Identification of ZmEXO70s gene family in maize and its correlation with heat tolerance at seedling stage

【Objective】To identify the members of maize ZXO70s(ZmEXO70s) gene family and analyze the correlation between their genetic variation and maize heat tolerance, and lay the foundation for defining its functional and molecular mechanisms in maize heat tolerance.【Method】With the sequences of Arabidopsis thaliana EXO70s(AtEXO70s) gene family members as reference, the members of the ZmEXO70s gene family were identified from the whole maize genome by MaizeGDB and NCBI databases, and the gene structure and phylogenetic analysis were carried out, and the members were systematically named according to their positions in the phylogenetic tree. Based on transcriptome sequencing(RNA-Seq) data, the expression patterns of ZmEXO70s gene family members under different tissues and abiotic stresses were analyzed. Finally, the SNP loci of ZmEXO70s gene in maize self-crossing population AM368 were identified, and gene family association analysis was conducted in combination with the heat resistance survival rate of maize AM368 at seedling stage.【Result】A total of 36 ZmEXO70s genes were identified from the whole genome sequence of maize, distributed on 10 chromosomes, with lengths ranging from 228 to 3726 bp, encoding 75-1241 amino acids, molecular weight of proteins ranging from 8.6 to 136.6 kD, theoretical isoelectric point(pI) ranging from 4.5 to 9.9, and pI of most proteins was less than 7.0. The EXO70s proteins of A. thaliana, rice and maize were divided into 9 groups(groups A to I), with ZmEXO70s proteins distributed in each group. Most ZmEXO70s genes did not contain introns, and only a few did, with varying numbers of introns. The 36 ZmEXO70s proteins contained 8 motifs(Motif 1-Motif 8), which were mainly concentrated in the C terminal, indicating that the ZmEXO70s C terminal sequence was relatively conservative, and Motif 1-Motif 8 constituted the conservative domain of Exo70. The expression level of the ZmEXO70s gene varied greatly in different tissues. The ZmEXO70s gene was highly expressed in panicle and anther, leaf and filaments, ZmEXO70C1a gene was only highly expressed in panicle and anther, and ZmEXO70D2b gene was lowly expressed in endosperm and germinal seed. ZmEXO70G1c gene was lowly expressed in filaments and germinated seeds. Twenty-three ZmEXO70s genes could respond to at least one abiotic stress, suggesting that these ZmEXO70s genes were involved in the abiotic stress response process. The genetic variation of ZmEXO70D2a and ZmEXO70E1 was significantly correlated with maize seedling heat resistance(P≤0.01). 【Conclusion】The members of the ZmEXO70s gene family are relatively conserved in phylogenetic evolution, and the genome replication event may occur after the differentiation of Gramineae, and most of the ZmEXO70s genes are retained, while some of them are lost. ZmEXO70s may play an important role in abiotic stress response, ZmEXO70D2a and ZmEXO70E1 genes may be important candidate genes for the regulation of maize seedling heat tolerance.