Abstract
The lotus (Nelumbonaceae: Nelumbo Adans.) is a highly desired ornamental plant, comprising only two extant species, the sacred lotus (N. nucifera Gaerten.) with red flowers and the American lotus (N. lutea Willd.) with yellow flowers. Flower color is the most obvious difference of two species. To better understand the mechanism of flower color differentiation, the content of anthocyanins and the expression levels of four key structural genes (e.g., DFR, ANS, UFGT and GST) were analyzed in two species. Our results revealed that anthocyanins were detected in red flowers, not yellow flowers. Expression analysis showed that no transcripts of GST gene and low expression level of three UFGT genes were detected in yellow flowers. In addition, three regulatory genes (NnMYB5, NnbHLH1 and NnTTG1) were isolated from red flowers and showed a high similarity to corresponding regulatory genes of other species. Sequence analysis of MYB5, bHLH1 and TTG1 in two species revealed striking differences in coding region and promoter region of MYB5 gene. Population analysis identified three MYB5 variants in Nelumbo: a functional allele existed in red flowers and two inactive forms existed in yellow flowers. This result revealed that there was an association between allelic variation in MYB5 gene and flower color difference. Yeast two-hybrid experiments showed that NnMYB5 interacts with NnbHLH1, NlbHLH1 and NnTTG1, and NnTTG1 also interacts with NnbHLH1 and NlbHLH1. The over-expression of NnMYB5 led to anthocyanin accumulation in immature seeds and flower stalks and up-regulation of expression of TT19 in Arabidopsis. Therefore, NnMYB5 is a transcription activator of anthocyanin synthesis. This study helps to elucidate the function of NnMYB5 and will contribute to clarify the mechanism of flower coloration and genetic engineering of flower color in lotus.
Highlights
In most species, flower color is attributed to the accumulation of anthocyanins (Quattrocchio et al, 2006)
To test whether there is a relationship between flower color difference of two species and regulatory genes, a total of 97 individuals from 16 wild lotus populations were further collected, including 49 individuals from eight populations of N. nucifera with red flower collected from China and Thailand and 48 individuals from eight populations of N. lutea with yellow flowers collected from United States of America (USA) (Table S1)
In order to study the regulatory gene involved in flower color, so in our subsequent analyses in N. nucifera, we only focused on the three regulatory genes (NnMYB5, NnbHLH1 and NnTTG1) expressed in red flower
Summary
Flower color is attributed to the accumulation of anthocyanins (Quattrocchio et al, 2006). Anthocyanin-regulating R2R3-MYB genes were isolated from many ornamental plants and economic crops, such as grape VvMybA1, tomato ANT1, apple MdMYB10, gerbera GMYB10, orchid OgMYB1, gentian GtMYB3, pear PyMYB10, lily LhMYB12, crabapple McMYB10 and peach PpMYB10.1 (Kobayashi et al, 2002; Mathews et al, 2003; Espley et al, 2007; Chiou & Yeh, 2008; Laitinen et al, 2008; Nakatsuka et al, 2008; Feng et al, 2010; Yamagishi, Yoshida & Nakayama, 2012; Tian et al, 2015; Tuan et al, 2015). Examples of the interaction between anthocyanin-related MYBs and bHLHs include Zea mays ZmC1 and ZmB, gerbera GMYC1 and GMYB10, Arabidopsis PAP1, PAP2, MYB113 & MYB114 and TT8, petunia AN2 and AN1, apple MdMYB10 and MdbHLH33, and litchi LcMYB1 and LcbHLH1, LcbHLH3 (Goff, Cone & Chandler, 1992; Elomaa et al, 2003; Zimmermann et al, 2004; Quattrocchio et al, 2006; Espley et al, 2007; Lai et al, 2016)
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