Identification of Chlorophyll Metabolism- and Photosynthesis-Related Genes Regulating Green Flower Color in Chrysanthemum by Integrative Transcriptome and Weighted Correlation Network Analyses

Integrative analysis of transcriptome and target metabolites uncovering flavonoid biosynthesis regulation of changing petal colors in Nymphaea 'Feitian 2'

BACKGROUND

Nymphaea (waterlily) is known for its rich colors and role as an important aquatic ornamental plant globally. Nymphaea atrans and some hybrids, including N. 'Feitian 2,' are more appealing due to the gradual color change of their petals at different flower developmental stages. The petals of N. 'Feitian 2' gradually change color from light blue-purple to deep rose-red throughout flowering. The mechanism of the phenomenon remains unclear.

RESULTS

In this work, flavonoids in the petals of N. 'Feitian 2' at six flowering stages were examined to identify the influence of flavonoid components on flower color changes. Additionally, six cDNA libraries of N. 'Feitian 2' over two blooming stages were developed, and the transcriptome was sequenced to identify the molecular mechanism governing petal color changes. As a result, 18 flavonoid metabolites were identified, including five anthocyanins and 13 flavonols. Anthocyanin accumulation during flower development is the primary driver of petal color change. A total of 12 differentially expressed genes (DEGs) in the flavonoid biosynthesis pathway were uncovered, and these DEGs were significantly positively correlated with anthocyanin accumulation. Six structural genes were ultimately focused on, as their expression levels varied significantly across different flowering stages. Moreover, 104 differentially expressed transcription factors (TFs) were uncovered, and three MYBs associated with flavonoid biosynthesis were screened. The RT-qPCR results were generally aligned with high-throughput sequencing results.

CONCLUSIONS

This research offers a foundation to clarify the mechanisms underlying changes in the petal color of waterlilies.

Expression of RsPORB Is Associated with Radish Root Color

Radish (Raphanus sativus) plants exhibit varied root colors due to the accumulation of chlorophylls and anthocyanins compounds that are beneficial for both human health and visual quality. The mechanisms of chlorophyll biosynthesis have been extensively studied in foliar tissues but remain largely unknown in other tissues. In this study, we examined the role of NADPH:protochlorophyllide oxidoreductases (PORs), which are key enzymes in chlorophyll biosynthesis, in radish roots. The transcript level of RsPORB was abundantly expressed in green roots and positively correlated with chlorophyll content in radish roots. Sequences of the RsPORB coding region were identical between white (948) and green (847) radish breeding lines. Additionally, virus-induced gene silencing assay with RsPORB exhibited reduced chlorophyll contents, verifying that RsPORB is a functional enzyme for chlorophyll biosynthesis. Sequence comparison of RsPORB promoters from white and green radishes showed several insertions and deletions (InDels) and single-nucleotide polymorphisms. Promoter activation assays using radish root protoplasts verified that InDels of the RsPORB promoter contribute to its expression level. These results suggested that RsPORB is one of the key genes underlying chlorophyll biosynthesis and green coloration in non-foliar tissues, such as roots.

The genomic and bulked segregant analysis of Curcuma alismatifolia revealed its diverse bract pigmentation

Compared with most flowers where the showy part comprises specialized leaves (petals) directly subtending the reproductive structures, most Zingiberaceae species produce showy "flowers" through modifications of leaves (bracts) subtending the true flowers throughout an inflorescence. Curcuma alismatifolia, belonging to the Zingiberaceae family, a plant species originating from Southeast Asia, has become increasingly popular in the flower market worldwide because of its varied and esthetically pleasing bracts produced in different cultivars. Here, we present the chromosome-scale genome assembly of C. alismatifolia "Chiang Mai Pink" and explore the underlying mechanisms of bract pigmentation. Comparative genomic analysis revealed C. alismatifolia contains a residual signal of whole-genome duplication. Duplicated genes, including pigment-related genes, exhibit functional and structural differentiation resulting in diverse bract colors among C. alismatifolia cultivars. In addition, we identified the key genes that produce different colored bracts in C. alismatifolia, such as F3'5'H, DFR, ANS and several transcription factors for anthocyanin synthesis, as well as chlH and CAO in the chlorophyll synthesis pathway by conducting transcriptomic analysis, bulked segregant analysis using both DNA and RNA data, and population genomic analysis. This work provides data for understanding the mechanism of bract pigmentation and will accelerate breeding in developing novel cultivars with richly colored bracts in C. alismatifolia and related species. It is also important to understand the variation in the evolution of the Zingiberaceae family.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-022-00081-6.

