A Preliminary Study On The Accumulation And Translocation Of Microcystin MC - LR In Rice

Cyanobacterial bloom leads to severe pollution of microcystin in the fresh water of Taihu Lake.The rice field irrigated with MC-LR-contaminated water from Taihu Lake is an important part of southern rice area in China. MC-LR is the most toxic and frequently occurring isoform of microcystin. The molecular mechanism for MC-LR transport has been clearly identified in mammalian cells, but little is known about the mechanism for MC-LR uptake in plants as well as the related health risk evaluation. To address these gaps, rice has been selected to finish the following investigation, (1)Analyzing the correlation between MC-LR accumulation and the phytotoxicity in rice seedlings; (2) Detecting MC-LR content in rice grains collected from Taihu Lake region for health risk evaluation of MC-LR through consuming rice; (3) Cloning the first potentially plant MC-LR transporter gene from rice.(1) The investigation on the correlation MC-LR accumulation and the phytotoxicity in rice seedlings. Rice seeds and seedlings were treated with MC-LR at different concentrations (0-5μg/mL) in a hydroponic study. Treatment with MC-LR at high concentrations led to the remarkable inhibition of seed germination, which may resulted from the decrease in the activity of amylase under MC-LR stress. The accumulation of MC-LR in rice seedlings showed negative and positive correlation with growth and oxidative injury (indicated by MDA,POD, CAT, and SOD), respectively. These results suggested that oxidative injury resulted from the MC-LR accumulation contributed to the growth inhibition of rice seedlings under MC-LR stress.(2) The health risk evaluation of MC-LR through consuming rice. ELISA was applied to detect the MC-LR content in 44 rice grains collected Taihu Lake region. MC-LR was detected at 21 of the total 44 sited rice grain samples with a range of the concentration from 0.04 to 3.19 mg/kg DW. The detection rate was 48%. The health risk evaluation suggested that MC-LR uptake by consuming these rice did not exceed the Tolerable Daily Intake (TDI) of MC-LR recommended by World Health Organization (WHO). These results suggested that MC-LR in natural waterbody could accumulate in rice grains, posing potentially health risk. Currently, the rice grains from Taihu Lake region accumulate relative lower content of MC-LR, which may attribute to two aspects. First, the concentration of MC-LR in Taihu Lake is not high enough for its significant accumulation in rice grains. Second, the metabolism of MC-LR during the long-distance transportation could limit its final accumulation in rice grains.(3) The cloning and analysis of OsOATPM gene coding for a possible MC-LR transporter in rice. A unique rice locus LOC_Os04g44430 (named as OsOATPM) was retrieved from rice genome by blasting against mammalian MC-LR transporter gene. RT-PCR was performed to obtain the full-length cDNA of this gene (1794 bp). The upstream of this gene contained abundant c/s-elements for transcription regulation. Sequence analysis suggested that OsOATPM belonged to typical transmembrane protein with the ability of oligopeptide or ploypeptide transportation. Then we introduced this gene to Escherichia coli (E. coli) for prokaryotic expression, which helped us to understand that OsOATPM may function as a potential efflux transporter of MC-LR. The detailed molecular function of OsOATPM needs to be confirmed by further genetic evidences, but our current results provide scientific basis for the health risk evaluation of MC-LR accumulation in rice.