Basic leucine zipper (bZIP), a conserved transcription factor widely found in eukaryotes, has important regulatory roles in plant growth. To understand the information related to the bZIP gene family in walnut, 88 JrbZIP genes were identified at the genome-wide level and classified into 13 subfamilies (A, B, C, D, E, F, G, H, I, J, K, M, and S) using a bioinformatic approach. The number of exons in JrbZIPs ranged from 1 to 12, the number of amino acids in JrbZIP proteins ranged from 145 to 783, and the isoelectric point ranged from 4.85 to 10.05. The majority of JrbZIP genes were localized in the nucleus. The promoter prediction results indicated that the walnut bZIP gene contains a large number of light-responsive and jasmonate-responsive action elements. The 88 JrbZIP genes were involved in DNA binding and nucleus and RNA biosynthetic processes of three ontological categories, molecular functions, cellular components and biological processes. The codon preference analysis showed that the bZIP gene family has a stronger bias for AGA, AGG, UUG, GCU, GUU, and UCU than other codons. Moreover, the transcriptomic data showed that JrbZIP genes might play an important role in floral bud differentiation. The results of a protein interaction network map and kegg enrichment analysis indicated that bZIP genes were mainly involved in phytohormone signaling, anthocyanin synthesis and flowering regulation. qRT-PCR demonstrated the role of the bZIP gene family in floral bud differentiation. Co-expression network maps were constructed for 29 walnut bZIP genes and 6 flowering genes, and JrCO (a homolog of AtCO) was significantly correlated (p < 0.05) with 13 JrbZIP genes in the level of floral bud differentiation expression, including JrbZIP31 (homolog of AtFD), and JrLFY was significantly and positively correlated with JrbZIP10,11,51,59,67 (p < 0.05), and the above results suggest that bZIP family genes may act together with flowering genes to regulate flower bud differentiation in walnut. This study was the first genome-wide report of the walnut bZIP gene family, which could improve our understanding of walnut bZIP proteins and provide a solid foundation for future cloning and functional analyses of this gene family.
Keywords: Juglans regia L.; bZIP gene family; evolutionary analyses; expression pattern analysis; flower bud differentiation.