Involvement of Oxalate Oxidase In Aphid Resistance.

Oxalate oxidase (OXO), a cereal-specific protein which produces hydrogen peroxide in the presence of oxalate, has been a subject of interest among crop geneticists due to its apparent ability to confer improved resistance to many highly-destructive fungal and bacterial pathogens.  Previous research has shown that OXO-genes and are up-regulated in wheat, rice, and barley during pathogen infection. Additionally, transfer of OXO-genes from wheat or barley to non-cereal plants, such as peanut, sunflower, and potato, increases plant resistance levels to Sclerotinia sclerotiorum, Phytopthera infestans, and Streptomyces reticuliscabiei, among other pathogens. It is believed that OXO confers enhanced pathogen resistance in part by producing hydrogen peroxide, which may act as a signaling molecule or as a direct microbicide, or participate in strengthening of cell walls. Additionally, OXO is a particularly potent defense against oxalate-producing fungal pathogens, which secrete large amounts of toxic oxalate and weaken plant defenses. Crops producing OXO not only nullify the effects of oxalate from pathogens, but also use it to produce defensive reactive oxygen species.

Based on the numerous studies supporting the role of OXO in defense against pathogens, it was hypothesized that it may also enhance plant defense against insect pathogens, specifically aphids. For this experiment, plants from a T5 transgenic OXO-knockout rice line were tested for aphid resistance against wild-type Kitaake plants. Aphid reproductive ability was tested by caging adult bird cherry oat aphids in individual leaf cages until nymphs were produced. All but one nymph, designated the foundress, in each cage were removed and the progeny of the foundress counted and removed for fourteen days.  Plant susceptibility to aphids was determined by scoring of symptoms. Expression of OXO was determined by real-time PCR before and after aphid infestation.