Bacterio-opsin gene overexpression fails to elevate fungal disease resistance in transgenic poplar (Populus)

Overexpression of the bacterio-opsin (bO) gene in tobacco had previously been shown to induce hypersensitive-response-like lesions, increase viral and bacterial disease resistance, and stimulate pathogenesis-related gene expression. To see if this gene enhanced resistance to fungal pathogens of poplar, we generated a total of 35 transgenic lines in two clones of Populus trichocarpa Torr. & A. Gray × Populus deltoides Bartr. ex Marsh. and one clone of P. trichocarpa × Populus nigra L. and challenged them with the fungal pathogens Melampsora occidentalis H. Jack (leaf rust), Venturia populina (Vuill.) Fabric. (leaf and shoot blight), Septoria musiva Peck, and Septoria populicola Peck (leaf spot and stem canker) in greenhouse, field, or laboratory inoculations. Northern analysis showed that the bO gene was expressed in the transgenic poplars; however, no increase in expression of phenylalanine ammonia-lyase (PAL1) or two wound-inducible poplar chitinase genes (WIN6 and WIN8) were observed, even in one line that showed very high bO expression, intensive lesion development, and retarded growth. Poplars required a high threshold of bO expression for lesion development, and susceptibility to all of the pathogens tested was unaffected by bO overexpression.