Responding Mechanism To Different Water Conditions In Seedlings Of Metasequoia Glyptostroboides

In recent years,global climate change has changed precipitation patterns in most regions,it changed the water condition of ecosystems,thus affecting plant growth and reproduction.Research on the response mechanism of endangered species under different water conditions is benefited to understand its resistance and defense protection mechanism,and to protect Metasequoia glyptostroboides better.M.glyptostroboides are precious relic plant,which used to be widely distributed in the northern hemisphere,but now only in the narrow distribution area between Chongqing,Hunan,and Hubei.The change of water conditions is probably the main reason for the change of distribution.In this study,the seedlings cultivated from seeds of wild M.glyptostroboides.Then the seedings were treated with different water gradients:Normal water content（25%）,low water content（15%）,high water content recovery（32%）and complete submergence treatment,as well as the recovery treatment of water conditions: Low water content recovery（15%→25%）,high water content recovery（32%→25%）,and complete submergence recovery treatment（complete submergence→25%）.Survival/death conditions were analyzed,also the morphological,photosynthetic and fluorescence indexes of seedlings under normal water content,low water content and complete submergence treatment time were measured,as well as the related indexes of hormone,antioxidant system,non-structural carbohydrate,osmotic regulatory substances,and anaerobic metabolism in roots.Transcriptome sequencing was performed on the samples treated with normal water content,low water content and complete submergence at the previous two treatment time points,in order that we can further study the adaptability and response mechanism to water stress.The results are as follows:1)In the low water content treatment,all the 5 duplicated plants sampled after 12 days were dead,while in the full flooding treatment,all the plants sampled after 21 days were dead,indicating that seedlings were more tolerant to flooding environment.However,the mortality rate of seedlings recovered after 8 days of low water content treatment（7 plants）was slightly lower than that of seedlings of full flooding treatment（11 plants）,indicating that seedlings recovered more easily after 8 days of low water content treatment compared to complete submergence treatment.2)The plant height of seedlings was significantly decreased under both complete submergence treatment and low water content treatment（P<0.05）,indicating that both flooding,and drought inhibited the growth of seedlings.3)Under complete submergence treatment,Photosynthetic and fluorescence system indexes of seedling leaves began to decrease on the first day.The contents of starch（ST）and soluble sugar（SS）in roots also decreased on the first day,In the process of glucose metabolism,the up-regulation of the gene encoding alcohol dehydrogenase（ADH）increased the activity of ADH in seedlings,while the down-regulate genes encoding aldehyde decarboxylase（ALDH）decreased the activity of ALDH,indicated that seedlings could obtain certain energy through ethanol fermentation and resist flooding environment.Although ACS and ACO genes related to ethylene（ETH）synthesis precursors were down-regulated,and EIN3 genes related to gibberellic acid（GA）synthesis were down-regulated,there were no significant differences in ethylene,gibberellin,abscisic acid and other hormones between complete submergence treatment and normal water content.The enzyme content of the antioxidant system did not change significantly with the normal water content,but the proline（Pro）content increased significantly after 9 days of inundation,indicating that the harmful substances such as reactive oxygen species were removed by osmotic regulation.After recovery treatment,the seedlings showed rapid recovery of normal water content mainly through photosynthetic system,chlorophyll fluorescence system and peroxidase（POD）.In addition,glutamic acid decarboxylase（GAD）related genes,Cinnamyl-Co A reductase（CCR）related genes and walls are thin 1-related protein（WAT1）related genes were strongly upregulated,suggesting that they may also be important genes in the response to flooding.4)Under low water content,photosynthetic and fluorescence system indexes of seedling leaves began to decrease on the first day,while the content of ST and SS in root began to increase on the first day,which may be caused by rapid water loss in plants.Although genes encoding ACS and ACO related to ethylene（ETH）synthesis precursors were down-regulated,and genes encoding GA and ABA synthesis were upregulated,ethylene,gibberellic acid and other hormones did not respond significantly in seedlings.The genes related to enzyme activity of antioxidant system were not up regulated at the beginning of drought,but the contents of superoxide dismutase（SOD）was significantly increased,and proline were significantly increased at the 8th day.The results showed that seedlings could eliminate reactive oxygen species and maintain osmotic pressure to resist drought through accumulation of Pro contents and increase of SOD activity.After rehydration treatment,seedlings recovered mainly through photosynthetic system,chlorophyll fluorescence system,NSC and Pro,and then all indexes returned to the normal water content level.In this study,we analyzed the phenotypic,physiological and transcriptome changes of M.glyptostroboides’ seedlings under different water conditions to clarify the response mechanism under water stress,deepen the understanding of the adaptation and resistance mechanism of rare species,and provide reference for the effective development of endangered species conservation measures.