Overexpression of a Gene Encoding Hydrogen Peroxide-Generating Oxalate Oxidase Evokes Defense Responses in Sunflower

Oxalate oxidase (OXO) converts oxalic acid (OA) and O(2) to CO(2) and hydrogen peroxide (H(2)O(2)), and acts as a source of H(2)O(2) in certain plant-pathogen interactions. To determine if the H(2)O(2) produced by OXO can function as a messenger for activation of defense genes and if OXO can confer resistance against an OA-producing pathogen, we analyzed transgenic sunflower (Helianthus annuus cv SMF3) plants constitutively expressing a wheat (Triticum aestivum) OXO gene. The transgenic leaf tissues could degrade exogenous OA and generate H(2)O(2). Hypersensitive response-like lesion mimicry was observed in the transgenic leaves expressing a high level of OXO, and lesion development was closely associated with elevated levels of H(2)O(2), salicylic acid, and defense gene expression. Activation of defense genes was also observed in the transgenic leaves that had a low level of OXO expression and had no visible lesions, indicating that defense gene activation may not be dependent on hypersensitive response-like cell death. To further understand the pathways that were associated with defense activation, we used GeneCalling, an RNA-profiling technology, to analyze the alteration of gene expression in the transgenic plants. Among the differentially expressed genes, full-length cDNAs encoding homologs of a PR5, a sunflower carbohydrate oxidase, and a defensin were isolated. RNA-blot analysis confirmed that expression of these three genes was significantly induced in the OXO transgenic sunflower leaves. Furthermore, treatment of untransformed sunflower leaves with jasmonic acid, salicylic acid, or H(2)O(2) increased the steady-state levels of these mRNAs. Notably, the transgenic sunflower plants exhibited enhanced resistance against the OA-generating fungus Sclerotinia sclerotiorum.