Arabidopsis transcription factor TCP4 represses chlorophyll biosynthesis to prevent petal greening

Green petals pose a challenge for pollinators to distinguish flowers from leaves, but they are valuable as a specialty flower trait. However, little is understood about the molecular mechanisms that underlie the development of green petals. Here, we report that CINCINNATA (CIN)-like TEOSINTE BRANCHED 1/CYCLOIDEA/PCF (TCP) proteins play key roles in the control of petal color. The septuple tcp2/3/4/5/10/13/17 mutant produced flowers with green petals due to chlorophyll accumulation. Expression of TCP4 complemented the petal phenotype of tcp2/3/4/5/10/13/17. We found that chloroplasts were converted into leucoplasts in the distal parts of wild-type petals but not in the proximal parts during flower development, whereas plastid conversion was compromised in the distal parts of tcp2/3/4/5/10/13/17 petals. TCP4 and most CIN-like TCPs were predominantly expressed in distal petal regions, consistent with the green-white pattern in wild-type petals and the petal greening observed in the distal parts of tcp2/3/4/5/10/13/17 petals. RNA-sequencing data revealed that most chlorophyll biosynthesis genes were downregulated in the white distal parts of wild-type petals, but these genes had elevated expression in the distal green parts of tcp2/3/4/5/10/13/17 petals and the green proximal parts of wild-type petals. We revealed that TCP4 repressed chlorophyll biosynthesis by directly binding to the promoters of PROTOCHLOROPHYLLIDE REDUCTASE (PORB), DIVINYL REDUCTASE (DVR), and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1), which are known to promote petal greening. We found that the conversion of chloroplasts to leucoplasts and the green coloration in the proximal parts of petals appeared to be conserved among plant species. Our findings uncover a major molecular mechanism that underpins the formation of petal color patterns and provide a foundation for the breeding of plants with green flowers.

Important Roles of Key Genes and Transcription Factors in Flower Color Differences of Nicotianaalata

Nicotiana alata is an ornamental horticultural plant with a variety of flower colors and a long flowering period. The genes in four different colored N. alata (white, purple, red, and lemon green) were analyzed to explain the differences in flower color using transcriptomes. A total of 32 differential expression genes in the chlorophyll biosynthesis pathway and 41 in the anthocyanin biosynthesis pathway were identified. The enrichment analysis showed that the chlorophyll biosynthesis pathway and anthocyanin biosynthesis pathway play critical roles in the color differences of N. alata. The HEMA of the chlorophyll biosynthesis pathway was up-regulated in lemon green flowers. Compared with white flowers, in the red and purple flowers, F3H, F3'5'H and DFR were significantly up-regulated, while FLS was significantly down-regulated. Seventeen differential expression genes homologous to transcription factor coding genes were obtained, and the homologues of HY5, MYB12, AN1 and AN4 were also involved in flower color differences. The discovery of these candidate genes related to flower color differences is significant for further research on the flower colors formation mechanism and color improvements of N. alata.

CmNAC73 Mediates the Formation of Green Color in Chrysanthemum Flowers by Directly Activating the Expression of Chlorophyll Biosynthesis Genes HEMA1 and CRD1

Chrysanthemum is one of the most beautiful and popular flowers in the world, and the flower color is an important ornamental trait of chrysanthemum. Compared with other flower colors, green flowers are relatively rare. The formation of green flower color is attributed to the accumulation of chlorophyll; however, the regulatory mechanism of chlorophyll metabolism in chrysanthemum with green flowers remains largely unknown. In this study, we performed Illumina RNA sequencing on three chrysanthemum materials, Chrysanthemum vestitum and Chrysanthemum morifolium cultivars ‘Chunxiao’ and ‘Green anna’, which produce white, light green and dark green flowers, respectively. Based on the results of comparative transcriptome analysis, a gene encoding a novel NAC family transcription factor, CmNAC73, was found to be highly correlated to chlorophyll accumulation in the outer whorl of ray florets in chrysanthemum. The results of transient overexpression in chrysanthemum leaves showed that CmNAC73 acts as a positive regulator of chlorophyll biosynthesis. Furthermore, transactivation and yeast one-hybrid assays indicated that CmNAC73 directly binds to the promoters of chlorophyll synthesis-related genes HEMA1 and CRD1. Thus, this study uncovers the transcriptional regulation of chlorophyll synthesis-related genes HEMA1 and CRD1 by CmNAC73 and provides new insights into the development of green flower color in chrysanthemum and chlorophyll metabolism in plants.