Heme-heme oxygenase 1 system is involved in ammonium tolerance by regulating antioxidant defence in Oryza sativa

Despite substantial evidence showing the ammonium-altered redox homeostasis in plants, the involvement and molecular mechanism of heme-heme oxygenase 1 (heme-HO1), a novel antioxidant system, in the regulation of ammonium tolerance remain elusive. To fill in these gaps, the biological function of rice HO1 (OsSE5) was investigated. Results showed that NH4Cl up-regulated rice OsSE5 expression. Oxidative stress and subsequent growth inhibition induced by excess NH4Cl was partly mitigated by pretreatment with carbon monoxide (CO, a by-product of HO1 activity) or intensified by zinc protoporphyrin (ZnPP, a potent inhibitor of HO1 activity). Pretreatment with HO1 inducer hemin, not only up-regulated OsSE5 expression and HO activity, but also rescued the down-regulation of antioxidant transcripts, total and related isozymatic activities, thus significantly counteracting the excess NH4Cl-triggered reactive oxygen species overproduction, lipid peroxidation and growth inhibition. OsSE5 RNAi transgenic rice plants revealed NH4Cl-hypersensitive phenotype with impaired antioxidant defence, both of which could be rescued by CO but not hemin. Transgenic Arabidopsis plants over-expressing OsSE5 also exhibited enhanced tolerance to NH4Cl, which might be attributed to the up-regulation of several antioxidant transcripts. Altogether, these results illustrated the involvement of heme-HO1 system in ammonium tolerance by enhancing antioxidant defence, which may improve plant tolerance to excess ammonium fertilizer. The molecular mechanism of heme-heme oxygenase1 (heme-HO1) in the regulation of ammonium tolerance is still lacking. Our results showed that rice OsSE5 expression was up-regulated by NH4Cl. Oxidative stress and subsequent growth inhibition induced by excess NH4Cl was partly mitigated by pretreatment with hemin (HO1 inducer) or carbon monoxide (CO, a by-product of HO1 activity), or intensified by zinc protoporphyrin (ZnPP, a potent inhibitor of HO1 activity). Combined with pharmacological, genetic, and molecular analyses, we proposed that heme-HO1 system was involved in the regulation of NH4+ tolerance by manipulation of downstream antioxidant defence in rice and Arabidopsis.