Ornamental trees such as Chionanthus retusus enrich landscapes worldwide, valued for their abundant spring blossoms and symbolic beauty. In China, the species is widely cultivated and cherished for its resilience, longevity, and medicinal and cultural uses. Yet, its striking diversity in petal shapes has long puzzled breeders. Floral morphology is closely tied to vasculature development, but detailed genetic resources for C. retusus have been lacking. Earlier studies in other plants hinted that vascular patterning and auxin signaling strongly affect organ morphogenesis. Based on these challenges, there is a need to conduct in-depth research to reveal the molecular and genomic basis of petal shape formation.
Scientists from Shandong Agricultural University and collaborators have assembled the first telomere-to-telomere gap-free reference genome of Chionanthus retusus. Their findings, published (DOI: 10.1093/hr/uhae249) on September 3, 2024, in Horticulture Research, reveal how auxin-regulated gene networks shape petal vasculature and floral form. By analyzing varieties with flat, semi-closed, and spiral petals, the study identified CrAUX/IAA20 as a central regulator of vein strength and distribution, offering new insight into the molecular mechanisms behind petal diversity.
The team generated a gap-free genome assembly of C. retusus using PacBio HiFi, Oxford Nanopore, and Hi-C sequencing, achieving two complete haplotypes (687 and 683 Mb). Annotation predicted over 42,000 protein-coding genes, establishing the most complete Oleaceae genome to date. Comparative genomics showed that C. retusus underwent fewer whole-genome duplication events than close relatives, providing clues to its evolutionary trajectory.
Morphological and anatomical analyses of three floral varieties—'Xuezaohua' (flat petals), 'Xuedenglong' (semi-closed petals), and 'Xuexuan' (spiral petals)—revealed that petal shape diversity is primarily governed by differences in vascular patterning. Flat petals exhibited stronger and thicker veins, while curled or spiral petals had weaker, asymmetrical vasculature.
Transcriptome profiling across four flowering stages identified thousands of differentially expressed genes, with hormone signaling pathways, especially auxin response, strongly enriched. Among candidate regulators, CrAUX/IAA20 stood out: intron retention produced two transcript variants with distinct expression profiles, directly linked to petal vein traits. Overexpression of CrAUX/IAA20 in Arabidopsis thaliana reduced vein length and thickness, confirming its inhibitory effect on vascular development. These findings establish CrAUX/IAA20 as a pivotal genetic switch for floral morphology.
“This study represents a milestone for ornamental genomics,” said Professor Jihong Li, corresponding author of the paper. “By delivering the first telomere-to-telomere genome for Chionanthus retusus, we not only provide a foundational resource for Oleaceae research but also uncover how auxin signaling fine-tunes petal vasculature. Identifying CrAUX/IAA20 as a central regulator of flower shape bridges the gap between genome biology and horticultural practice. These insights will accelerate the molecular breeding of novel varieties with tailored ornamental traits, benefiting both science and the landscape industry.”
The availability of a gap-free reference genome and the discovery of CrAUX/IAA20 open new opportunities for breeding programs targeting petal traits in C. retusus and related Oleaceae species. By manipulating auxin-related pathways, breeders may be able to design flowers with desired shapes, enhancing ornamental appeal and market value. Beyond horticulture, the study contributes to fundamental plant biology by linking vasculature development to organ morphology. As landscape preferences shift and urban greening expands, genomic insights into ornamental plants will support more sustainable, diversified, and aesthetically pleasing ecosystems, while also providing a genetic blueprint for conserving biodiversity.
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References
DOI
10.1093/hr/uhae249
Original Source URL
https://doi.org/10.1093/hr/uhae249
Funding information
This work was supported by the Subject of Key R & D Plan of Shandong Province (Major Scientific and Technological Innovation Project) Mining, Accurate Identification of Forest Tree Germplasm Resources (No. 2021LZGC023) and Agricultural science and Technology Fund Project of Shandong province (No. 2019LY001-4).
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.