Nitric oxide: a signaling molecule which activates cell wall-associated defense of tomato against Rhizoctonia solani

We investigated the role of sodium nitroprusside (SNP), as a NO donor, in activating cell wall-related defense responses of tomato against R. solani. Based on previous knowledge on the cross talk of NO with various pathways and its effect on cell wall components, experiments were carried out to investigate the function of cell wall-related defense responses, octadecanoid and phenylpropanoid pathways (which are associated with cell wall modification) not only in SNP-activated defense but also in basal resistance of tomato to the pathogen. In detached leaves and intact plants, we observed a considerable decrease in disease progress on both partially resistant (CH Falat) and susceptible (Mobil) tomato cultivars treated with SNP. The SNP treatment regulated malondialdehyde (MDA), H2O2 and O-2 (-)levels in plant cells, whereas SNP primed callose depsition, phenolics and lignification, as defense responses related to cell wall modification in the inoculated tomato plants. Priming in the activity and expression of lipoxygenase (LOX) and phenylalanine ammonia-lyase (PAL) as key markers of octadecanoid and phenylpropanoid pathways, respectively, was observed in SNP-treated inoculated plants. Co-application of LOX or PAL inhibitors and SNP completely suppressed SNP-activated defense responses. Using DEA-NONOate, as an independent NO releaser, supported the data obtained by SNP on disease progress and the role of PAL and LOX in NO-related protection of tomato against the pathogen. These findings highlight the role of phenylpropanoid and octadecanoid pathways and involvement of cell wall modifications in NO-associated induction of defense as novel mechanisms of SNP-IR in tomato-R. solani pathosystem.