Genome-wide Dissection Of Heat Shock Transcriptional Factor Family Genes And Their Expression Profile Analysis During Watermelon Fruit Development

Watermelon [Citrullus lanatus(Thunb.)Matsum.& Nakai] has a great nutritional and economic importance.Fruit is the organ for consumption.Unveiling the underlying mechanisms regulating fruit development of watermelon can provide valuable information for genetic improvement of watermelon.Heat shock transcription factors(Hsfs)are key regulators that play a diverse regulatory role in plant developmental processes and response to environmental stresses.Whether ClHsfs are involved in regulation of watermelon fruit development is still unknown.To address this problem,a genome-wide comprehensive analysis of the Hsf family genes was carried out in this study to identify ClHsf family genes in watermelon and analyze their expression profiles during fruit development.The main results were summarized below:1.Twenty-four ClHsf genes were identified in watermelon genome by using hmmsearch program.Encoded peptides ranged from 192 to 782 amino acid residues.Sequence logo comparison of watermelon,cucumber and Arabidopsis DNA-binding domain showed that the residue distribution is conserved among three species.Based on linker length between the DBD and oligomerization domain,and number of amino acid bases inserted into the HR-A/B regions,ClHsfs were classified into three classes,namely A(54.17%),B(37.5%)and C(8.33%)which were class specific in their structure,evolution,and functions.2.The ClHsfs genes were distributed unevenly on 11 watermelon chromosomes.16.67% ClHsf genes were located on chr5 and chr1,12.5% ClHsfs on chr10,8.3% ClHsfs on chr3,chr8,chr7,chr9 and chr11,and 4.16% ClHsfs on chr2,chr4,and chr6,respectively.In watermelon,58.33% of Hsf genes have passed through duplication events.Duplication model analysis revealed that ClHsfs contains nine pairs of segmental duplicates and three pairs of transposed duplicate genes.Small-scale segmental and transposed duplications among ClHsf genes have played a vital role in the moderate expansion of this gene family.Selection pressure analysis revealed that all duplicated ClHsf genes have mainly undergone a periodic strong to moderate purifying selection.The segmental duplication pairs have undergone more strict purifying selection and retained their original function,whereas transposed duplicates may evolve new functions in case of neo-functionalization and sub-functionalization.The divergent time of ClHsfs duplicates dates back to approximately 179 Mya to 312 Mya.It can be speculated that Hsf-B class members are ancient and expansion happened in Hsfs after evolution to land plants.3.Furthermore,the expression pattern of ClHsf family genes was analyzed in rind,fruit flesh and seed tissues of watermelon.In rind,ClHsf-17(A2)had a higher expression at 10 days after pollination proposed its role in rind development.In fruit flesh,ClHsf-02(B1)and ClHsf-23(C1)expressed at moderately higher expression level during immature white pink and red flesh stage.In seed tissues,ClHsf-24(A9)showed a higher expression at seed maturation stage.By analyzing their promoter sequences,Skn-1 motif,GCN4 motif,and CAT-box were detected at a higher frequency.Gene annotation also predicted these four sequences functions related to cellular development.Hence,it can be speculated that these four candidate ClHsf genes might have functions in fruit development.In summary,this study provides insights to elucidate the expression profiles of ClHsf genes during watermelon fruit development and contribute towards better understanding of the ClHsf gene family structure and evolution.