As phytoalexins

The functions of phenylpropanoid derivatives are as diverse as their structural variations. Phenylpropanoids serve as phytoalexins, UV protectants, insect repellents, flower pigments, and signal molecules for plant-microbe interactions. They also function as polymeric constituents of support and surface structures such as lignins and suberins [1]. Therefore, biosynthesis of phenylpropanoids has received much interest in relation to these functions. In addition, the biosynthesis of these compounds has been intensively studied because they are often chiral, and naturally occurring samples of these compounds are usually optically active. Elucidation of these enantioselective mechanisms may contribute to the development of novel biomimetic systems for enantioselective organic synthesis. 

Falcarinol and falcarindiol concentration changes are small compared to those in myristicin and 6-MM content, and do not appear related to radiation (Table II). These polyacetylenes are present in fresh carrots. Immediately upon wounding, they are translocated to the surface through oil ducts (3 ) > and, therefore, their function as phytoalexins apparently does not depend upon novo synthesis. 

Tiburzy (22,31) obtained similar results by application of the PAL inhibitor aminooxyacetic acid (AOA). However, AOA does not specifically inhibit PAL (99), and PAL is not only involved in lignin biosynthesis (100). Thus, AOA and the related inhibitor aminooxyphenyl propionic acid (AOPP) (101,102) inhibit the biosynthesis of lignin (103,104), anthocyanins (105), other flavonoids (106), and conjugates of cinnamic acids (107) via PAL, as well as ethylene (108-110) via a pyridoxal phosphate dependent enzyme (110,111). In view of the possible function of phenolic compounds as phytoalexins (21,112,113) and the well documented role of ethylene in some resistance reactions (114-116), the above cited experiments with AOA (22,... 

Two known dihydroflavonols, 3-hydroxy-5-methoxy-6,7-methylenedioxyflavanone and 3,5,7,4 -tetrahydroxy-3 -methoxyflavanone, are produced as phytoalexins by sugarbeet roots Beta vulgaris, Chenopodiaceae) when inoculated with Rhizoctonia solani. They had not been previously reported for B. vulgaris ... 

Numerous catechols and hydroquinones in both glycoside-masked and -unmasked forms are useful metabolites in plant chemical defense. Many such metabolites are present in concentrations that can prove detrimental due to oxygenation of the tissue accompanying wounding of the plant in the infection process or in other direct physical injury. Some agents are also synthesized subsequent to enzyme induction in association with infection to mediate chemical defense, as in the broad class of defensive substances known as phytoalexins.12 Some of these induced substances are oxidizable polyphenols, while others are not (Figure 8.1). 

The dihydrofurans ( )-ipomeamarone (482) and ( )-epiipomeamarone (483) were isolated from Myoporum desertii and exist in mould-damaged sweet potatoes as phytoalexins (73AJC375). The tetrahydrofuran derivatives (484) and (485) are potent antidepressants (74JOC1042). 

Compounds formed in response to stress may occur in at least two ways. In one response, the plant may form compounds throughout the tissue at a considerable distance from the infection site (Hammerschmidt, 1999). In another response, the plant may form compounds specifically at the infection site. This may include only a few cells and in rare cases, as few as one or two cells. (Snyder and Nicholson, 1990 Nicholson and Wood, 2001). In general, such compounds are referred to as either stress metabolites or more often as phytoalexins. By definition phytoalexins are formed in response to infection (Aguero et al., 2002 Lo et al., 2002 Hammerschmidt and Nicholson, 2001 Lo and Nicholson, 1998). Phytoalexins often exhibit toxicity to specific pathogens. In this case there is a genetic relationship between the expression of phytoalexin synthesis and the organism that induces that synthesis (Essenberg et al., 1985). 

Aside from being UV-protectants, in a number of species certain stilbenes act as phytoalexins. Resveratrol (6.24 trans-3,5,4 -trihydroxy-stilbene), its cA-isomer, as well as their glucosides and dehydrodimer mm-e- iniferin (6.25) are present in grape leaves and berries and play a role in the defense against gray rot caused by the fungal pathogen Botrytis cinerea. 

