Genome wide and evolutionary analysis of heat shock protein 70 proteins in tomato and their role in response to heat and drought stress

Heat shock protein 70 (HSP70) proteins play a crucial role in mitigating the detrimental effects of abiotic stresses in plants. In the present study, 21 full length non-redundant SlHSP70 genes were detected and characterized in tomato (Solanum lycopersicum L.). The SlHSP70 genes were classified into four groups based on phylogenetic analysis. Similarities were observed in gene features and motif structures of SlHSP70s belonging to the same group. SlHSP70 genes were unevenly and unequally mapped on 11 chromosomes. Segmental and tandem duplication are the main events that have contributed to the expansion of the SlHSP70 genes. A large number of groups and sub-groups were generated during comparative analysis of HSP70 genes in multiple plant species including tomato. These findings indicated a common ancestor which created diverse sub-groups prior to a mono-dicot split. The selection pressure on specific codons was identified through a maximum-likelihood approach and we found some important coding sites in the coding region of all groups. Diversifying positive selection was indirectly associated with evolutionary changes in SlHSP70 proteins and suggests that gene evolution modulated the tomato domestication event. In addition, expression analysis using RNA-seq revealed that 21 SlHSP70 genes were differentially expressed in response to drought and heat stress. SlHSP70-5 was down-regulated by heat treatment and up-regulated by drought stress. Furthermore, the expression of some of the duplicate genes was partially redundant, while others showed functional diversity. Our results indicate the diverse role of HSP70 gene family in S. lycopersicum under drought and heat stress conditions and open the gate for further investigation of HSP70 gene family functions, especially under drought and heat stress.