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In animals, it has also been suggested that various conditions associated with increased production of ROS and reactive nitrogen species (RNS) favor the activation of the heme oxygenase (HO, EC 1.14.99.3)/carbon monoxide (CO) signal pathway, which is now regarded as an important cellular stratagem to counteract and resist different stress insults. Transcriptional regulation of heat shock on HO-1 has been observed in certain mammalian cell lines. Recent studies have shown that the HO/CO signal system also plays a critical role in plant responses to abiotic stresses. However, it has not been elucidated whether HO was able to respond to ROS or heat shock stress in planta.As a member of reactive oxygen species (ROS), hydrogen peroxide (H2O2) exerts a double-sward function in plant cells, a toxic component as well as a signal molecular. In the present study, an effort was made to elucidate a possible interrelationship among endogenous H2O2, acclimation to oxidative stress and HO. Pretreatment of lower H2O2 doses (0.05,0.5 and 5 mM) for 24 h was able to dose-dependently attenuate lipid peroxidation in wheat seedling leaves mediated by further oxidative damage elicited by higher dose of H2O2 (150 mM) for 6 h, also 0.5 mM H2O2 being the most effective concentrations. Further results illustrated that 0.5 mM H2O2 pretreatment triggered the biphasic production of H2O2 during 24-h period. We also noticed that only peakⅠ(0.25 h) rather than peakⅡ(4 h) was approximately consistent with the enhancement of heme oxygenase (HO) activity, HO-1 gene expression. Meanwhile, enhanced SOD activity, Mn-SOD and Cu,Zn-SOD transcripts might be a potential source of peak I of endogenous H2O2. Further results confirmed that 0.5 mM H2O2 treatment for 0.5 h was able to up-regulate HO gene expression, which was detected by enzyme activity determination, semi-quantitative RT-PCR and western blotting. Meanwhile, the application of dimethylthiourea (DMTU), a trap for endogenous H2O2, not only blocked the up-regulation of HO, but also reversed the corresponding oxidation attenuation. Together, above results suggested that endogenous H2O2 production (peak I) plays a positive role in the induction of HO by enhancing its mRNA level and protein expression, thus leading to the acclimation to oxidative stress.In this study, we have demonstrated for the first time that heat shock treatment on tomato (Lycopersicon esculentum) seedlings did not only time-dependently induce lipid peroxidation or stem elongation inhibition, but also upregulate HO activity and HO-1 protein expression. Furthermore, ZnPPIX, the potent inhibitor of HO-1, was able to aggravate the heat-induced oxidative damage. In order to elucidate the antioxidative mechanism of HO under heat shock, we further examined the effect of ZnPPIX on total catalase (CAT), ascorbate peroxidase (APX) activities as well as their transcripts, and transcript levels of LeHSP17.7, LeHSP20, LeHSP70 along with LeHSP101. We discovered that as the upreglation of HO by heat shock was partly blocked by ZnPPIX, activities of the two antioxidant enzymes were significantly inhibited. Similarly, upregulation of LeHSP70 was weakened and induction of LeHSP20 was delayed, but LeHSP17.7, LeHSP101,CAT or APX transcripts level were not affected. In sum, our results confirmed tomato HO was able to respond to heat shock and might be involved in heat-induced antioxidative defense.
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