Zhaoyu Gu’s research while affiliated with China Agricultural University and other places

Temporal decline in microRNA miR156 expression is crucial for the transition to, and maintenance of, the adult phase and flowering competence in flowering plants. However, the molecular mechanisms underlying the temporal regulation of miR156 reduction remain largely unknown. Here, we investigated the epigenetic mechanism regulating the temporal silencing of cin‐MIR156 in wild chrysanthemum (Chrysanthemum indicum), focusing on the role of the lysine‐specific demethylase CiLDL1 and the nuclear factor Y complex. CiLDL1 and CiNF‐YB8 interact with the classical histone‐like fold domain (HFD) of CiNF‐YC1 and CiNF‐YA3, which form distinct heterotrimers binding to the ‘CCAAT’ box in the promoter region of cin‐MIR156ab. CiLDL1 and CiNF‐YB8 have opposing effects on cin‐MIR156ab expression, with influencing histone 3 lysine 4 demethylation (H3K4me2) levels at the cin‐MIR156ab locus. During aging, decreased CiNF‐YB8 expression leads to a quantitative switch from the CiNF‐YA3–CiNF‐YC1–CiNF‐YB8 heterotrimer to the CiNF‐YA3–CiNF‐YC1–CiLDL1 heterotrimer, which reduces H3K4me2 levels at the cin‐MIR156ab locus, thus temporal silencing its expression. Our results thus reveal that the dynamic regulatory shift between CiLDL1 and CiNF‐YB8 ensures proper aging‐dependent flowering in chrysanthemum.

The gibberellins (GA) receptor GA INSENSITIVE DWARF1 (GID1) plays a central role in GA signal perception and transduction. The typical photoperiodic plant chrysanthemum (Chrysanthemum morifolium) only flowers when grown in short-day photoperiods. In addition, chrysanthemum flowering is also controlled by the aging pathway, but whether and how GAs participate in photoperiod- and age-dependent regulation of flowering remain unknown. Here, we demonstrate that photoperiod affects CmGID1B expression in response to GAs and developmental age. Moreover, we identified PHOTOLYASE/BLUE LIGHT RECEPTOR2 (PHR2), an atypical photocleavage synthase, as a CRYPTOCHROME-INTERACTING bHLH1 (CIB1) interactor with which it forms a complex in response to short days to activate CmGID1B transcription. Knocking down CmGID1B raised endogenous bioactive GA contents and GA signal perception, in turn modulating the expression of the aging-related genes MicroRNA156 (miR156) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE3 (SPL3). We propose that exposure to short days accelerates the juvenile-to-adult transition by increasing endogenous GA contents and response to GAs, leading to entry into floral transformation.

Chrysanthemum is one of the four major cut flowers in the world, with high ornamental and economic value. Fragrance is an important ornamental character of chrysanthemum flowers, especially those consumed as tea and other foods, and the flower fragrance is the major determinant of the commercial value of chrysanthemum cultivars. Currently, however, the research on chrysanthemum flower fragrance is mainly focused on the composition and content of fragrant compounds, and a clear classification of fragrance types is lacking. Here, we divided chrysanthemum fragrance into six categories based on sensory evaluation and determined the identity and content of fragrant compounds of chrysanthemum accessions representative of each fragrance type by GC-MS. In addition, we analyzed the conserved aromatic substances responsible for the fruity fragrance type chrysanthemum with multi-functional development potential, providing a theoretical basis for creating new chrysanthemum germplasm with specific fragrance types. The results of this study can accelerate the breeding process of chrysanthemum accessions with new fragrance types.

Chrysanthemum morifolium is one of the four major cut flowers in the world and has important ornamental and economic value. Flower fragrance is an important ornamental character of chrysanthemum, especially for tea and edible chrysanthemum, and the excellent fragrance determines its commercial value. At present, however, chrysanthemum is mostly chrysanthemum fragrance and medicinal fragrance, and the research on chrysanthemum flower fragrance mainly focuses on the composition and content of fragrancetic substances, and there is no clear classification of fragrance types. Here, we divided chrysanthemum fragrance into six categories by sensory evaluation, and detected the fragrance components and contents of every representative chrysanthemum with different fragrance types by GC-MS. In addition, we analyzed the genetic conservative substances of fruity fragrance chrysanthemum with multi-functional development potential, which provided a theoretical basis for creating new chrysanthemum germplasm with specific fragrance types, which was conducive to accelerating the biological breeding process of chrysanthemum with new fragrance types.

