Difference In Cold Tolerance And Its Enigenetic Basis Of Populations In The Northern And Southern Margin Of The Distribution Region Of Alternanthera Philoxeroides In China

Biological invasion is one of the three major environmental problems in the world,and has attracted the attention of the majority of scientists.Alligator weed(Alternanthera philoxeroides)is listed as one of the most harmful alien invasive plants in China,which has caused huge economic losses and ecological damage.Alligator weed originated in South America,it was introduced into Shanghai in 1930’s as a military horse feed,and then it was widely introduced and escaped into wild population,mainly distributed in the provinces south of the Yellow River.It rapidly spread from south to north,occupying a vast geographical distribution in China,Jinan is the northernmost distribution point known.Previous studies have found that Alligator weed is mainly clonal propagation in China,and its genetic diversity is very low,so it is considered to be the same clonal reproduction.As an invasive plant lacking genetic diversity advantage,alligator weed has a strong adaptability to heterogeneous environment,and it is considered that epigenetic variation plays a key role in its environmental adaptation.We hypothesized that epigenetic variation contributed to the expansion of alligator weed from south to north.In this study,two populations(Jinan and Haikou)located in the northern and southern margin of the distribution area of alligator weed were used as research materials to study the difference of cold tolerance and its epigenetic basis through common garden planting,physiological tolerance analysis,transcriptome analysis and epigenetic analysis.The main results are as follows:(1)The comparison of cold tolerance,Survival rate and physiological experiment(electrolyte leakage,proline content,chlorophyll fluorescence parameter Fv/Fm)of two populations of JN and HK showed that,the cold tolerance of Jinan(JN)was significantly higher than that of Haikou(HK);The cold tolerance of JN was still higher than that of HK with the extension of cultivation time in common garden(JN-G1 and HK-G1,JN-G2 and HK-G2)after continuous cultivation for one year and two years,and the difference of cold tolerance among populations was gradually reduced.The results showed that some traits such as cold tolerance were stable in a short period of time,but the stability would be weaker with the extension of cultivation time.(2)The expression patterns and differential genes of cold response genes in two populations were compared by transcriptome.The results showed that the gene expression patterns were similar but the expression level was different among populations under cold stress.The differential expression genes are mainly involved in signal transduction,transcription factors,plant hormones and metabolite biosynthesis.JN and HK have significant differences in transcription levels of many transcription factors,and JN expression level is significantly higher than HK.Real time quantitative PCR further verified that the expression levels of JN in Ap WRKY28,Ap WRKY40,Ap WRKY51,Ap WRKY60,Ap TIFY10 A and Ap LBD37 were significantly higher than those in HK.(3)The results showed that the cDNA sequences of Ap WRKY40,Ap TIFY10 A and Ap LBD37 were almost identical between the two populations,which indicated that the expression differences of these genes were not completely determined by the variation of their gene sequences.Further research on alligator weed from the perspective of epigenetics,the histone modification of three candidate genes was verified by chromatin immunoprecipitation chip technology.The histone modification of three candidate genes was detected by specific primers after immunoprecipitation.The results showed that there were significant differences in histone H3K4 trimethylation and H3K9 acetylation of three candidate genes Ap WRKY40,Ap TIFY10 A and Ap LBD37 among populations,and the degree of JN modification was significantly higher than that of HK,which was positively correlated with their gene expression,indicating that the difference in cold tolerance of alligator weed might be related to histone modification.(4)The results of DNA methylation revealed by MSAP showed that there were significant variations in DNA methylation among populations(JN-G0 and HK-G0)in the original habitat,and the epigenetic distance between populations was far.After one year and two years of continuous cultivation in common garden(JN-G1 and HK-G1,JN-G2 and HK-G2),with the extension of cultivation time in the common garden,the variation of DNA methylation gradually decreased among populations,and Phi PT value: G0>G1>G2.There was no significant difference in the epigenetic distance between the two populations after two years of cultivation.There was a positive correlation between DNA methylation and cold tolerance among populations.The difference of cold tolerance between the two populations may be related to DNA methylation.After cold stress,the DNA methylation of the two populations changed significantly.Under the same environment,the DNA methylation of the two populations tended to be the same,cold stress could induce rapid changes of DNA methylation.These results suggest that long-term environmental adaptation can induce the change of DNA methylation level,which may be related to the difference of cold tolerance.The results showed that the cold tolerance of alligator weed in Jinan and Haikou was significantly different after years of adaptation and domestication,and these differences were at least partly regulated by epigenetic DNA methylation and histone modification.In conclusion,there are significant differences in cold tolerance between the northern and southern populations of alligator weed,at least part of which may be related to epigenetic modification.This study revealed the adaptive mechanism of invasive plant alligator weed from the perspective of epigenetics,which has a certain reference value for understanding the ecological adaptation of clonal plants.