Induced resistance in plants to insect herbivory.

摘要

Although induced resistance in plants against insects has been studied for three decades, understanding of the resistance mechanisms is still limited. Induced resistance and associated biochemical mechanisms in soybean and cotton to herbivory by Helicoverpa zea were studied in this research. Induced resistance was indicated by decreased larval growth when larvae fed on previously damaged foliage or reproductive tissues compared to the respective controls. Correlated with the induced resistance was a shift of the oxidative status of the host plant as shown by increased levels of lipid peroxidation, prooxidant phenolics, hydroxyl radical formation, and oxidative enzymes with a concomitant decrease in the level of antioxidants. Moreover, herbivory caused a decline in host nutritional quality as indicated by a reduction in protein and most amino acids in cotton. Oxidative damage to the larval midgut epithelium was correlated with oxidative changes in the host plants. These results suggest that oxidative stress and nutritional stress are mechanistic components of induced resistance.; Herbivory markedly enhanced salicylic acid (SA) levels in cotton. SA accumulation is known to be essential for the expression of systemic-acquired resistance against phytopathogens but its role in insect resistance is unknown. To test if salicylic acid acts as a signal for induced resistance against insects, SA or its methyl derivative, MeSA, was applied to cotton. The application of these compounds did not affect foliar resistance against H. zea.; Herbivory increased phenylalanine ammonia lyase (PAL) activity and certain phenolics in soybean or cotton. Plant phenolics have long been proposed as anti-herbivore reagents based on correlational or artificial-diet incorporated studies. To test the direct impact of phenolics on plant anti-herbivory defense, transgenic tobacco with differential-expression of PAL was tested against Manduca sexta and Heliothis virescens larvae. Foliar phenolics such as chlorogenic acid, rutin and total flavonoids were differentially accumulated in the respective tobacco lines. However, the dosage of those phenolics and larval growth were not correlated. Results suggest that phenolics are not the determining factor in host plant resistance against insects in this system.