The Structure And Expression Analysis Of Hsp70 Gene Family In Brassica Napus And Its Ancestors

Polyploidization is a common phenomenon in the evolutionary process and an important way of new species formation of plants.As the most common type of polyploids,the heterozygosity origins of allopolyploids give them more obvious evolutionary advantages.Brassica napus is a model plant for the study of allopolyploidy.It originated from the interspecific hybridization between Brassica rapa and Brassica oleracea about 7500 years ago and the subsequent chromosome doubling.As an important stress resistant molecular chaperon,Hsp70 protein widely present in various tissues and organs of plants,which is crucial to plant growth and development and stress response.Hsp70 gene sequences are highly conserved in different plant,which makes them exist in the form of gene families.The allotetraploid B.napus and its diploid ancestors were selected as experimental materials in this research.According to the published genomic data,we analyzed the structural characteristics of Hsp70 gene family and its gene expression patterns under high temperature stress,and explored the effect of whole-genome triplication and allopolyploidization on the Hsp70 gene family in Brassica.In this study,a total of 47 Hsp70 genes were identified in B.napus(A_nA_nC_nC_ngenome),named"Bn Hsp70 gene",of which 22 genes from A_n subgenome and 25 genes from C_nsubgenome.Meanwhile,29 and 20 Hsp70 genes were explored in B.rapa(A_rA_rgenome)and B.oleracea(C_oC_o genome),respectively named"Br Hsp70 gene"and"Bo Hsp70 gene".Based on phylogenetic analysis of the encoded products,a total of114 Hsp70 proteins derived from B.napus,B.rapa,B.oleracea and Arabidopsis thaliana,were divided into 6 clades.The Hsp70 gene structure and protein motif in each clade were highly similar,and the subcellular localization prediction of members in different clades were also different.However,it was also found that intron gain of Bn Hsp70 gene may be of relevance to intron evolution in the course of polyploidization.Moreover,it was showed that 17 Hsp70 genes maintained their relative position in A_rand A_n,and 9 genes maintained their relative position in C_o and C_nby comparing the gene distribution of Brassica napus and its ancestors.Allopolyploidization and segmental duplication events played essential role in the expansion of Bn Hsp70 gene family.Also,the majority of the Bn Hsp70 genes have experienced purifying selection pressure after the duplication events.In addition,combining transcriptome data analysis,nearly all Hsp70 genes were expressed in stems,leaves,flowers and siliques among three species,indicating that Hsp70 genes were widely involved in the growth and development of various tissues of plants.Also,most Hsp70 genes were tissue-specific expression.However,some Bn Hsp70 genes showed significantly different expression patterns in four tissues from homologous genes of ancestors.Compared with the orthologs of diploid ancestors,some Bn Hsp70 genes showed biased expression connected with tissue and subgenomic interaction.In this work,gene expression pattern of Hsp70 gene family in B.napus and its diploid ancestors under high temperature stress was analyzed by q RT-PCR.The results showed that Hsp70 gene expression level in three species changed during the stress period.Most of them had the same expression trend in B.napus and diploid ancestors,which their expression levels increased significantly and reached the highest relative expression level at the early stage of stress,which may be the rapid response of Hsp70 genes to high temperature.Compared with the homologs of diploid ancestors,the expression of Bn Hsp70 gene appeared increase specifically,which may play a role in the high temperature tolerance of allopolyploid.In conclusion,this study discussed the structure and expression of the Hsp70 gene family in Brassica napus and its diploid ancestors.These findings revealed evolution of the Hsp70 gene family polyploid,laid a foundation for exploring the molecular mechanism of allopolyploid adaptability,and had a guiding role in breeding for stress resistance of B.napus.