Identification of the PmWEEP locus controlling weeping traits in Prunus mume through an integrated genome-wide association study and quantitative trait locus mapping

Comparative population genomics reveals convergent and divergent selection in the apricot-peach-plum-mei complex

The economically significant genus Prunus includes fruit and nut crops that have been domesticated for shared and specific agronomic traits; however, the genomic signals of convergent and divergent selection have not been elucidated. In this study, we aimed to detect genomic signatures of convergent and divergent selection by conducting comparative population genomic analyses of the apricot-peach-plum-mei (APPM) complex, utilizing a haplotype-resolved telomere-to-telomere (T2T) genome assembly and population resequencing data. The haplotype-resolved T2T reference genome for the plum cultivar was assembled through HiFi and Hi-C reads, resulting in two haplotypes 251.25 and 251.29 Mb in size, respectively. Comparative genomics reveals a chromosomal translocation of ~1.17 Mb in the apricot genomes compared with peach, plum, and mei. Notably, the translocation involves the D locus, significantly impacting titratable acidity (TA), pH, and sugar content. Population genetic analysis detected substantial gene flow between plum and apricot, with introgression regions enriched in post-embryonic development and pollen germination processes. Comparative population genetic analyses revealed convergent selection for stress tolerance, flower development, and fruit ripening, along with divergent selection shaping specific crop, such as somatic embryogenesis in plum, pollen germination in mei, and hormone regulation in peach. Notably, selective sweeps on chromosome 7 coincide with a chromosomal collinearity from the comparative genomics, impacting key fruit-softening genes such as PG, regulated by ERF and RMA1H1. Overall, this study provides insights into the genetic diversity, evolutionary history, and domestication of the APPM complex, offering valuable implications for genetic studies and breeding programs of Prunus crops.

Genomic region and origin for selected traits during differentiation of small-fruit cultivars in Japanese apricot (Prunus mume)

The Japanese apricot (Prunus mume) is a popular fruit tree in Japan. However, the genetic factors associated with fruit trait variations are poorly understood. In this study, we investigated nine fruit-associated traits, including harvesting time, fruit diameter, fruit shape, fruit weight, stone (endocarp) weight, ratio of stone weight to fruit weight, and rate of fruit gumming, using 110 Japanese apricot accessions over four years. A genome-wide association study (GWAS) was performed for these traits and strong signals were detected on chromosome 6 for harvesting time and fruit diameters. These peaks were shown to undergo strong artificial selection during the differentiation of small-fruit cultivars. The genomic region defined by the GWAS and XP-nSL analyses harbored several candidate genes associated with plant hormone regulation. Furthermore, the alleles of small-fruit cultivars in this region were shown to have genetic proximity to some Chinese cultivars of P. mume. These results indicate that the small-fruit trait originated in China; after being introduced into Japan, it was preferred and selected by the Japanese people, resulting in the differentiation of small-fruit cultivars.

Genome-wide association study provides genetic insights into natural variation in watermelon rind thickness and single fruit weight

Rind thickness and fruit weight are agronomic traits closely related to quality and yield, which have attracted much attention from consumers and breeders. However, the genetic mechanism of these two traits is still not well understood in natural populations. In this study, rind thickness and single fruit weight in 151 watermelon accessions were determined in 2019 and 2020, and genome-wide association analysis was performed by integrating phenotypic and genotype data. Abundant phenotypic variation was found in the test population, and the watermelon with thinner rind thickness tended to have smaller fruit weights. Five significant SNPs were closely associated with rind thickness on chromosome 2 by Genome-wide association study (GWAS), i.e., 32344170, 32321308, 32304738, 32328501, and 32311192. And there were 21 genes were annotated in the candidate interval, most notably, Cla97C02G044160 belonged to the MADS family, and part of the genes in this family played an important role in regulating organ size, further analysis of gene structure, gene expression level, and phylogenetic tree showed that Cla97C02G044160 was a candidate gene for regulating target traits. In conclusion, our study provides molecular insights into the natural variation of watermelon rind thickness and single fruit weight, meanwhile, providing data support for molecular marker-assisted breeding.

Genome-wide identification of Aux/IAA gene family and their expression analysis in Prunus mume

AUXIN/INDOLE ACETIC ACIDs (Aux/IAAs), an early auxin-responsive gene family, is important for plant growth and development. To fully comprehend the character of Aux/IAA genes in woody plants, we identified 19 PmIAA genes in Prunus mume and dissected their protein domains, phylogenetic relationship, gene structure, promoter, and expression patterns during floral bud flushing, auxin response, and abiotic stress response. The study showed that PmIAA proteins shared conserved Aux/IAA domain, but differed in protein motif composition. 19 PmIAA genes were divided into six groups (Groups Ⅰ to Ⅵ) based on phylogenetic analysis. The gene duplication analysis showed that segmental and dispersed duplication greatly influenced the expansion of PmIAA genes. Moreover, we identified and classified the cis-elements of PmIAA gene promoters and detected elements that are related to phytohormone responses and abiotic stress responses. With expression pattern analysis, we observed the auxin-responsive expression of PmIAA5, PmIAA17, and PmIAA18 in flower bud, stem, and leaf tissues. PmIAA5, PmIAA13, PmIAA14, and PmIAA18 were possibly involved in abiotic stress responses in P. mume. In general, these results laid the theoretical foundation for elaborating the functions of Aux/IAA genes in perennial woody plant development.

