Involvement of haem oxygenase-1 in hydrogen peroxide-induced lateral root formation in tomato

The objective of this study was to test whether haem oxygenase-1 (HO-1) is involved in hydrogen peroxide (H₂O₂)-induced lateral root (LR) formation. The results showed that 0.1 mM H₂O₂ mimicked the effects of the HO-1 inducer, haemin, on the up-regulation of tomato HO-1 (SlHO1) expression, increased carbon monoxide (CO) synthesis and LR formation. However, 1.0 mM H₂O₂ resulted in inhibitory responses. The above inducible or inhibitory responses elicited by 0.1 and 1.0 mM H₂O₂ were noticeably blocked or rescued by the HO-1 inhibitor zinc protoporphyrin IX (ZnPP) or haemin, and then separately reversed by CO or ZnPP. Further trials showed that haemin-induced responses were not altered by the H₂O₂ trap, dimethylthiourea (DMTU). When applied alone, DMTU not only decreased H₂O₂ contents but also inhibited SlHO1 expression and LR development. These responses were recovered by the application of haemin or CO. Molecular evidence revealed that H₂O₂-modulated expression of the target genes responsible for LR formation was blocked by ZnPP, but rescued by CO. Salinity-induced up-regulation of HO-1 expression and thereafter LR formation were also dependent on the H₂O₂ generation. Overall, these results demonstrated a possible role of HO-1 in the H₂O₂-induced tomato LR formation.