Strawflower (Helichrysum bracteatum) capitula have papery bracts and thus have the qualities of a naturally dried flower. The involucral bract cells have a secondary cell wall (SCW) of which a crucial component is lignin. Although the constituents of SCWs have been studied extensively in plants, little is known of the mechanism regulating SCW formation, especially lignin biosynthesis in the involucral bracts of strawflower. In this study, a homolog of NAC SECONDARY WALL THICKENING PROMOTING FACTOR 1, designated HbNST1, was identified as a positive regulator of lignin biosynthesis in strawflower. The transcript level of HbNST1 was the highest in the involucral bracts. Subcellular localization analysis indicated that HbNST1 was localized to the nucleus. Overexpression of HbNST1 in Chrysanthemum indicum promoted the expression of a gene related to lignin biosynthesis, a homolog of cinnamyl alcohol dehydrogenase, designated CiCAD, suggesting that HbNST1 was associated with the accumulation of lignin in the SCW of the involucral bracts. Taken together, the results suggested that HbNST1 positively regulated lignin accumulation in the involucral bracts and mediated the expression of lignin biosynthesis-related genes in strawflower.

Chrysanthemum (Chrysanthemum morifolium) is well known as a photoperiod‐sensitive flowering plant. However, it has also evolved into a temperature‐sensitive ecotype. Low temperature can promote the floral transition of the temperature‐sensitive ecotype, but little is known about the underlying molecular mechanisms. Here, we identified MADS AFFECTING FLOWERING 2 (CmMAF2), a putative MADS‐box gene, which induces floral transition in response to low temperatures independent of day length conditions in this ecotype. CmMAF2 was shown to bind to the promoter of the GA biosynthesis gene CmGA20ox1 and to directly regulate the biosynthesis of bioactive GA1 and GA4. The elevated bioactive GA levels activated LEAFY (CmLFY) expression, ultimately initiating floral transition. In addition, CmMAF2 expression in response to low temperatures was directly activated by CmC3H1, a CCCH‐type zinc‐finger protein upstream. In summary, our results reveal that the CmC3H1–CmMAF2 module regulates flowering time in response to low temperatures by regulating GA biosynthesis in the temperature‐sensitive chrysanthemum ecotype.

The carotenoids biosynthesis pathway in plants has been studied extensively, yet little is known about the regulatory mechanisms underlying this process, especially for ornamental horticulture plants. In this study, a natural variation of chrysanthemum with orange coloration was identified and compared with the wild type with pink coloration; the content and component of carotenoids were largely enriched in the mutant with orange coloration. CmCCD4a-5, the DNA sequence in both ‘Pink yan’ and the mutant, was identified and shown to function as a carotenoid degradation enzyme. Compared with ‘Pink yan’, the mutant shows lower expression level of CmCCD4a-5. Furthermore, CmGATA4 was found to have an opposite expression trend to CmCCD4a-5, and it could directly bind with the CmCCD4a-5 promoter. Taken together, this study demonstrates that CmGATA4 acts as a negative regulator of CmCCD4a-5 and, furthermore, low expression of CmCCD4a-5 resulted in carotenoid accumulation in the mutant.

Volatile composition is an important feature that determines flavor, which actively affects the overall evaluation of chrysanthemum tea. In this study, HS-GC-IMS (headspace-gas chromatography-ion mobility spectrometry) was performed to characterize the volatile profiles of different chrysanthemum tea subtypes. Forty-seven volatiles of diverse chemical nature were identified and quantified. Partial least squares discriminant analysis (PLS-DA) revealed that four chrysanthemum teas were distinct from each other based on their volatile compounds. Furthermore, this work provides reference methods for detecting novel volatile organic compounds in chrysanthemum tea plants and products.