Identification and Expression Analysis of NAC Gene Family in Weeping Trait of Lagerstroemia indica

Lagerstroemia indica is a widely used ornamental plant in summer gardens because of its desirable plant shape. The weeping traits of plants are related to secondary cell wall thickness and hormone signaling. NAC (NAM-ATAF1/2-CUC2), as one of the plant-specific transcription factors, is a switch for the secondary cell wall and also involved in leaf senescence, phytohormone signaling, and other growth processes. We identified a total of 21 LiNAC genes from the transcriptome data, which we divided into 14 subgroups and 2 groups. The physicochemical characteristics of amino acids, subcellular localization, transmembrane structure, GO and KEGG enrichment, and expression patterns were also examined. The qRT-PCR analysis showed that the expressions of LiNAC8 and LiNAC13 in upright L. indica 'Shaoguifei' and weeping L. indica 'Xiariwuniang' were significantly higher from the beginning to the end of growth stage (S1-S3), and the expressions of 'Shaoguifei' were always higher than those of 'Xiariwuniang'. However, LiNAC2 showed a downward trend in S1-S3 and the relative expression level of 'Shaoguifei' was lower than that of 'Xiariwuniang'. It is hypothesized that these LiNAC genes may be involved in the regulation of weeping traits in L. indica. The results of this study provide a basis for analyzing the functions of LiNAC genes and help to explore the molecular regulatory mechanisms related to the weeping traits in L. indica.

Integration of Transcriptome and Methylome Analyses Provides Insight Into the Pathway of Floral Scent Biosynthesis in Prunus mume

DNA methylation is a common epigenetic modification involved in regulating many biological processes. However, the epigenetic mechanisms involved in the formation of floral scent have rarely been reported within a famous traditional ornamental plant Prunus mume emitting pleasant fragrance in China. By combining whole-genome bisulfite sequencing and RNA-seq, we determined the global change in DNA methylation and expression levels of genes involved in the biosynthesis of floral scent in four different flowering stages of P. mume. During flowering, the methylation status in the “CHH” sequence context (with H representing A, T, or C) in the promoter regions of genes showed the most significant change. Enrichment analysis showed that the differentially methylated genes (DMGs) were widely involved in eight pathways known to be related to floral scent biosynthesis. As the key biosynthesis pathway of the dominant volatile fragrance of P. mume, the phenylpropane biosynthesis pathway contained the most differentially expressed genes (DEGs) and DMGs. We detected 97 DMGs participated in the most biosynthetic steps of the phenylpropane biosynthesis pathway. Furthermore, among the previously identified genes encoding key enzymes in the biosynthesis of the floral scent of P. mume, 47 candidate genes showed an expression pattern matching the release of floral fragrances and 22 of them were differentially methylated during flowering. Some of these DMGs may or have already been proven to play an important role in biosynthesis of the key floral scent components of P. mume, such as PmCFAT1a/1c, PmBEAT36/37, PmPAL2, PmPAAS3, PmBAR8/9/10, and PmCNL1/3/5/6/14/17/20. In conclusion, our results for the first time revealed that DNA methylation is widely involved in the biosynthesis of floral scent and may play critical roles in regulating the floral scent biosynthesis of P. mume. This study provided insights into floral scent metabolism for molecular breeding.

High-Altitude Genetic Selection and Genome-Wide Association Analysis of Yield-Related Traits in Elymus sibiricus L. Using SLAF Sequencing

The genetic adaptations to harsh climatic conditions in high altitudes and genetic basis of important agronomic traits are poorly understood in Elymus sibiricus L. In this study, an association population of 210 genotypes was used for population structure, selective sweep analysis, and genome-wide association study (GWAS) based on 88,506 single nucleotide polymorphisms (SNPs). We found 965 alleles under the natural selection of high altitude, which included 7 hub genes involved in the response to UV, and flavonoid and anthocyanin biosynthetic process based on the protein-protein interaction (PPI) analysis. Using a mixed linear model (MLM), the GWAS test identified a total of 1,825 significant loci associated with 12 agronomic traits. Based on the gene expression data of two wheat cultivars and the PPI analysis, we finally identified 12 hub genes. Especially, in plant height traits, the top hub gene (TOPLESS protein) encoding auxins and jasmonic acid signaling pathway, shoot apical meristem specification, and xylem and phloem pattern formation was highly overexpressed. These genes might play essential roles in controlling the growth and development of E. sibiricus. Therefore, this study provides fundamental insights relevant to hub genes and will benefit molecular breeding and improvement in E. sibiricus and other Elymus species.