When metalaxyl was used, necrotic reactions in susceptible crop cultivars similar to those of resistant ones, as well as phytoalexin accumulation, have been reported (12, 13, 21, 37, 41, 45). Here the use of metalaxyl-resistant strains could also clarify the signifi-... 

The downstream synthesis of two coumarins, ayapin (6,7-methylenedioxy-coumarin) and sco-poletin (6-methoxy-7-hydroxy-coumarin), is of biological interest due to their apparent roles as phytoalexins. Their accumulation has been correlated with resistance to pathogens in Helianthus (Tal and Robeson, 1986a, 1986b) and as feeding deterrents (Olson and Roseland, 1991). Both compounds are found in Jerusalem artichoke tubers (Cabello-Hurtado et al., 1998), and their synthesis via the phenylpropanoid pathway involves the first three steps leading to the activation of 4-coumaroyl CoA (Werck-Reichhart, 1995). Several routes have been postulated ayapin does not appear to be derived from scopoletin (Cabello-Hurtado et al., 1998). 

When plants undergo various stresses, certain secondary metabolites, including defense compounds, accumulate. Several secondary metabolites such as terpenoid indole alkaloids, indole glucosinolate, nicotine alkaloids, and polyamines are known to accumulate through the induction of biosynthetic genes by jasmonates.898-900 MeJA also induces genes involved in the formation of tryptophan derivatives, terpenoid indole alkaloids.901 These compounds are known to be involved in defense response to pathogen attack as phytoalexins. 

There has recently been considerable Interest In the use of ellcitors to activate biosynthetic pathways leading to relatively exotic plant secondary metabolites (19,95-98). Indeed, It may be possible to efficiently Isolate rare metabolites from cultured plant cells following their exposure to appropriate ellcitors. For example, Tyler et a1. (90) used fungal ellcitors to produce sangulnarine from Papaver cell suspension cultures and reused the same cells with a new round of medium and ellcitor. Such semicontinuous production schemes may permit the economic Isolation of rare secondary metabolites that behave as phytoalexins. 

Summarizing this section, we developed the time- and spectrally-resolved fluorescence imaging system based on line illumination, which is capable of rapid acquisition of fluorescence intensities as a function of Em, x, and %y-positions. We applied this method to the analysis of a plant defense response, accumulation of antimicrobial compounds of phytoalexin in oat leaves, induced by the elicitor. In addition to the strong fluorescence from chlorophyll molecules, weakly fluorescent components, one of which possibly originated from avenanthramide A as phytoalexin, were observed in oat leaves treated with an elicitor. 

Oxidation and demethylation of C yielded 44. Phenolic compounds are frequently produced by plants as phytoalexins. 

Research in recent years has shown that protective chemicals in plants can operate in disease resistance. Some of these chemicals occur in disease-free plants, but fungicidal compounds can build up in tissues in response to attempted infection. When this occurs, these defense chemicals are known as phytoalexins. Papers to be presented at this conference by Cutler et al., Harborne, Bailey,... 

Yet another family producing polyacetylenes is the Composltae two acetylenics safynol and dehydrosafynol are formed in the diseased safflower (24). Our own experiments at Reading Indicate that acetylenes are formed In other members. This is however a very large family and other responses have been detected as well. TVo sesquiterpene lactones have been encountered as phytoalexins In Lac-tuca (25) while coumarins are reported as such In the sunflower Hellanthus annuus (26) ... 

Concentrations of constitutive terpenoids in the root epidermis of cotton are unrelated to differences in resistance. But concentrations of terpenoid aldehydes formed in the vicinity of the pericycle, near the head of the animal, act as phytoalexins and are closely correlated with levels of resistance (11, 48). Little or no phytoalexin is formed in the pericycle of susceptible cultivars. Mixtures of terpenoid phytoalexins are more toxic to the nematode than gossypol alone, and mixtures containing methylated terpenoid phytoalexins (from 6. hirsutum) are more toxic than those that contain only nonmethyTated phytoalexins (from 6. arboreum) (49). Thus, the structure of terpenoid phytoalexins also is importan for resistance to root knot nematode. 

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