Abstract
Chinese narcissus (Narcissus tazetta var. chinensis) is one of the ten traditional flowers in China and a famous bulb flower in the world flower market. However, only white color tepals are formed in mature flowers of the cultivated varieties, which constrains their applicable occasions. Unfortunately, for lack of genome information of narcissus species, the explanation of tepal color formation of Chinese narcissus is still not clear. Concerning no genome information, the application of transcriptome profile to dissect biological phenomena in plants was reported to be effective. As known, pigments are metabolites of related metabolic pathways, which dominantly decide flower color. In this study, transcriptome profile and pigment metabolite analysis methods were used in the most widely cultivated Chinese narcissus “Jinzhanyintai” to discover the structure of pigment metabolic pathways and their contributions to white tepal color formation during flower development and pigmentation processes. By using comparative KEGG pathway enrichment analysis, three pathways related to flavonoid, carotenoid and chlorophyll pigment metabolism showed significant variations. The structure of flavonoids metabolic pathway was depicted, but, due to the lack of F3ʹ5ʹH gene; the decreased expression of C4H, CHS and ANS genes; and the high expression of FLS gene, the effect of this pathway to synthesize functional anthocyanins in tepals was weak. Similarly, the expression of DXS, MCT and PSY genes in carotenoids synthesis sub-pathway was decreased, while CCD1/CCD4 genes in carotenoids degradation sub-pathway was increased; therefore, the effect of carotenoids metabolic pathway to synthesize adequate color pigments in tepals is restricted. Interestingly, genes in chlorophyll synthesis sub-pathway displayed uniform down-regulated expression, while genes in heme formation and chlorophyll breakdown sub-pathways displayed up-regulated expression, which also indicates negative regulation of chlorophyll formation. Further, content change trends of various color metabolites detected by HPLC in tepals are consistent with the additive gene expression patterns in each pathway. Therefore, all three pathways exhibit negative control of color pigments synthesis in tepals, finally resulting in the formation of white tepals. Interestingly, the content of chlorophyll was more than 10-fold higher than flavonoids and carotenoids metabolites, which indicates that chlorophyll metabolic pathway may play the major role in deciding tepal color formation of Chinese narcissus.
Highlights
There are many different colored flowers in the world
As T2 to T5 stages show the most significant tepal color changes, they should cover the most important regulations that result in white mature tepals; tepals at these four color stages were collected for subsequent RNA-seq and pigment metabolites analysis
The expression of CCD7 and CCD8 genes always showed extremely low level (Figure 8). These results indicate that CCD1 and CCD4 should play the major role in carotenoids degradation/cleavage sub-pathway
Summary
There are many different colored flowers in the world. Color make flowers attractive and facilitates the reproduction of flowering plants. Anthocyanins, as the chief flavonoid pigments, confer a diverse range of color from orange, red to violet and blue. Other flavonoids such as flavones and flavonols cover the range from ivory white to pale yellow or invisible to human beings, act as accessory pigments [11]. Studies discovered that the concentration of pigments [17], the mixed effect of diverse pigments [18], the chelation effect of pigments with metal ions [19,20], the pH value of vacuoles [21], and other physical (such as light, temperature and humidity) or chemical (such as vacuolar ion species) factors have effects on the output of flower color [22]; the composition of pigments in flower is the most important determinant [22]. White color can be the result of various factors [30,31], and the systematic explanation of white flower formation in plants is still fragmentary, the color innovation of this kind of plants is still difficult
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