While NO is often considered a cytotoxic compound, it can also function as an antioxidant in cells by its reaction with other radical molecules, thereby breaking the chain of free radical propagation (Wink et al., 1993; Halliwell et al., 1999). Several reports of NO as a protectant against senescence and oxidative stress in plants have appeared. In a variety of both climacteric and non-climacteric fruits and flowers, of vegetables and legume species, NO emission decreases with maturation and during senescence (Leshem and Haramaty, 1996; Leshem et al., 1998). Moreover, exogenous application of NO markedly delays senescence and maturation of these tissues (Leshem et al., 1998). NO is thought to delay senescence both by down-regulating ethylene emission and by acting as an antioxidant. NO can, therefore, be regarded as a naturally occurring plant growth effector. More recently, the same authors (Leshem et al., 1998) have shown that NO fumigation can be advantageously replaced, in the case of cut flowers, by the NO donor, sildenafil citrate (marketed under the trade name Viagra). Viagra application increases the vase life of cut flowers by as much as a week (Siegel-Itzkovich, 1999).
Other studies have demonstrated more directly that NO is able to counteract the toxicity of oxidative stress in plants. For example, NO-treated potato leaves were resistant to chlorosis, ion leakage, DNA fragmentation and apoptotic-like cell death produced by treatment with the AOS-generating herbicide, diquat, and by invasion with the pathogen Phytophtora infestans (Laxalt et al., 1997; Beligni and Lamattina, 1999a,b).
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