Low temperature is one of the common abiotic stresses that adversely affect the growth and development of plants. In this study, we used RNA-Seq to identify low-temperature-responsive genes in birch and further analyzed the underlying molecular mechanism. Birch seedlings were treated by the low temperature (6 °C) for 0, 1, 1.5, 2, 2.5, and 3 h, respectively. A total of 3491 genes were differentially expressed after low-temperature stress. Gene Ontology (GO) and Kyoto Encyclopedia of Gene and Genomes (KEGG) functional enrichment analysis were performed for the differentially expressed genes (DEGs). GO analysis indicated that 3491 DEGs were distributed into 1002 categories, and these DEGs were enriched in “cell process”, “metabolic process”, and “stimulus response”, under the “biological process” category; in “organelles” and “cell components”, under the “cell component” category; and in “catalytic activity” and “adhesion”, under the “molecular function” category. The KEGG enrichment indicated that 119 DEGs were involved in Ca²⁺ and plant hormone signal transduction; 205 DEGs were involved in secondary metabolic processes, such as lipid metabolism and phenylpropanoid biosynthesis pathway; and 20 DEGs were involved in photosynthesis. In addition, a total of 362 transcription factors (TFs) were differentially expressed under low-temperature stress, including AP2/ERF, C2H2, MYB-HB-like, WRKY, bHLH, WD40-like, and GRAS families. Gene Bpev01.c0480.g0081 (calmodulin-like CML38), Bpev01.c1074.g0005 (calmodulin-like CML25), Bpev01.c1074.g0001 (Calcium-binding EF-hand family protein), Bpev01.c2029.g0005 (calmodulin-like protein), Bpev01.c0154.g0008 (POD), Bpev01.c0015.g0143 (N-acetyl-1-glutamate synthase), and Bpev01.c0148.g0010 (branched chain amino acid transferase) were up-regulated at a high level, under low-temperature stress.

The objective of this research was to study the differences in endogenous hormone levels and the genes related to reproductive development in Chinese pinenut (Pinus koraiensis) trees of different ages. The apical buds of P. koraiensis were collected from 2-, 5-, 10-, 15-, and 30-year-old plants and also from grafted plants. There were three replicates from each group used for transcriptome sequencing. After assembly and annotation, we identified the differentially expressed genes (DEGs) and performed enrichment analysis, pathway analysis, and expression analysis of the DEGs in each sample. The results showed that unigenes related to reproductive development, such as c64070.graph_c0 and c68641.graph_c0, were expressed at relatively low levels at young ages, and that the relative expression gradually increased with increasing plant age. In addition the highest expression levels were reached around 10 and 15 years of age, after which they gradually decreased. Moreover, some unigenes, such as c61855.graph_c0, were annotated as abscisic acid hydroxylase genes, and the expression of c61855.graph_c0 gradually declined with increasing age in P. koraiensis.

Cold and freezing stress is one of the most harmful environmental stresses, especially in temperate and subtropical areas, that adversely affects plant growth, development, and yield production. Betula platyphylla Sukaczev, also known as white birch, is one of the most valuable, important, and widely distributed tree species in East Asia. This study explored the effects of cold acclimation (CA) in reducing the destructive effect of freezing stress in B. platyphylla seedlings. We measured the physiological and biochemical characteristics of B. platyphylla seedlings, such as chlorophyll content, electrolyte leakage (EL), malondialdehyde (MDA), antioxidant enzymes (such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)), and proline content before and after freezing stress to observe the contribution of CA in reducing the detrimental effects of freezing stress. The results showed that CA increased physiological and biochemical characteristics of B. platyphylla seedlings before and after freezing stress, except for chlorophyll content. Antioxidant enzymes were significantly positively correlated with proline, MDA, and EL content, and negatively correlated with chlorophyll content. Moreover, histochemical detection (H₂O₂ and O₂⁻) and cell death were revealed to be induced by cold stress in B. platyphylla seedlings. Furthermore, it was revealed that increased time and decreased temperature of the CA process significantly influenced the physiological and biochemical parameters. Overall, the CA process significantly reduced the detrimental effects of freezing stress compared to the control treatment in B. platyphylla seedlings. Taken together, these findings provide beneficial information toward understanding the mechanism of CA and freezing stress in B. platyphylla. Furthermore, the substantial activity of physiological and biochemical results could be used as selection criteria for screening time and temperature points of cold/freezing stress in further omics analyses. In addition, the combination of current study results, further omics analyses, and genetic engineering techniques directly contribute to sustainable forest management systems, tree plantations, and conservation of tree species, especially non-cold/non-freezing tolerant tree species.