Functional Study Of Nicotiana Attenuata’s Alternative Oxidase In Plant Defense Against Herbivore And Pathogen

Alternative oxidase (AOX) is an important component of mitochondrial electron transport chain in plants. In contrast to main cytochrome respiratory pathway, AOX encodes a cyanide-insensitive electron transport pathway branched from ubiquinone electron acceptor. Therefore, this pathway is also known as the "cyanide-resistant alternative pathway". Many studies have indicated that AOX is involved in plants’ resistance to wide range of abiotic stresses, including low temperature, drought, salt, oxidative stress, for example by maintaining mitochondria redox homeostasis. Because of its important physiological function in plants, AOX-related research is an important area in plant science.Using transgenic plants with attenuated transcript levels of AOX, we examined the function of AOX in resistance of plants to (ⅰ) chewing insect (Manduca sexta), (ⅱ) bacterial pathogen (Pseudomonas syringae pv tomato DC3000) and (ⅲ) piercing-sucking insect (Empoasca sp.). M. sexta feeding highly elevated the transcript levels of AOX in N. attenuata plants. However, silencing AOX in N. attenuata did not compromise defense-related phytohormone and secondary metabolite levels and therefore did not result in altered defense response of N. attenuata plants against M. sexta. We conclude that AOX is not required for N. attenauta’s resistance to chewing herbivores.In nature and in the laboratory, piercing-sucking leafhopper Empoasca sp. caused larger cell death areas and induced higher salicylic acid levels in AOX-silenced plants compared to wild-type plants. The decreased levels of defense-related secondary metabolites in AOX-silenced plants imply that leafhopper up-regulated SA levels may suppress JA signaling pathway, which alleviates JA-dependent defenses. Since these insects not only produce feeding damage but also may transmit pathogens due to specific feeding mechanism, this study suggests a possible dual role of AOX during plant defense against insects and pathogens.In comparison to wild-type plants, silencing AOX caused accelerated cell death after pathogen inoculation; consequently, AOX-silenced plants showed increased levels of salicylic acid (SA) and hydrogen peroxide in infected leaf tissues; also relatively decreased mount of bacteria, suggesting that AOX is involved in HR-like PCD, probably by diminishing reactive oxygen species produced in mitochondria and affects plant immunity to pathogen indirectly.This work demonstrates multiple functions of AOX in plants’ resistance to both insect and pathogen attack and highlights the physiological and ecological functions of AOX.