No abstract

Many environmental factors may affect herbivore/ plant interactions by increasing the level of resistance or susceptibility of the plant to the herbivore. Among the factors with a demonstrated capability to induce changes in levels of resistance are temperature, solar radiation, water stress, soil fertility, insecticides, herbicides, fungicides, growth regulators, pathogen infection, weed competition, and previous or concurrent herbivore attack. Most chemical factors, responsible for the resistance of plants to arthropods studied to date, have involved the genetically controlled, injury-independent accumulation of metabolites with allomonal activity.A completely different plant defense strategy has been demonstrated for many pathogens for which phytoalexin accumulation is the result of a post-challenge response.The aforementioned environmental factors, including pestrelated injury, can induce de novo synthesis and accumulation of compounds with allomonal properties (phytoalexins) or change the relative concentration of both nutrient and non-nutrient compounds.The focus in this paper is on the post-infestation induction of resistance by arthropods, in a manner up until recently known to result only from pathogen infection. The insect-resistance role of phytoalexins is discussed and reference is made to other possible natural sources of inducers of resistance.Antiherbivory in plants has been ascribed mainly to the presence of physical defenses or to the injury-independent accumulation of secondary metabolites that have allomonal

Effects of soybean phytoalexins on the feeding of the soybean looper and Mexican bean beetle were investigated to test the hypothesis that phytoalexins might be a defense mechanism of plants against insects as well as against pathogens. Short-term behavioral responses to the phytoalexins were analyzed using dual-choice tests with phytoalexin-rich and phytoalexin-poor (control) tissues. Phytoalexin production was elicited with ultraviolet radiation. Results from the dual-choice tests indicated that 6th instar soybean looper larvae fed equally on the control and phytoalexin-rich tissues. Feeding by adult and 4th instar Mexican bean beetles, however, was strongly deterred by the phytoalexins as evidenced by "single-bite" mandible scars on the phytoalexin-rich cotyledon discs. Nutritional effects of the isoflavonoid phytoalexin glyceollin on early instar soybean looper larvae were tested by incorporating the phytoalexin into an artificial medium at a level of 1% dry weight (0.15% fresh weight). The larvae were reared for 7 days from emergence on diets of control and glyceollin-containing media. Although survival on the glyceollin diets was initially less than on the control diets, under the experimental conditions glyceollin had no significant effect on the growth, development, or subsequent survival of the larvae. Efficiency of food utilization (ECI) was reduced, indicating that the phytoalexins may be a mild digestibility-reducing factor for the loopers. Implications of the results for host-plant resistance are discussed.

Cell wail preparations (elicitors) from Phytophthora megasperma var. sojae increase C2H4 formation, phenylalanine ammonia lyase activity, and glyceollin accumulation in soybean cotyledons within about 1.5, 3, and 6 hours after treatment, respectively. The immediate precursor of C2H4, 1-aminocyclopropane-1-carboxylic acid, stimulates C2H4 formation like the elicitor within 1.5 hours after administration, whereas phenylalanine ammonia lyase activity and glyceollin concentration remain unchanged. Aminoethoxyvinylglycine, a specific inhibitor of C2H4 formation in higher plants, inhibits elicitor-induced C2H4 formation by about 95% but has no effects on phenylalanine ammonia lyase or glyceollin accumulation. It was concluded that C2H4 is a signal accompanying the specific recognition process which finally leads to the induction of phytoalexin formation, but it is not functioning as a link or messenger in the induction sequence of glyceollin accumulation.

Laboratory studies showed that the soybean phytoalexin glyceollin, an isoflavonoid previously investigated as an inhibitor of fungal pathogens, is an effective antifeedant for some insect species. Glyceollins extracted from soybean, Glycine max Merrill, cotyledons were applied to the surface of common bean leaves, Phaseolus vulgaris L., in five concentrations, including physiological concentration, for feeding preference tests. Leaves treated with glyceollin at concentrations below physiological levels were less acceptable to the southern corn root worm, Diabrotica undecimpunctata howardi Barber, and the Mexican bean beetle, Epilachna varivestis Mulsant, than untreated leaves. Feeding deterrence was positively correlated with increasing concentrations. The bean leaf beetle, Cerotoma trifurcata Forster, was not affected even by very high doses. An ethological concentration (EC ••) was computed based on the log dose-reduction of acceptance of treated disks. The EC •• for the Mexican bean beetle was 6.1~g/mg leaf dry weight and 3.5~g/mg for the southern corn rootworm. Results indicate that soybean phytoalexins may represent a common defense against microorganisms and insect herbivores.