Genome-Wide Identification and Expression Analysis of HSP70 Gene Family Under High-Temperature Stress in Lettuce (Lactuca sativa L.)

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 <i>LsHSP70</i> gene family, and the expression patterns in response to high-temperature stress were analyzed. The mechanism of <i>LsHSP70-19</i> 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 <i>LsHSP70</i> genes were identified by the Hidden Markov Model (HMM) and Protein Family Database (Pfam). These 37 <i>LsHSP70</i> 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 <i>LsHSP70</i> 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 <i>LsHSP70</i> genes existed between <i>LsHSP70</i> members, and lettuce <i>LsHSP70</i> and sunflower <i>HaHSP70</i> 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 <i>LsHSP70</i>. In addition, the results of the expression pattern analysis for all <i>LsHSP70</i> genes under high-temperature stress showed that 28 out of 37 <i>LsHSP70</i> genes were able to respond to heat stress, and only <i>LsHSP70-8</i>, <i>LsHSP70-14</i>, <i>LsHSP70-19</i>, <i>LsHSP70-23</i>, and <i>LsHSP70-24</i> were able to respond rapidly to heat stress (2 h). The expression of <i>LsHSP70-19</i> was higher at different periods under high-temperature stress; the overexpression of <i>LsHSP70-19</i>, the plant fresh weight, and the root weight were better than the control (CK); and the heat resistance was better. These results suggest that <i>LsHSP70-19</i> may play an important role under high-temperature stress in lettuce.