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Summary: Glycine‐rich RNA‐binding proteins (GRPs) have been implicated in the responses of plants to environmental stresses, but the function of GRP genes involved in salt stress and the underlying mechanism remain unclear. In this study, we identified BpGRP1 (glycine‐rich RNA‐binding protein), a Betula platyphylla gene that is induced under salt stress. The physiological and molecular responses to salt tolerance were investigated in both BpGRP1‐overexpressing and suppressed conditions. BpGRF3 (growth‐regulating factor 3) was identified as a regulatory factor upstream of BpGRP1. We demonstrated that overexpression of BpGRF3 significantly increased the salt tolerance of birch, whereas the grf3‐1 mutant exhibited the opposite effect. Further analysis revealed that BpGRF3 and its interaction partner, BpSHMT, function upstream of BpGRP1. We demonstrated that BpmiR396c, as an upstream regulator of BpGRF3, could negatively regulate salt tolerance in birch. Furthermore, we uncovered evidence showing that the BpmiR396c/BpGRF3 regulatory module functions in mediating the salt response by regulating the associated physiological pathways. Our results indicate that BpmiR396c regulates the expression of BpGRF3, which plays a role in salt tolerance by targeting BpGRP1. [ABSTRACT FROM AUTHOR]

The microRNAs, which are small RNAs of 18–25 nt in length, act as key regulatory factors in posttranscriptional gene expression during plant growth and development. However, little is known about their regulatory roles in response to stressful environments in birch (Betula platyphylla). Here, we characterized and further explored miRNAs from osmotic- and salt-stressed birch. Our analysis revealed a total of 190 microRNA (miRNA) sequences, which were classified into 180 conserved miRNAs and 10 predicted novel miRNAs based on sequence homology. Furthermore, we identified Bp-miR408a under osmotic and salt stress and elucidated its role in osmotic and salt stress responses in birch. Notably, under osmotic and salt stress, Bp-miR408a contributed to osmotic and salt tolerance sensitivity by mediating various physiological changes, such as increases in reactive oxygen species accumulation, osmoregulatory substance contents and Na+ accumulation. Additionally, molecular analysis provided evidence of the in vivo targeting of BpBCP1 (blue copper protein) transcripts by Bp-miR408a. The overexpression of BpBCP1 in birch enhanced osmotic and salt tolerance by increasing the antioxidant enzyme activity, maintaining cellular ion homeostasis and decreasing lipid peroxidation and cell death. Thus, we reveal a Bp-miR408a – BpBCP1 regulatory module that mediates osmotic and salt stress responses in birch. [ABSTRACT FROM AUTHOR]

SWEET proteins play important roles in plant growth and development, sugar loading in phloem and resistance to abiotic stress through sugar transport. In this study, 13 BpSWEET genes were identified from birch genome. Collinearity analysis showed that there were one tandem repeating gene pair (BpSWEET1b/BpSWEET1c) and two duplicative gene pairs (BpSWEET17a/BpSWEET17b) in the BpSWEET gene family. The BpSWEET gene promoter regions contained several cis-acting elements related to stress resistance, for example: hormone-responsive and low-temperature-responsive cis-elements. Analysis of transcriptome data showed that BpSWEET genes were highly expressed in several sink organs, and the most BpSWEET genes were rapidly up-regulated under cold stress. BpSWEET1c, which was highly expressed in cold stress, was selected for further analysis. It was found that BpSWEET1c was located on the cell membrane. After 6 h of 4 °C stress, sucrose content in the leaves and roots of transient overexpressed BpSWEET1c was significantly higher than that of the control. MDA content in roots was significantly lower than that of the control. These results indicate that BpSWEET1c may play a positive role in the response to cold stress by promoting the metabolism and transport of sucrose. In conclusion, 13 BpSWEET genes were identified from the whole genome level. Most of the SWEET genes of birch were expressed in the sink organs and could respond to cold stress. Transient overexpression of BpSWEET1c changed the soluble sugar content and improved the cold tolerance of birch. [ABSTRACT FROM AUTHOR]

The basic leucine zipper (bZIP) gene, which plays a significant role in the regulation of tolerance to biotic/abiotic stresses, has been characterized in many plant species. Betula platyphylla is a significant afforestation species. To elucidate the stress resistance mechanism of birch, previous studies identified some stress resistance genes. However, the genome-wide identification and characterization of bZIP gene family in the birch have not been reported. Here, the 56 BpbZIP genes were identified and classified into 13 groups in birch. Cis-element analysis showed that the promoters of 56 family genes contained 108 elements, of which 16 were shared by 13 groups. There were 8 pairs of fragment repeats and 1 pair of tandem repeats, indicating that duplication may be the major reason for the amplification of the BpbZIP gene family. Tissue-specific of BpbZIP genes showed 18 genes with the highest expression in roots, 15 in flowers, 11 in xylem and 9 in leaves. In addition, five differentially expressed bZIP genes were identified from the RNA-seq data of birch under low-temperature stress, and the co-expressed differentially expressed genes were further screened. The analysis of gene ontology (GO) enrichment of each co-expression regulatory network showed that they were related to membrane lipids and cell walls. Furthermore, the transient overexpression of BpChr04G00610 decreased the ROS scavenging ability of birch under low-temperature stress, suggesting that it may be more sensitive to low-temperature. In conclusion, this study provides a basis for the study of the function of BpbZIP genes. • Fifty-six bZIP genes were identified in Betula platyphylla. • The response of BpbZIPs to low-temperature may be related to cell wall. • Bpchr04g00610 may play a negative regulatory role in low-temperature stress. • This work provide insights into the future characterization of BpbZIP genes. [ABSTRACT FROM AUTHOR]