Petal power unveiled: genome breakthrough reveals secrets behind flower shape

Researchers have decoded the complete genome of Chionanthus retusus, a widely cultivated ornamental tree known for its striking flower shapes. Using state-of-the-art sequencing, scientists identified a key gene, CrAUX/IAA20, that regulates petal morphology by modulating the development of petal vasculature. The study found that different expression patterns of CrAUX/IAA20 transcripts lead to variations in petal vein strength, resulting in distinct flower shapes such as flat, spiral, or inward-curved petals. This work not only provides an invaluable genomic resource but also uncovers a molecular mechanism linking auxin signaling to floral aesthetics, offering new possibilities for ornamental plant breeding.

Chionanthus retusus, also called "April Snow" in China, is cherished for its dense spring blooms and diverse floral forms. Its flowers and leaves are also used in traditional Chinese tea and medicine. However, the genetic mechanisms governing its petal shape diversity have remained largely unknown due to the lack of a reference genome. In flowering plants, petal morphology is closely linked to the development of vascular tissues, yet few studies have explored how vascular patterning affects petal shape. Moreover, the role of auxin signaling—a key regulator of plant organ development—in petal vasculature remains under-investigated. Due to these challenges, an in-depth study of C. retusus genetics and flower development is urgently needed.

A research team from Shandong Agricultural University and collaborators has published a new study (DOI: 10.1093/hr/uhae249) on September 3, 2024, in Horticulture Research. The researchers assembled the first telomere-to-telomere (T2T) gap-free genome of C. retusus and used it to explore the molecular mechanisms underlying petal shape diversity. By integrating transcriptome data and functional analysis, they identified CrAUX/IAA20 as a key gene controlling petal vasculature strength and morphology, offering insights into auxin-responsive regulatory networks that shape flower architecture.

To understand the basis of petal shape diversity, researchers focused on three C. retusus varieties with distinct flower forms: XZH (flat petals), XDL (inward-curved petals), and XX (spiral petals). Genome assembly using PacBio HiFi and ONT reads produced a complete reference with 42,864 annotated genes. Comparative morphological and histological analyses revealed that petal shape differences stem from the strength and symmetry of petal vasculature. Flat-petaled XZH showed more robust and symmetrical vein patterns than the other two varieties.

Transcriptome analysis across four flower developmental stages identified differentially expressed auxin-related genes, especially CrAUX/IAA20. The gene produces two transcripts—long (CrAUX/IAA20-L) and short (CrAUX/IAA20-S)—depending on intron retention. The long transcript, predominantly expressed in flowers with curled petals, suppresses vascular development. Mutation in the promoter’s auxin response element of the flat-petaled variety renders it less responsive to auxin, lowering CrAUX/IAA20-L expression. Overexpression experiments in Arabidopsis confirmed that both transcripts reduce vein number and length, with CrAUX/IAA20-L showing stronger effects. This gene acts as a brake on auxin-mediated vascular development, directly impacting petal structure.

“Our work provides the first direct link between petal vasculature patterning and flower shape diversity through auxin signaling,” said Dr. Xiaojiao Han, corresponding author of the study. “By pinpointing how CrAUX/IAA20 modulates the balance between auxin responsiveness and vein formation, we’ve uncovered a regulatory switch that shapes the floral identity of C. retusus. This genome and mechanistic insight open new avenues for ornamental plant breeding and evolutionary developmental biology.”

This high-quality T2T reference genome lays the foundation for molecular breeding in C. retusus, allowing precise selection of traits linked to floral aesthetics. Understanding how CrAUX/IAA20 controls petal vein architecture offers a novel target for manipulating flower shape in breeding programs. The insights into auxin-regulated vasculature development may also apply to other ornamental or crop species, aiding efforts in landscape design, commercial floriculture, and even functional studies of lateral organ morphogenesis. Future applications could include genome editing or promoter engineering to create designer floral patterns suited to specific horticultural demands.

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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.