Detection and function of nitric oxide during the hypersensitive response in Arabidopsis thaliana: Where there’s a will there’s a way

•NO is a key component of signalling pathways during the hypersensitive response.•We report NO emission in Arabidopsis plants during HR using chemiluminescence.•NR-deficient plants display NO emission during HR following nitrite supplementation.•Nitrite plays a central role as NO source during HR.

Nitric oxide (NO) was identified as a key player in plant defence responses approximately 20 years ago and a large body of evidence has accumulated since then supporting its role as a signalling molecule. However, there are many discrepancies in current NO detection assays and the enzymatic pathways responsible for its synthesis have yet to be determined. This has provoked strong debates concerning the function of NO in plants, even questioning its existence in planta. Here we gather data obtained using the model pathosystem Arabidopsis/Pseudomonas, which confirms the production of NO during the hypersensitive response and supports is role as a trigger of hypersensitive cell death and a mediator of defence gene expression. Finally, we discuss potential sources of NO synthesis, focusing on the role of nitrite as major substrate for NO production during incompatible interactions.