This study investigates the role of nitric oxide (NO) and its signaling molecules, cyclic GMP (cGMP) and cyclic ADP-ribose (cADPR), in plant defense responses. The researchers found that infection of resistant tobacco plants with tobacco mosaic virus (TMV) increased the activity of nitric oxide synthase (NOS). Administration of NO donors or recombinant mammalian NOS to tobacco plants or suspension cells triggered the expression of defense-related genes, including those encoding pathogenesis-related 1 (PR-1) protein and phenylalanine ammonia lyase (PAL). These genes were also induced by cGMP and cADPR, which can serve as second messengers for NO signaling in animals. The study demonstrated that NO treatment induced transient increases in endogenous cGMP levels and that the activation of PAL was blocked by inhibitors of guanylate cyclase. The findings suggest that several key components of animal NO signaling are also operative in plants, highlighting the conservation of defense mechanisms across different kingdoms.This study investigates the role of nitric oxide (NO) and its signaling molecules, cyclic GMP (cGMP) and cyclic ADP-ribose (cADPR), in plant defense responses. The researchers found that infection of resistant tobacco plants with tobacco mosaic virus (TMV) increased the activity of nitric oxide synthase (NOS). Administration of NO donors or recombinant mammalian NOS to tobacco plants or suspension cells triggered the expression of defense-related genes, including those encoding pathogenesis-related 1 (PR-1) protein and phenylalanine ammonia lyase (PAL). These genes were also induced by cGMP and cADPR, which can serve as second messengers for NO signaling in animals. The study demonstrated that NO treatment induced transient increases in endogenous cGMP levels and that the activation of PAL was blocked by inhibitors of guanylate cyclase. The findings suggest that several key components of animal NO signaling are also operative in plants, highlighting the conservation of defense mechanisms across different kingdoms.