Abstract: Carbon monoxide (CO), an odorless, tasteless and colorless gas, has recently proved to be an important bioactive or signal molecule in mammalian cells, with its effects mediated mainly by nitric oxide (NO). In the present report, we show that exogenous CO induces lateral root (LR) formation, an NO-dependent process. Administration of the CO donor hematin to rapeseed (Brassica napus L. Yangyou 6) seedlings for 3 days, dose-dependently promoted the total length and number of LRs. These responses were also seen following the application of gaseous CO aqueous solutions of different saturated concentrations. Furthermore, the actions of CO on seedlings were fully reversed when the CO scavenger hemoglobin (Hb) or the CO-specific synthetic inhibitor zinc protoporphyrin-IX (ZnPPIX) were added. Interestingly, depletion of endogenous NO using its specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO) or the nitric oxide synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME), led to the complete abolition of LR development, illustrating an important role for endogenous NO in the action of CO on LR formation. However, the induction of LR development by 200 μmol/L sodium nitroprusside (SNP), an NO donor, was not affected by the presence or absence of ZnPPIX. Furthermore, using an anatomical approach combined with laser scanning confocal microscopy with the NO-specific fluorophore 4,5-diaminofluorescein diacetate, we observed that both hematin and SNP increased NO release compared with control samples and that the NO signal was mainly distributed in the LR primordia (LRP), especially after 36 h treatment. The LRP were found to have similar morphology in control, SNP- and hematin-treated seedlings. Similarly, the enhancement of the NO signal by CO at 36 h was differentially quenched by the addition of cPTIO, l-NAME, ZnPPIX and Hb. In contrast, the induction of NO caused by SNP was not affected by the application of ZnPPIX. Therefore, we further deduced that CO induces LR formation probably mediated by the NO/NOS pathway and NO may act downstream of CO signaling, which has also been shown to occur in animals.

Key words: carbon monoxide, lateral root, nitric oxide, rapeseed, signal transduction