Abstract: Traditionally, reactive oxygen species (ROS) were considered to be toxic by products of aerobic metabolism. However, in recent years , it has become apparent that plants actively produce ROS as signaling molecules to control processes such as programmed cell death , abiotic stress responses, pathogen defense and systemic signaling . Emerging evidence suggests that hormone signaling pathways regulated by abscisic acid, salicylic acid, jasmonic acid and ethylene , as well as ROS signaling pathways, play key roles in the crosstalk between biotic and abiotic stress signaling. Reactive oxygen species (ROS) were originally thought to only be released by phagocytic cells during their role in host defence . It is now clear that ROS have a cell signalling role in many biological systems , both in animals and in plants . ROS induce programmed cell death or necrosis, induce or suppress the expression of many genes, and activate cell signalling cascades, such as those involving . Recent biochemical and genetic studies confirm that hydrogen peroxide is a signalling molecule in plants that mediates responses to abiotic and biotic stresses . The synthesis and action of hydrogen peroxide appear to be linked to those of nitric oxide . Downstream signalling events that are modulated by hydrogen peroxide include calcium mobilization, protein phosphorylation and gene expression .

Key words: ROS