Nitric oxide generation during the interaction with Phytophthora capsici of two Capsicum annuum varieties showing different degrees of sensitivity

The response to the pathogenic oomycete, Phytophthora capsici, in plants of two varieties of pepper (Capsicum annuum L.), Serrano Criollo de Morelos (SCM) and Americano (AM), was studied. Both varieties developed a hypersensitive reaction and generated nitric oxide (NO) as a defence, but only SCM was able to overcome the infection by inhibiting the pathogen's growth, while AM succumbed to the disease. Bio-imaging of NO in vivo was carried out using diaminofluorescein diacetate (DAF-2DA) in conjunction with confocal laser scanning microscopy. NO-induced fluorescence was always higher in SCM than in AM tissues, and was also detected at trace level in decapitated non-infected control plants as a result of wounding. In var. SCM, the highest intensity was detected 3 days after infection in the first four sections following the necrosis zone, which corresponded to the mould invasion zone. In AM, extensive necrosis was followed by general invasion of the hyphae, accompanied by minimal NO production. These findings suggest that NO is involved in the resistance reaction against this water mould and that it is induced by the isoenzyme, nitric oxide synthase (iNOS), whose activity is qualitatively greater in the tissues of SCM. This is the first time that the involvement of NO and NOS has been reported in the defensive signalling reaction between pepper plants and P. capsici.