Objective To identify S1-bZIP subfamily members in Populus tomentosa and analyze the expression profiles of PtoS1-bZIP genes in different tissues in response to abiotic stress.

Method Members of PtoS1-bZIP subfamily were systematically analyzed using bioinformatic approaches. Real-time PCR was performed to explore the expression patterns of PtoS1-bZIP genes in different tissues, as well as under different hormonal or abiotic stress treatment.

Result A total of ten intron-lacking PtoS1-bZIP genes were identified in the genome of Populus tomentosa, which were distributed to eight distinct chromosomes. According to phylogenetic analysis, PtoS1-bZIP subfamily members were divided into three sub-clades, designated as Clades I to III. 12 pairs of fragment replicates were identified in the genome of Populus tomentosa. Cis-acting element prediction showed that the promoter regions of PtoS1-bZIP genes were rich of various regulatory elements involving light, hormone or abiotic stress response. qRT-PCR analysis revealed that the members of PtoS1-bZIP subfamily were specifically expressed across different tissues. The majority of PtoS1-bZIP genes in the first and second clades were upregulated under ABA and drought treatments, while expression levels were downregulated under salt treatment. In contrast, all of the PtoS1-bZIP members in the third clades were upregulated under ABA, drought, and salt treatments.

Conclusion In the genome of Populus tomentosa, ten PtoS1-bZIP genes are identified and divided into three clades. The expression of major PtoS1-bZIP genes in the first and second clades were induced by drought stress and inhibited by salt stress. The expression of members in the third clades were induced by both drought and salt stress. It indicates that different clades of PtoS1-bZIP genes may have functional differentiation, which play different roles in response to abiotic stresses. Our results lay a foundation for further investigation of the biological functions of PtoS1-bZIP genes and their molecular mechanisms involved in regulating stress resistance of poplars.