Nitric oxide modulates cadmium influx during cadmium-induced programmed cell death in tobacco BY-2 cells

Nitric oxide (NO) is a bioactive gas and functions as a signaling molecule in plants exposed to diverse biotic and abiotic stresses including cadmium (Cd2+). Cd2+ is a non-essential and toxic heavy metal, which has been reported to induce programmed cell death (PCD) in plants. Here, we investigated the role of NO in Cd2+-induced PCD in tobacco BY-2 cells (Nicotiana tabacum L. cv. Bright Yellow 2). In this work, BY-2 cells exposed to 150 μM CdCl2 underwent PCD with TUNEL-positive nuclei, significant chromatin condensation and the increasing expression of a PCD-related gene Hsr203J. Accompanied with the occurring of PCD, the production of NO increased significantly. The supplement of NO by sodium nitroprusside (SNP) had accelerated the PCD, whereas the NO synthase inhibitor Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME) and NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) alleviated this toxicity. To investigate the mechanism by which NO exerted its function, Cd2+ concentration was measured subsequently. SNP led more Cd2+ content than Cd2+ treatment alone. By contrast, the prevention of NO by l-NAME decreased Cd2+ accumulation. Using the scanning ion-selective electrode technique, we analyzed the pattern and rate of Cd2+ fluxes. This analysis revealed the promotion of Cd2+ influxes into cells by application of SNP, while l-NAME and cPTIO reduced the rate of Cd2+ uptake or even resulted in net Cd2+ efflux. Based on these founding, we concluded that NO played a positive role in CdCl2-induced PCD by modulating Cd2+ uptake and thus promoting Cd2+ accumulation in BY-2 cells.