Soil salinization is the main abiotic stressor faced by crops. An improved understanding of the transcriptional response to salt stress in roots, the organ directly exposed to a high salinity environment, can inform breeding strategies to enhance tolerance and increase crop yield. Here, RNA-sequencing was performed on the roots of salt-tolerant wheat breeding line CH7034 at 0, 1, 6, 24, and 48 h after NaCl treatment. Based on transcriptome data, a weighted gene co-expression network analysis (WGCNA) was constructed, and five gene co-expression modules were obtained, of which the blue module was correlated with the time course of salt stress at 1 and 48 h. Two GO terms containing 249 differentially expressed genes (DEGs) related to osmotic stress response and salt-stress response were enriched in the blue module. These DEGs were subsequently used for association analysis with a set of wheat germplasm resources, and the results showed that four genes, namely a Walls Are Thin 1-related gene (TaWAT), an aquaporin gene (TaAQP), a glutathione S-transfer gene (TaGST), and a zinc finger gene (TaZFP), were associated with the root salt-tolerance phenotype. Using the four candidate genes as hub genes, a co-expression network was constructed with another 20 DEGs with edge weights greater than 0.6. The network showed that TaWAT and TaAQP were mainly co-expressed with fifteen interacting DEGs 1 h after salt treatment, while TaGST and TaZFP were mainly co-expressed with five interacting DEGs 48 h after salt treatment. This study provides key modules and candidate genes for understanding the salt-stress response mechanism in wheat roots.

Soil salinization is a major abiotic stressor that significantly reduces wheat yield. Identifying novel salt-tolerance genes and integrating them into wheat breeding programs can enhance wheat productivity in saline soils. Ethylene response factor (ERF) plays an important role in plant response to salt stress, and thus far, four wheat ERF genes have been identified to be involved in salt stress response. To systematically identify salt tolerance-related ERF genes in wheat, in this study, 213 ERF sequences were isolated from the whole genome of common wheat and classified into 54 members based on subgenome homology, named TaERF1 to TaERF54. Transcriptome sequencing results showed different expression patterns of TaERF members in leaves after 1, 6, 24, and 48 h of NaCl treatment. Based on association analysis, nine TaERF genes were correlated with the leaf salt injury index. Among them, five SNPs of TaERF16-B formed two haplotypes: Hap1 and Hap2. RT-qPCR results showed that the expression level of TaERF16-B was significantly higher in Hap2-typed germplasms than that in Hap1-typed germplasms after 1 and 6 h of NaCl treatment. A Kompetitive Allele-Specific PCR marker K52 was developed for genotyping TaERF16-B haplotypes, which further confirmed the significant correlation between TaERF16-B and salt tolerance-related phenotypes in mapping population and wheat germplasms. This study provides new genes and molecular markers for improving salt tolerance in wheat.

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.