Regulatory mechanisms of nitric oxide and reactive oxygen species generation and their role in plant immunity

Rapid production of nitric oxide (NO) and reactive oxygen species (ROS) has been implicated in diverse physiological processes, such as programmed cell death, development, cell elongation and hormonal signaling, in plants. Much attention has been paid to the regulation of plant innate immunity by these signal molecules. Recent studies provide evidence that an NADPH oxidase, respiratory burst oxidase homolog, is responsible for pathogen-responsive ROS burst. However, we still do not know about NO-producing enzymes, except for nitrate reductase, although many studies suggest the existence of NO synthase-like activity responsible for NO burst in plants. Here, we introduce regulatory mechanisms of NO and ROS bursts by mitogen-activated protein kinase cascades, calcium-dependent protein kinase or riboflavin and its derivatives, flavin mononucleotide and flavin adenine dinucleotide, and we discuss the roles of the bursts in defense responses against plant pathogens.