Abstract: [Objective] Poplar (Populus) is widely planted in China and usually used as a model tree. As conserved molecular chaperon, plant BAG proteins play critical roles in growth, development and stress responses. Study on the evolution and biological functions of Populus BAG genes is useful for genetic improvement of tree species.[Method] Arabidopsis BAG proteins were used as keywords to search online database (https://phytozome.jgi.doe.gov/) in order to identify the homologous BAGs from poplar (Populus trichocarpa), rice (Oryza sativa) and moss (Physcomitrella patens). Protein structures and evolutionary relationship of these BAG proteins were analyzed using the bioinformatic method. Tissue expression patterns and stress (drought and heat) responses of Populus BAG genes were detected based on the microarray (http://bar.utoronto.ca/) and qRT-PCR data.[Result] P. trichocarpa contains fourteen BAG genes, respectively named as PtrBAG1 to PtrBAG14 according to the international rules for nomenclature (ordered on their positions on the chromosome). BAG proteins are relatively conserved among poplar, Arabidopsis, rice and moss. These Populus BAG proteins are divided into two subfamilies. The members in subfamily Ⅰ contains an UBL domain, suggesting that these proteins might function as bridges to assist the degradation of some proteins. The members in subfamily Ⅱ have an IQ domain, implying the potential roles of these proteins in Ca²⁺ signaling. Poplar BAG genes exhibited different stress (drought and heat) responses. For example, heat treatment resulted in visible changes in BAG expression. Of them, BAG1, BAG4, BAG6, BAG7 and BAG8 were activated whereas others were suppressed by heat stress. By contrast, there were few alterations in the expression of most BAG genes under the treatment of drought. Particularly, heat treatment induced up regulated expression of BAG4 and BAG6 by 90 folds. Tissue expression analysis indicated that most of BAGs had little expression in detected tissues (root, leaves, shoot, seedling, female catkins, male catkins and xylem). However, BAG2 and BAG12 were highly expressed in xylem.[Conclusion] Poplar BAG family contains 14 members, which are basically divided into two subfamilies. These BAG proteins are closer to BAG homologs from Arabidopsis and rice than those from moss. All poplar BAG genes are responsive to heat stress but not to drought stress. In particular, BAG4 and BAG6 are most highly induced by heat treatment. BAG2 and BAG12 are likely involved in wood formation.

Key words: Populus, BAG gene family, gene expression, phylogeny