**1. Introduction**

Soil salinization is worldwide problem that can alter the soil osmotic potential to the point where it inhibits the uptake of water by plants, severely impacting agricultural production and the ecological environment. It has been reported that more than 6% of the world's land is affected by salt [1–3], and increased salinization may lead to the loss of 30% arable land in the next 25 years. It has been reported that more than 6% of the world's land is affected by salt [1–3], and increased salinization may lead to the loss of 30% arable land in the next 25 years and up to 50% by 2050 [1–3]. Therefore, soil salinization is a serious threat to the growth and development of plants. At present, it is particularly urgent to search for salinity resistant varieties of plants and screen for salt tolerant gene alleles or transform them genetically to enable plants to grow and reproduce with increasing salinity stress [4]. Moreover, understanding the mechanism of salt tolerance in plants can provide valuable information for effective engineering strategies.

In plants, the salt resistance mechanism is very complicated and involves a complex of processes at the molecular, cellular, metabolic, physiological, and whole-plant levels. Once the plant is under salt stress, multiple signal transduction pathways are activated to cope with salt stress [2,4–6]. In recent years, although extensive studies among ion uptake and transport, osmotic regulation, hormone metabolism, antioxidant metabolism, and stress signaling have made significant progress [4–11], the molecular mechanisms involved in salt tolerance remain to be elucidated. In addition, next-generation high-throughput sequencing based RNA-seq analysis has been widely used to uncover expression patterns under abiotic stress, and it provides a comprehensive means of identifying and studying the differential expression genes [12–15].

*Betula halophila* is a haloduric species in China, belonging to the family Betulaceae. It was first discovered in a swamp with extremely high salinity in Xinjiang province [16] in 1956 by Professor Renchang Qin. *B. halophila* is a critically endangered plant, which has high salt tolerance, and high ecological and economic value in promoting the afforestation of saline soil in arid and semi-arid areas16. Thus *B. halophila* is a potent source of salt tolerant genes. However, to the best of our knowledge, there is no published information on genes associated with salt tolerance in *B. halophila*. Understanding the molecular mechanisms of salt tolerance are potentially important for breeding salt tolerant varieties. With the aim of identifying the genes in response to increasing salt concentration and potentially the molecular mechanisms of salt tolerance in *B. halophila*, we constructed transcriptome libraries from the leaves of control *B. halophila* plants and plants subjected to salt treatment. The aims were to detect salt responsive genes from *B. halophila* and explore their roles in response to salt stress. Our results provide insight into the molecular mechanisms of salt tolerance in *B. halophila*. A better understanding of these tolerance mechanisms can be used to breed crops with improved yield performance under salinity stress.
