Heme Oxygenase-1 Is Involved In UV-B Tolerance In Arabidopsis By Regulation Of Flavonoids Biosynthesis

Heme oxygenase (HO, EC 1.14.99.3) catalyses the oxidative cleavage of heme to BV (biliverdin), iron and carbon monoxide. Numerous studies have already shown that HO plays an important role in the metabolism and defense in plants. Rencently, it was reported that HO is up-regulated by ultraviolet-B irradiation and has antioxidant properties in soybean plants. However, the detailed molecular mechanism remains unclear. In this study, we explored the possible roles of HO in the response of Arabidopsis to ultraviolet-B irradiation.Arabidopsis contains four HO genes (HY1, HO2-4). To characterize the contribution of each individual gene to UV-B tolerance, we tested the effects of UV-B irradiation on seedling growth including fresh weight and primary root growth in Arabidopsis wild-type and four HO mutants(hy1-100, ho2, ho3 and ho4). The results showed that hy1-100 mutant displayed more sensitivity to UV-B. After UV-B irradiation, HYI was up-regulated significantly, while HO2, HO3, HO4 was only slightly increased. Furthermore, redundancy test showed knock-down of HO2, HO3, HO4 genes didn’t affect the UV-B tolerance in Arabidopsis.Ultraviolet-B radiation (UV-B; 280-315 nm) is an intrinsic part of the sunlight. Plants need sunlight and are thus inevitably, exposed to UV-B radiation. However, Organisms have evolved mechanisms to protect against UV-B. The most common protective meachanism is the biosynthesis of UV-absorbing compounds. The contents of flavonoids and anthocyanin in hy1-100 mutants are lower drasticly than wild-type under normal growth conditions. When exposured to UV-B radiation, both flavonoids and anthocyanin were increased significantly in wild-type, but only slightly in hy1-100 mutant. Expression analysis of flavoniods biosynthetic genes by qRT-PCR revealed the UV-B induced up-regulation of flavoniods biosynthetic genes was blocked partially in hy1-100 mutant compaired with wild-type. We noticed that chalcone synthase (CHS), the key enzymes in flavonoid biosynthetic pathway, whose expression both in transcription and protein level was impaired in hy1-100 mutant irrespective of exposuring to UV-B or not. Aditionally, CORM-2 (donor of CO, which is the product of HO), rescured the decrease of fresh weight, chlorophyll, and increase the contents of flavonids, in hy1-100 mutant under UV-B irradiation.Taken together, our molecular and genetic evidences suggested HO-1 is an important component in the response of Arabidopsis to UV-B irradiation, especially by the regulation of flavonoids accumulation.