NO signals in the haze: Nitric oxide signalling in plant defence

Nitric oxide (NO) is gaining increasing attention as a regulator of diverse (patho-)physiological processes in plants. Although this molecule has been described as playing a role in numerous conditions, its production, turnover and mode of action are poorly understood. Recent studies on NO production have tended to highlight the questions that still remain unanswered rather than telling us more about NO metabolism. But regarding NO signalling and functions, new findings have given an impression of the intricacy of NO-related signalling networks. Different targets of protein S-nitrosylation have been characterised and enzymatic routes controlling this posttranslational modification are emerging, along with their physiological implications. Evidence is also accumulating for protein tyrosine nitration and cGMP as important components of NO-related signal transduction.