmune and antistress responses in plants, bactericide antifungul, and antiviral effects towards microorganisms, and the wide medical application for a number of mammalian pathologies. The main achievements in the metabolic engineering of flavonoids in microbial biotechnologies are discussed.

The Arabidopsis PAD4 gene was previously shown to be required for synthesis of camalexin in response to infection by the virulent bacterial pathogen Pseudomonas syringae pv maculicola ES4326 but not in response to challenge by non-host fungal pathogen Cochliobolus carbonum. In this study, we show that pad4 mutants exhibit defects in defense responses, including camalexin synthesis and pathogenesis-related PR-1 gene expression, when infected by P. s. maculicola ES4326. No such defects were observed in response to infection by an isogenic avirulent strain carrying avirulence gene avrRpt2. In P. s. maculicola ES4326-infected pad4 plants, synthesis of salicylic acid (SA) was found to be reduced and delayed when compared with SA synthesis in wild-type plants. Moreover, treatment of pad4 plants with SA partially reversed the camalexin deficiency and PR-1 gene expression phenotypes of P. s. maculicola ES4326-infected pad4 plants. These findings support the hypothesis that PAD4 acts upstream from SA accumulation in regulating defense response expression in plants infected with P. s. maculicola ES4326. A working model of the role of PAD4 in governing expression of defense responses is presented

A full-length cDNA encoding phenylalanine ammonia-lyase (PAL) from Zea mays L. was isolated and the coding region was expressed in Escherichia coli as a C-terminal fusion to glutathione S-transferase. After purification by glutathione-Sepharose chromatography, the glutathione S-transferase moiety was cleaved off and the resulting PAL enzyme analyzed. In contrast to PAL from dicots, this maize PAL isozyme catalyzed the deamination of both L-phenylalanine (PAL activity) and L-tyrosine (tyrosine ammonialyase activity). These results provide unequivocal proof that PAL and tyrosine ammonia-lyase activities reside in the same polypeptide. In spite of large differences in the Michaelis constant and turnover number of the two activities, their catalytic efficiencies are very similar. Also, both activities have the same pH and temperature optima. These results imply that maize can produce p-coumaric acid from both phenylalanine and tyrosine

Plant defense against microbial pathogens and herbivores relies heavily on the induction of defense proteins and low molecular weight antibiotics. The signals between perception of the aggression, gene activation, and the subsequent biosynthesis of secondary compounds are assumed to be pentacylic oxylipin derivatives. The rapid, but transient, synthesis of cis-jasmonic acid was demonstrated after insect attack on a food plant and by microbial elicitor addition to plant suspension cultures. This effect is highly specific and not caused by a number of environmental stresses such as light, heavy metals, or cold or heat shock. Elicitation of Eschscholtzia cell cultures also led to a rapid alkalinization of the growth medium prior to jasmonate formation. Inhibition of this alkalinization process by the protein kinase inhibitor staurosporine also inhibited jasmonate formation. The induction of specific enzymes in the benzo [c] phenanthridine alkaloid pathway leading to the antimicrobial sanguinarine was induced to a qualitatively and quantitatively similar extent by fungal elicitor, methyl jasmonate, and its linolenic acid-derived precursor 12-oxophytodienoic acid. It is herein proposed that a second oxylipid cascade may exist in plants starting from linoleic acid via 15,16-dihydro-12-oxophytodienoic acid to 9,10-dihydrojasmonate. Experiments with synthetic trihomojasmonate demonstrated that p-oxidation is not a prerequisite for biological activity and that 12-oxophytodienoic acid and derivatives are most likely fully active as signal transducers. Octadecanoic acid-derived compounds are essential elements in modulating the synthesis of antibiotic compounds and are thus integral to plant defense.

The biological activity of resveratrol, a stilbenic compound synthesized by grapevines in response to various stresses, was reevaluated against Botrytis cinerea using a novel in vitro system that enabled direct observation of the fungus with an inverted microscope. We determined that 90 micrograms resveratrol/ml reduced germination of B. cinerea conidia by ca. 50%. Moreover, resveratrol was shown to significantly reduce mycelial growth of B. cinerea at concentrations ranging from 60 to 140 micrograms/ml. Exposure to resveratrol at concentrations ranging from 60 to 140 micrograms/ml resulted in cytological changes in B. cinerea, such as production of secondary or tertiary germ tubes by conidia, cytoplasmic granulations, protoplasmic retractation in the hyphal tip cells, and formation of curved germ tubes. These data reinforce the role played by this compound in the B. cinerea-grapevine interaction

Systemic acquired resistance is an important component of the disease-resistance arsenal of plants, and is associated with an enhanced potency for activating local defense responses upon pathogen attack. Here we demonstrate that pretreatment with benzothiadiazole (BTH), a synthetic activator of acquired resistance in plants, augmented the sensitivity for low-dose elicitation of coumarin phytoalexin secretion by cultured parsley (Petroselinum crispum L.) cells. Enhanced coumarin secretion was associated with potentiated activation of genes encoding Phe ammonia-lyase (PAL). The augmentation of PAL gene induction was proportional to the length of pretreatment with BTH, indicating time-dependent priming of the cells. In contrast to the PAL genes, those for anionic peroxidase were directly induced by BTH in the absence of elicitor, thus confirming a dual role for BTH in the activation of plant defenses. Strikingly, the ability of various chemicals to enhance plant disease resistance correlated with their capability to potentiate parsley PAL gene elicitation, emphasizing an important role for defense response potentiation in acquired plant disease resistance

