Phytoalexins, induced synthesis

Phenvlpropanoid Metabolism and Phvtoalexins. The induced synthesis of phytoalexins in many species of plants due to attack by... 

In 1973, a group of Department of Chemistry, Colorado State University, USA examined phytoalexins induced in Beta vulgaris leaves with Cercospora leaf spot of beet caused by Cercospora beticola infection. Generally, phytoalexins are produced by de novo synthesis in higher plants after the infection of fungal organism [38]. 

Induced lignification is blocked by treatments which inhibit protein synthesis (10), and it seems likely therefore that the increases in activities observed for enzymes involved in lignin biosynthesis (7,8) depend on enhanced translation, and probably enhanced transcription of the appropriate genes, as has been shown for the phytoalexin response of some plants (27). However, the molecular biology of the response in wheat awaits investigation. 

De Novo Synthesis of Phytoalexins. Phytoalexins have been studied in great depth by plant pathologists. Excellent review papers are available in Hedin s ACS symposium volume. Host Plant Resistance to Plants (42), and more recently in the book edited by Horsfall and Cowling, Plant Disease (43). The antiherbivory effect of phytoalexins, however, is only now beginning to be fully appreciated. It is apparent that pathogen induced phytoalexins do have a definite effect on insect herbivores. There is mounting evidence that herbivore-inflicted injury may also result in the induction of phytoalexin production and accumulation. 

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). 

HAGMANN, M.-L., HELLER, W., GRISEBACH, H Induction of phytoalexin synthesis in soybean. Stereospecific 3,9-dihydroxypterocarpan 6a-hydroxylase from elicitor-induced soybean cell cultures. Eur. J.Biochem., 1984,142,127-131. 

ZAHRINGER, U EBEL, J., MULHEIRN, L.J., LYNE, R.L., GRISEBACH, H., Induction of phytoalexin synthesis in soybean - dimethylallyl pyrophosphate trihydroxypterocarpan dimethylallyltransferase from elicitor-induced cotyledons. FEBSLett., 1979,101,90-92. 

Direct-defense compounds can be either constitutively present in (specific parts of) the plant or be produced after induction by pathogens or herbivores. The latter compound will be less costly for the plant. For example, elicitor-induced accumulation of the antimicrobial sesquiterpenoid capsidiol correlated with the induction of 5-ept-aristolochene synthase, which is a branch-point sesquiterpene cyclase involved in the synthesis of sesquiterpene phytoalexins (8). In rice Oryza saliva L.), 14 diterpenoid phytoalexins have been identified. All these compounds are accumulated in rice leaves after inoculation with the... 

The importance of terpenoid phytoalexins in resistance to wilts was further demonstrated in studies of temperature effects on resistance and in studies of induced resistance. Increase in temperature from 25 to 30 C causes a marked increase in resistance. This temperature change also slows the rate of sporulation of the fungus and increases the rate of phytoalexin formation by cotton (56). Likewise, treatments that induce resistance also induce pEy toalexin synthesis (57). Phytoalexin synthesis, therefore, is also important to explain environmental effects on disease resistance. 

It is envisioned that the defensive systems of plants include preformed physical and chemical barriers as well as inducible defenses that also can be either physical or chemical. An example of an inducible defense is the synthesis of specific antimicrobial compounds such as phytoalexins or phytoanticipins. However, in other instances plants react to pests by developing long-lasting, broad-spectrum systemic resistance to later attacks by pathogens. This type of resistance is referred to as systemic acquired resistance, and was discovered in tobacco by Ross (1961). This form of resistance is almost universally... 

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