Genome-wide association screening and verification of potential genes associated with root architectural traits in maize (Zea mays L.) at multiple seedling stages.

BACKGROUND: Breeding for new maize varieties with propitious root systems has tremendous potential in improving water and nutrients use efficiency and plant adaptation under suboptimal conditions. To date, most of the previously detected root-related trait genes in maize were new without functional verification. In this study, seven seedling root architectural traits were examined at three developmental stages in a recombinant inbred line population (RIL) of 179 RILs and a genome-wide association study (GWAS) panel of 80 elite inbred maize lines through quantitative trait loci (QTL) mapping and genome-wide association study. RESULTS: Using inclusive composite interval mapping, 8 QTLs accounting for 6.44-8.83 % of the phenotypic variation in root traits, were detected on chromosomes 1 (qRDWv3-1-1 and qRDW/SDWv3-1-1), 2 (qRBNv1-2-1), 4 (qSUAv1-4-1, qSUAv2-4-1, and qROVv2-4-1), and 10 (qTRLv1-10-1, qRBNv1-10-1). GWAS analysis involved three models (EMMAX, FarmCPU, and MLM) for a set of 1,490,007 high-quality single nucleotide polymorphisms (SNPs) obtained via whole genome next-generation sequencing (NGS). Overall, 53 significant SNPs with a phenotypic contribution rate ranging from 5.10 to 30.2 % and spread all over the ten maize chromosomes exhibited associations with the seven root traits. 17 SNPs were repeatedly detected from at least two growth stages, with several SNPs associated with multiple traits stably identified at all evaluated stages. Within the average linkage disequilibrium (LD) distance of 5.2 kb for the significant SNPs, 46 candidate genes harboring substantial SNPs were identified. Five potential genes viz. Zm00001d038676, Zm00001d015379, Zm00001d018496, Zm00001d050783, and Zm00001d017751 were verified for expression levels using maize accessions with extreme root branching differences from the GWAS panel and the RIL population. The results showed significantly (P < 0.001) different expression levels between the outer materials in both panels and at all considered growth stages. CONCLUSIONS: This study provides a key reference for uncovering the complex genetic mechanism of root development and genetic enhancement of maize root system architecture, thus supporting the breeding of high-yielding maize varieties with propitious root systems.

Identification and Molecular Characterization of RWP-RK Transcription Factors in Soybean.

The RWP-RK is a small family of plant-specific transcription factors that are mainly involved in nitrate starvation responses, gametogenesis, and root nodulation. To date, the molecular mechanisms underpinning nitrate-regulated gene expression in many plant species have been extensively studied. However, the regulation of nodulation-specific NIN proteins during nodulation and rhizobial infection under nitrogen starvation in soybean still remain unclear. Here, we investigated the genome-wide identification of RWP-RK transcription factors and their essential role in nitrate-inducible and stress-responsive gene expression in soybean. In total, 28 RWP-RK genes were identified from the soybean genome, which were unevenly distributed on 20 chromosomes from 5 distinct groups during phylogeny classification. The conserved topology of RWP-RK protein motifs, cis-acting elements, and functional annotation has led to their potential as key regulators during plant growth, development, and diverse stress responses. The RNA-seq data revealed that the up-regulation of GmRWP-RK genes in the nodules indicated that these genes might play crucial roles during root nodulation in soybean. Furthermore, qRT-PCR analysis revealed that most GmRWP-RK genes under Phytophthora sojae infection and diverse environmental conditions (such as heat, nitrogen, and salt) were significantly induced, thus opening a new window of possibilities into their regulatory roles in adaptation mechanisms that allow soybean to tolerate biotic and abiotic stress. In addition, the dual luciferase assay indicated that GmRWP-RK1 and GmRWP-RK2 efficiently bind to the promoters of GmYUC2, GmSPL9, and GmNIN, highlighting their possible involvement in nodule formation. Together, our findings provide novel insights into the functional role of the RWP-RK family during defense responses and root nodulation in soybean.

Genome-Wide Association Study of Root and Shoot Related Traits in Spring Soybean (Glycine max L.) at Seedling Stages Using SLAF-Seq.

Root systems can display variable genetic architectures leading to nutrient foraging or improving abiotic stress tolerance. Breeding for new soybean varieties with efficient root systems has tremendous potential in enhancing resource use efficiency and plant adaptation for challenging climates. In this study, root related traits were analyzed in a panel of 260 spring soybean with genome-wide association study (GWAS). Genotyping was done with specific locus amplified fragment sequencing (SLAF-seq), and five GWAS models (GLM, MLM, CMLM, FaST-LMM, and EMMAX) were used for analysis. A total of 179,960 highly consistent SNP markers distributed over the entire genome with an inter-marker distance of 2.36 kb was used for GWAS analysis. Overall, 27 significant SNPs with a phenotypic contribution ranging from 20 to 72% and distributed on chromosomes 2, 6, 8, 9, 13, 16 and 18 were identified and two of them were found to be associated with multiple root-related traits. Based on the linkage disequilibrium (LD) distance of 9.5 kb for the different chromosomes, 11 root and shoot regulating genes were detected based on LD region of a maximum 55-bp and phenotypic contribution greater than 22%. Expression analysis revealed an association between expression levels of those genes and the degree of root branching number. The current study provides new insights into the genetic architecture of soybean roots, and the underlying SNPs/genes could be critical for future breeding of high-efficient root system in soybean.