| Mangrove is a special kind of woody plant community,which is naturally distributed in the intertidal zone of tropical and subtropical coastal areas.It has important ecological functions such as windbreak and shoreline protection.Recently,in China,mangrove plants have been moved northward to the coastal intertidal zone in temperate regions to protect the coastline there.Many researches showed that the typical mangrove plant Kandelia obovata may be the most cold-resistant mangrove plant,so K.obovata is the main species used in the artificial northern transplanting.However,the cold resistance mechanism and low temperature adaptation mechanism of K.obovata remain unclear.K.candel and K.obovata are the only two species of the genus Kandelia.Studies have shown that the morphological similarities between the two species are great,but there may be great differences in the distribution area and cold tolerance.K.candel shows typical cold intolerance,the internal reasons for these differences are still unknown.Based on the above information,the research topics involved in this paper are divided into two aspects,namely,the cold acclimation mechanism of K.obovata and genomic polymorphism of Kandelia genus.In first topic,the medium cold acclimation experiment was designed to simulate the natural cold acclimation conditions,the purpose of this study is to explore the mechanism of cold acclimation of K.obovata under natural conditions,and provide reference for the practices of mangrove migration.Cold acclimation refers to a natural ecological phenomenon that can increase the survival rate of plant overwintering.K.obovata is moved from the warm tropical and subtropical regions to the temperate zone with four seasons,and it must also undergo the process of low temperature acclimation.Natural low temperature acclimation usually occurs in the autumn of low temperature fluctuation,and the whole process of cpld acclimation can be divided into three parts:plants produce stress responses when accepted the low temperature signal for the first time,plants rebuild physiological dynamic balance after many times of low temperature acclimatization,finally,plants produce low temperature memory to complete cold acclimation process.In this topic,a total of six experimental groups with different number of cold cycles and treatment times were set up,and the physiological state of K.obovata seedlings in different cold acclimation process was analyzed by testing 6 physiological indexs.In the second part,the genome Survey of K.candel was sequenced.Compared with the K.obovata genome assembled initially in our laboratory,the genome polymorphism information of the two species was obtained,and the functional annotation and pathway enrichment analysis of the polymorphic genes were carried out.The results were as follows:1.When the seedlings of K.obovata first perceived low temperature signals,the experimental groups produced similar stress responses.We speculate that seedlings of K.obovata can receive low temperature signals smoothly and produce roughly the same stress responses.2.After several times of simulated cold acclimation cycles,the content or level of each physiological index showed an inconsistent distribution before and after acclimatization,but in the process of subsequent low temperature acclimatization,the physiological indicators did not show a specific direction of change.We guess that it may be due to the fact that low temperature is a new stimulus for the K.obovata seedlings from subtropics,so they are in a state of exploration and search in response to the low temperature environment and attempt to reconstruct a new physiological balance,but the direction of this exploration is not consistent,indicating that the current acclimation conditions are still inadequate to help the plant to produce a specific memory of low temperature.However,after several times of acclimation cycles,the plants rebuilt the physiological balance,indicating that K.obovata has the ability to resist the low temperature environment,and has the ability to overwinter in the temperate region,and forming cold acclimatization.3.From the trend of Chlorophyll change,it is found that the seedlings of K.obovata tend to improve the content of photosynthetic pigments and improve the efficiency of photosynthesis in the process of low temperature acclimatization to resist the injury of low temperature.In the process of overwintering,woody plants usually adopt a relatively conservative strategy to reduce the content of photosynthetic pigments and reduce light and use efficiency,such as deciduous or dormancy,to survival in the low temperature environment.The results of this study were compared with the field investigation results,and it was found that the seedlings of K.obovata could also increase the content of photosynthetic pigments and improve the photosynthesis efficiency in the field environment.The trend of acclimatization of K.obovata is different from low temperature tolerant woody plants,but more similar to the acclimatization mode of annual herbaceous plants.We guess that K.obovata's unique and active coping with low temperature acclimatization may be an important reason for explaining the marvelous cold resistance of K.obovata.4.From the changes of physiological indexs,we can see that the physiological indexs of K.obovata have obvious changes in the low temperature and recovery stages of each acclimation cycle.We speculate that the effective recovery time is an essential part of the domestication during the cold resistance and acclimatization of K.obovata.Therefore,in the process of K.obovata moving northward,it is necessary to give the K.obovata seedlings low temperature acclimation treatments,and the effective recovery time was helpful to produce high survival rate of cold memory.5.A total of 710,241 genetic varients were detected betweeen K.candel and K.obovata,suggesting that polymorphism were widespread among the two species,of which 1,868 were high-impact,accounting for 0.1%.6.Comparing the annotation results of polymorphic genes,it was found that there were wide differences in cell components,molecular functions and biological processes between the two species.The differences in the KEGG pathway were mainly reflected in the metabolic process and gene expression,indicating that the physiological process of the two kinds of species are obvious differences.7.The results of GO functional enrichment and KEGG pathway enrichment of them showed the differences in biomembrane system,gene expression and transcriptional regulation,and metabolic pathway between them,but the genes related to stress resistance or cold resistance were not significantly enriched.It is difficult to reveal the intrinsic causes of the different distribution of the two species only from the gene polymorphism.The intrinsic mechanism of the difference in distribution of the two species still needs further exploration.In this paper,the cold acclimation mechanism of K.obovata was studied from simulated low temperature acclimation process.The results showed that the cold acclimation phases of K.obovata was similar to that of other plants,but positive cold resistance strategy was adopted in the process of cold resistance.The results of the experiment are more close to the actual situation than those of the short-term sustained low temperature stimulation.By exploring the low temperature acclimation mechanism of K.obovata under natural conditions,the aim is to provide reference for the practice of mangrove transplanting.The genetic differences between the two species were studied from the genomic polymorphism.Although the intrinsic reasons for the differences in cold resistance between the two species were not explained,the results of genetic polymorphism differences and annotation analysis were obtained,which provided a reference for further study of the genomic differences between the two species.The detection of SNP also provides an important reference for subsequent development of molecular markers. |
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