Balance Mechanisms Between Constitutive And Inducible Defenses To Specialist Herbivores In Plants

Investigations of the relationships between constitutive and inducible resistances to herbivores in plants may provide insight into the evolution of the plant-insect relationships as well as valuable background knowledge for breeding of crop resistance. In this study, I used wild cmcifers and its specialist herbivore the diamondback moth (DBM). Plutella xylostella (L.), as a model system. First, I examined the relationship between constitutive and induced resistances in cmcifers, by screening eleven wild species of cmcifers for level of constitutive resistance to DBM, and then examining the levels of induced resistance in six species of cmcifers that showed widely different levels of constitutive resistance. I used jasmonic acid (JA) and larvae of DBM as induction agents. Next, in order to explore the biochemical mechanisms underlying the changes between constitutive and induced resistances, I collected and analyzed the volatiles of cmcifers that showed a significant difference between constitutive and induced resistances.(1) The 11 species of cmcifers differed significantly in levels of constitutive resistance from relatively low, to medium and then to high. Comparatively, C. bursa-pastoris, C. impatiens and L. virginicum showed higher levels of constitutive resistance, and C. hirsute, C. limprichtiana and R. indica showed lower levels of constitutive resistance.(2) After feeding by larvae, the three plant species with higher constitutive resistance exhibited an induced susceptibility. In contrast, the three plant species with lower constitutive resistance exhibited an induced resistance. These results indicated that there was a balance mechanism between constitutive and induced resistances in cmcifers. However, all six species of cmcifers showed significant induced resistance by JA treatment, irrespective of levels of constitutive resistance.(3) Herbivory and JA treatments resulted in significant differences in the composition and releasing process of volatiles. In herbivory treatment, C. impatiens and L. virginicum released plentiful nitrile, and in contrast, they released nothing of nitrile in JA treatment. The release of plant volatile in herbivory treatment was transient. Immediately upon feeding by larvae, plants released plentiful volatiles. When feeding ceased, the emission of volatiles dropped rapidly and stopped within hours. In contrast, the release of plant volatile in JA treatment remained for a longer period of time. After application of jasmonic acid, the emission of volatiles was continued for up to 120 hours. The results of the present study demonstrated that there was a balance mechanism between constitutive and induced resistance in crucifers. In nature, the co-regulation of constitutive and induced resistances may help to address a balanced selection on specialist herbivore and thereby retard its counter-defences. So, in the course of crop-breeding for resistance, in addition to improving constitutive resistance of crop, more attentions should be paid for induced resistance. While intercropping in field, resistance crops should be intercropped with less resistance crops. When resistance crop is cultivated in monoculture over large areas, consistently high level of resistance may make counter-adaption by herbivores more likely.The plant responses elicited by JA differed from those induced by herbivory. Therefore, it must be cautious to use induced resistance elicited by chemicals, such as jasmonic acid, as a strategy of integrated pest management.