Nitric oxide production by the differentiating xylem of Zinnia elegans

• Nitric oxide (NO) is currently regarded as a signal molecule involved in plant cell differentiation and programmed cell death. • Here, we investigated NO production in the differentiating xylem of Zinnia elegans by confocal laser scanning microscopy to answer the question of whether NO is produced during xylem differentiation. • Results showed that NO production was mainly located in both phloem and xylem regardless of the cell differentiation status. However, there was evidence for a spatial NO gradient inversely related to the degree of xylem differentiation and a protoplastic NO burst was associated with the single cell layer of pro-differentiating thin-walled xylem cells. Confirmation of these results was obtained using trans-differentiating Z. elegans mesophyll cells. In this system, the scavenging of NO by means of 2-phenyl-4,4,5,5-tetramethyl imidazoline-1-oxyl-3-oxide (PTIO) inhibits tracheary element differentiation but increases cell viability. • These results suggest that plant cells, which are just predetermined to irreversibly trans-differentiate in xylem elements, show a burst in NO production, this burst being sustained as long as secondary cell wall synthesis and cell autolysis are in progress.