Abstract
The heat shock protein 70 (HSP70) family plays an important role in the growth and development of lettuce and in the defense response to high-temperature stress; however, its bioinformatics analysis in lettuce has been extremely limited. Genome-wide bioinformatics analysis methods such as chromosome location, phylogenetic relationships, gene structure, collinearity analysis, and promoter analysis were performed in the LsHSP70 gene family, and the expression patterns in response to high-temperature stress were analyzed. The mechanism of LsHSP70-19 in heat resistance in lettuce was studied by virus-induced gene silencing (VIGS) and transient overexpression techniques. The results showed that a total of 37 LsHSP70 genes were identified by the Hidden Markov Model (HMM) and Protein Family Database (Pfam). These 37 LsHSP70 genes were classified into groups A, B, C, and D by phylogenetic relationships. They were mainly localized on seven chromosomes except for chromosome 3; gene structure analysis showed that LsHSP70 contained 1-9 exons, and the protein structure domains of genes in the same group were highly conserved. The covariance analysis showed that nine pairs of LsHSP70 genes existed between LsHSP70 members, and lettuce LsHSP70 and sunflower HaHSP70 had been more conserved in the evolutionary process. The promoter analysis showed that there were a large number of cis-acting elements related to phytohormones, growth, development, stress, and light response in LsHSP70. In addition, the results of the expression pattern analysis for all LsHSP70 genes under high-temperature stress showed that 28 out of 37 LsHSP70 genes were able to respond to heat stress, and only LsHSP70-8, LsHSP70-14, LsHSP70-19, LsHSP70-23, and LsHSP70-24 were able to respond rapidly to heat stress (2 h). The expression of LsHSP70-19 was higher at different periods under high-temperature stress; the overexpression of LsHSP70-19, the plant fresh weight, and the root weight were better than the control (CK); and the heat resistance was better. These results suggest that LsHSP70-19 may play an important role under high-temperature stress in lettuce.