Identification of Salt Tolerance-Related NAC Genes in Wheat Roots Based on RNA-Seq and Association Analysis

Excavating new salt tolerance genes and utilizing them to improve salt-tolerant wheat varieties is an effective way to utilize salinized soil. The NAC gene family plays an important role in plant response to salt stress. In this study, 446 NAC sequences were isolated from the whole genome of common wheat and classified into 118 members based on subgenome homology, named TaNAC1 to TaNAC118. Transcriptome analysis of salt-tolerant wheat breeding line CH7034 roots revealed that 144 of the 446 TaNAC genes showed significant changes in expression levels at least two time points after NaCl treatment. These differentially expressed TaNACs were divided into four groups, and Group 4, containing the largest number of 78 genes, exhibited a successive upregulation trend after salt treatment. Single nucleotide polymorphisms (SNPs) of the TaNAC gene family in 114 wheat germplasms were retrieved from the public database and were subjected to further association analysis with the relative salt-injury rates (RSIRs) of six root phenotypes, and then 20 SNPs distributed on chromosomes 1B, 2B, 2D, 3B, 3D, 5B, 5D, and 7A were correlated with phenotypes involving salt tolerance (p < 0.0001). Combining the results of RT-qPCR and association analysis, we further selected three NAC genes from Group 4 as candidate genes that related to salt tolerance, including TaNAC26-D3.2, TaNAC33-B, and TaNAC40-B. Compared with the wild type, the roots of the tanac26-d3.2 mutant showed shorter length, less volume, and reduced biomass after being subjected to salt stress. Four SNPs of TaNAC26-D3.2 formed two haplotypes, Hap1 and Hap2, and germplasms with Hap2 exhibited better salt tolerance. Snp3, in exon 3 of TaNAC26-D3.2, causing a synonymous mutation, was developed into a Kompetitive Allele-Specific PCR marker, K3, to distinguish the two haplotypes, which can be further used for wheat germplasm screening or marker-assisted breeding. This study provides new genes and molecular markers for improvement of salt tolerance in wheat.