Sequence Analysis of the Hsp70 Family in Moss and Evaluation of Their Functions in Abiotic Stress Responses

The 70-kD heat shock proteins (Hsp70s) are highly conserved molecular chaperones that play essential roles in cellular processes including abiotic stress responses. Physcomitrella patens serves as a representative of the first terrestrial plants and can recover from serious dehydration. To assess the possible relationship between P. patens Hsp70s and dehydration tolerance, we analyzed the P. patens genome and found at least 21 genes encoding Hsp70s. Gene structure and motif composition were relatively conserved in each subfamily. The intron-exon structure of PpcpHsp70-2 was different from that of other PpcpHsp70s; this gene exhibits several forms of intron retention, indicating that introns may play important roles in regulating gene expression. We observed expansion of Hsp70s in P. patens, which may reflect adaptations related to development and dehydration tolerance, and results mainly from tandem and segmental duplications. Expression profiles of rice, Arabidopsis and P. patens Hsp70 genes revealed that more than half of the Hsp70 genes were responsive to ABA, salt and drought. The presence of overrepresented cis-elements (DOFCOREZM and GCCCORE) among stress-responsive Hsp70s suggests that they share a common regulatory pathway. Moss plants overexpressing PpcpHsp70-2 showed salt and dehydration tolerance, further supporting a role in adaptation to land. This work highlights directions for future functional analyses of Hsp70s.