A genetic approach was used to assess the extent to which a particular plant defense response, phytoalexin biosynthesis, contributes to Arabidopsis thaliana resistance to Pseudomonas syringae pathogens. The A. thaliana phytoalexin, camalexin, accumulated in response to infection by various P. syringae strains. No correlation between pathogen avirulence and camalexin accumulation was observed. A biochemical screen was used to isolate three mutants of A. thaliana ecotype Columbia that were phytoalexin deficient (pad mutants). The mutations pad1, pad2, and pad3 were found to be recessive alleles of three different genes. pad1 and pad2 were mapped to chromosome IV and pad3 was mapped to chromosome III. Infection of pad mutant plants with strains carrying cloned avirulence genes revealed that the pad mutations did not affect the plants' ability to restrict the growth of these strains. This result strongly suggests that in A. thaliana, phytoalexin biosynthesis is not required for resistance to avirulent P. syringae pathogens. Two of the pad mutants displayed enhanced sensitivity to isogenic virulent P. syringae pathogens, suggesting that camalexin may serve to limit the growth of virulent bacteria.

Salicylic acid (SA) is a natural inducer of disease resistance in some dicotyledonous plants. Rice seedlings (Oryza sativa L.) had the highest levels of SA among all plants tested for SA content (between 0.01 and 37.19 microgram/g fresh weight). The second leaf of rice seedlings had slightly lower SA levels than any younger leaves. To investigate the role of SA in rice disease resistance, we examined the levels of SA in rice (cv M-201) after inoculation with bacterial and fungal pathogens. SA levels did not increase after inoculation with either the avirulent pathogen Pseudomonas syringae D20 or with the rice pathogens Magnaporthe grisea, the causal agent of rice blast, and Rhizoctonia solani, the causal agent of sheath blight. However leaf SA levels in 28 rice varieties showed a correlation with generalized blast resistance, indicating that SA may play a role as a constitutive defense compound. Biosynthesis and metabolism of SA in rice was studied and compared to that of tobacco. Rice shoots converted [14C]cinnamic acid to SA and the lignin precursors p-coumaric and ferulic acids, whereas [14C]benzoic acid was readily converted to SA. The data suggest that in rice, as in tobacco, SA is synthesized from cinnamic acid via benzoic acid. In rice shoots, SA is largely present as a free acid; however, exogenously supplied SA was converted to beta-O-D-glucosylSA by an SA-inducible glucosyltransferase (SA-GTase). A 7-fold induction of SA-GTase activity was observed after 6 h of feeding 1 mM SA. Both rice roots and shoots showed similar patterns of SA-GTase induction by SA, with maximal induction after feeding with 1 mM SA.

The effect of chitosan on the growth and morphology of Fusarium oxysporum f. sp. albedinis (Foa), the causal agent of Bayoud disease, and its ability to elicit a defence reaction against this fungus in date palm roots were investigated. Chitosan at 1 mg mlE-1 reduced the growth of Foa on potato dextrose agar medium by an average of 75%, while mycelial growth was totally inhibited in a liquid medium. When added to a solid medium, chitosan caused morphological changes in Foa mycelium. In addition, when injected into roots at three concentrations (0.1, 0.5 and 1 mg mlE-1), chitosan elicited peroxidase (PO) and polyphenoloxidase (PPO) activity and, particularly at the concentration of 1 mg mlE-1, increased the level of phenolic compounds. Concerning phenolics, chitosan led to an accumulation of non-constitutive hydroxycinnamic acid derivatives, known to be of great importance in date palm resistance to Bayoud. The accumulation of hydroxycinnamic acid derivatives was greater in cv BSTN than in cv JHL. Chitosan could be used to protect date palm against this vascular disease [Sono stati studiati l´effetto del chitosano sulla crescita e la morfologia di Fusarium oxysporum f. sp. albedinis (Foa), agente causale della malattia del Bayoud, e la sua capacità di provocare una reazione di difesa contro questo fungo nella palma da datteri. Il chitosano a 1 mg mlE-1 ha ridotto la crescita di Foa su un substrato agar-patata-destrosio del 75% in media, mentre in substrato liquido la crescita del micelio era inibita completamente. Aggiunto a un substrato solido, il chitosano ha determinato cambiamenti morfologici nel micelio di Foa. Inoltre, quando veniva iniettato nelle radici a tre concentrazioni (0,1, 0,5 e 1 mg mlE-1), il chitosano stimolava l´attività della perossidasi (PO) e della polifenolossidasi (PPO) e, in particolare alla concentrazione di 1 mg mlE-1, determinava un aumento del livello di composti fenolici. Riguardo ai composti fenolici, il chitosano portava a un accumulo di derivati non costitutivi dell´acido idrossicinnamico, noti per la loro importanza notevole nella resistenza della palma da datteri al Bayoud. L´accumulo di derivati dell´acido idrossicinnamico è risultato maggiore nella cv BSTN rispetto alla cv JHL. Il chitosano potrebbe essere utilizzato nella difesa della palma da datteri da questa malattia vascolare.]