Induction of growth elongation in wheat root segments by heme molecules: a regulatory role of carbon monoxide in plants?

Recent studies suggest that carbon monoxide (CO), which is mainly produced by heme oxygenase (HO EC 1.14.99.3), may function as a physiological messenger or bioactive molecule by interacting with nitric oxide (NO) in animal cells. In this study, we report that application of the hematin and hemin, two heme molecules cleaved by HO to yield CO in animals, dose-dependently induced the significant increase in wheat root elongation as well as the actions of IAA and NO donor sodium nitroprusside (SNP). These responses were mimicked by the application of aqueous solution of CO with different saturation. Also, above heme molecule-induced effect is specific for CO since the potent inhibitor of HO-1, zinc protoporphyrin-IX (ZnPPIX) or CO scavenger hemoglobin (Hb) blocked the action of hematin and hemin, respectively. Further results proved that treatment with hematin or IAA could result in either the potent induction of HO-1 transcript or CO releasing in wheat root segments, both of which were reversed by the addition of ZnPPIX. ZnPPlX with lower concentration could prevent the elongation induced by IAA, while in the SNP-treatment the prevention of root growth occurred solely at higher concentrations. Also, wheat root segments elongation induced by IAA, SNP or hematin, was blocked by the specific NO scavenger, inhibitors of NO synthase (NOS) and guanylate cyclase (GC), respectively. Meanwhile, production of reactive oxygen species (ROS) could be demonstrated in the growing zone of wheat root segments treated by hematin or SNP using specific histochemical assay combined with the inhibitor investigation. Taken together, above results suggested that CO produced by HO might mediate the induction of growth elongation of wheat root segments by IAA, which might be also related to NO/cGMP- and even ROS-dependent pathways.