Objective  Abiotic stresses such as drought and salt seriously affect the growth and development of plants. Previous studies showed that histidine kinase played an important role during the process of plants responsing to abiotic stresses. In this study, we analyzed the expression of histidine kinase gene (PaHK3a) in roots, stems and leaves of poplar ‘84K’ (Poplus alba × P. glandulosa ‘84K’). And the expression of PaHK3a in leaves of in vitro ‘84K’ plants under various plant hormones and abiotic stresses was also detected. Together with the malondialdehyde (MDA) contents, superoxide dismutase (SOD) and peroxidase (POD) activities in leaves of poplar ‘84K’ in green house, the function of PaHK3a was preliminary proposed. The results in this study laid a foundation for molecular breeding of poplar for resistance.

  Method  Using in vitro poplar ‘84K’ as materials, the expression of PaHK3a gene in different organs, under plant hormone treatments (10 mmol/L ABA, 10 mmol/L 6-BA, 10 mmol/L IBA, 10 mmol/L GA3 and 10 mmol/L SA) and various abiotic stress conditions (42 ℃, 0 ℃, 200 mmol/L NaCl, and 5% PEG6000), was analyzed by qRT-PCR. Meanwhile, the expression of PaHK3a in leaves of green house grown poplar ‘84K’ plants under drought (6, 8, 10 d) and salt stress (2, 4, 6 d) was also detected, and the MDA contents, SOD and POD activities were measured, and the correlation between expression of PaHK3a gene and physiological indicators was analyzed to determine the function of PaHK3a preliminarily.

  Result  The expression of PaHK3a gene was highest in leaves, medium in roots and lowest in stems. The qRT-PCR results showed that the transcriptional levels of PaHK3a gene were about 2.63, 1.49, 1.54, 1.58 times of control, respectively under 42 ℃, 0 ℃, 200 mmol/L NaCl, and 5% PEG treatments. Under IBA treatment, the transcripts of PaHK3a were not significantly different from control. The expression of PaHK3a was down-regulated under the treatments of 6-BA, ABA, GA3 and SA, respectively. At different stress times of drought and salt treatment, the PaHK3a gene in leaves of greenhouse grown ‘84K’ plants increased significantly, with the style of increased first and then decreased; and the MDA content in leaves also had the similar style. SOD and POD activity were measured in drought and high salt, MDA content increased first and then decreased; SOD activity in leaves increased constantly during drought and salt stresses, and POD activity first increased and then decreased under drought stress, while increased constantly during salt stress. The correlation analysis between physiological indexes and PaHK3a gene expression found that under drought and salt stress, the expression of PaHK3a gene was positively correlated with MDA content, SOD activity and POD activity in leaves.

  Conclusion  The PaHK3a was expressed in roots, stems and leaves of poplar ‘84K’, with highest expression in leaves. The expression of PaHK3a was down-regulated by exogenous cytokinin (6-BA, GA3) and stress related plant hormones (ABA, SA), and up-regulated by temperature, salt and drought stresses. During the process of drought and salt stresses, the expression of PaHK3a increased significantly, with the increase of MDA content, SOD activity and POD activity, and it was positively correlated with MDA content, SOD activity and POD activity in leaves. Our results indicate that PaHK3a is involved in the response of poplar to plant hormones and plays an important role in poplar response to abiotic stresses.