Nitric oxide (NO) is an important signaling molecule that regulates a diverse range of physiological processes in a number of animal tissues and is generated by the action of nitric oxide synthase (NOS). In plants, NO plays a role in hormonal and defensive responses, but no plant homologs for mammalian NOS have been identified, indicating the presence of a specific plant NOS enzyme(s). In the October 3 Science, Fang-Qing Guo and colleagues at The University of California at San Diego show that the Arabidopsis AtNOS1 gene encodes a plant NOS—the AtNOS1 protein has been shown to function in snail NO synthesis. They propose a role for this enzyme in plant growth regulation and hormone signaling (Science, 302:100-103, October 3, 2003).

Guo et al. characterized an Arabidopsis thaliana AtNOS1 insertional mutant with 25% of wildtype (WT) NOS activity. WT NO levels increased following abscisic acid treatment and were...

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