Background: Plants have remarkable diversity in petal colour through the biosynthesis and accumulation of various pigments. To better understand the mechanisms regulating petal pigmentation inLonicera japonica, we used multiple approaches to investigate the changes in carotenoids, anthocyanins, endogenous hormones and gene expression dynamics during petal colour transitions, i.e., green bud petals (GB_Pe), white flower petals (WF_Pe) and yellow flower petals (YF_Pe).Results: Metabolome analysis showed that YF_Pe contained a much higher content of carotenoids than GB_Pe and WF_Pe, with α-carotene, zeaxanthin, violaxanthin and γ-carotene identified as the major carotenoid compounds in YF_Pe. Comparative transcriptome analysis revealed that the key differentially expressed genes (DEGs) involved in carotenoid biosynthesis, such asphytoene synthase,phytoene desaturaseandζ-carotene desaturase, were significantly upregulated in YF_Pe. The results indicated that upregulated carotenoid concentrations and carotenoid biosynthesis-related genes predominantly promote colour transition. Meanwhile, two anthocyanins (pelargonidin and cyanidin) were significantly increased in YF_Pe, and the expression level of ananthocyanidin synthasegene was significantly upregulated, suggesting that anthocyanins may contribute to vivid yellow colour in YF_Pe. Furthermore, analyses of changes in indoleacetic acid, zeatin riboside, gibberellic acid, brassinosteroid (BR), methyl jasmonate and abscisic acid (ABA) levels indicated that colour transitions are regulated by endogenous hormones. The DEGs involved in the auxin, cytokinin, gibberellin, BR, jasmonic acid and ABA signalling pathways were enriched and associated with petal colour transitions.Conclusion: Our results provide global insight into the pigment accumulation and the regulatory mechanisms underlying petal colour transitions during the flower development process inL. japonica.

The WRKY gene family, which is one of the largest transcription factor (TF) families, plays an important role in numerous aspects of plant growth and development, especially in various stress responses. However, the functional roles of the WRKY gene family in loquat are relatively unknown. In this study, a novel WRKY gene, EjWRKY17, was characterized from Eriobotrya japonica, which was significantly upregulated in leaves by melatonin treatment during drought stress. The EjWRKY17 protein, belonging to group II of the WRKY family, was localized in the nucleus. The results indicated that overexpression of EjWRKY17 increased cotyledon greening and root elongation in transgenic Arabidopsis lines under abscisic acid (ABA) treatment. Meanwhile, overexpression of EjWRKY17 led to enhanced drought tolerance in transgenic lines, which was supported by the lower water loss, limited electrolyte leakage, and lower levels of reactive oxygen species (ROS) and malondialdehyde (MDA). Further investigations showed that overexpression of EjWRKY17 promoted ABA-mediated stomatal closure and remarkably up-regulated ABA biosynthesis and stress-related gene expression in transgenic lines under drought stress. Overall, our findings reveal that EjWRKY17 possibly acts as a positive regulator in ABA-regulated drought tolerance.

Wild loquats (Eriobotrya japonica Lindl.) provide remarkable genetic resources for studying domestication and breeding improved varieties. Herein, we generate the first high-quality chromosome-level genome assembly of wild loquat, with the 45,791 predicted protein-coding genes. Analysis of comparative genomics indicated that loquat shared a common ancestor with apple and pear, and a recent whole-genome duplication event occurred in loquat prior to its divergence. Genome re-sequencing showed that the loquat germplasms were distinctly classified into wild and cultivated groups, and the commercial cultivars experienced allelic admixture. Compared with the cultivated loquats, the wild loquat genome showed very few selected genomic regions and had higher levels of genetic diversity. However, whole-genome scans of selective sweeps were mainly related to fruit quality, size, and flesh color during the domestication process. Large-scale transcriptome and metabolome analyses were further performed to identify the differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) in the wild and cultivated loquats, at various fruit development stages. Unlike that in wild loquat, the key DEGs and DAMs involved in carbohydrate metabolism, plant hormone signal transduction, flavonoid biosynthesis, and carotenoid biosynthesis were significantly regulated in cultivated loquats during fruit development. These high-quality reference genome, re-sequencing, and large-scale transcriptome/metabolome data provide valuable resources for elucidating fruit domestication and molecular breeding in loquat.

Flower development is a vital developmental process in the life cycle of woody perennials, especially fruit trees. Herein, we used transcriptomic, proteomic, and hormone analyses to investigate the key candidate genes/proteins in loquat (Eriobotrya japonica) at the stages of flower bud differentiation (FBD), floral bud elongation (FBE), and floral anthesis (FA). Comparative transcriptome analysis showed that differentially expressed genes (DEGs) were mainly enriched in metabolic pathways of hormone signal transduction and starch and sucrose metabolism. Importantly, the DEGs of hormone signal transduction were significantly involved in the signaling pathways of auxin, gibberellins (GAs), cytokinin, ethylene, abscisic acid (ABA), jasmonic acid, and salicylic acid. Meanwhile, key floral integrator genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) and floral meristem identity genes SQUAMOSA PROMOTER BINDING LIKE (SPL), LEAFY (LFY), APETALA1 (AP1), and AP2 were significantly upregulated at the FBD stage. However, key floral organ identity genes AGAMOUS (AG), AP3, and PISTILLATA (PI) were significantly upregulated at the stages of FBE and FA. Furthermore, transcription factors (TFs) such as bHLH (basic helix-loop-helix), NAC (no apical meristem (NAM), Arabidopsis transcription activation factor (ATAF1/2) and cup-shaped cotyledon (CUC2)), MYB_related (myeloblastosis_related), ERF (ethylene response factor), and C2H2 (cysteine-2/histidine-2) were also significantly differentially expressed. Accordingly, comparative proteomic analysis of differentially accumulated proteins (DAPs) and combined enrichment of DEGs and DAPs showed that starch and sucrose metabolism was also significantly enriched. Concentrations of GA₃ and zeatin were high before the FA stage, but ABA concentration remained high at the FA stage. Our results provide abundant sequence resources for clarifying the underlying mechanisms of the flower development in loquat.