The influence of R2R3-MYB transcription factors in Campanula medium and Petunia hybrida

Abstract

Flower color is one of the most important traits for ornamental plants. Anthocyanin is the main pigment in flowers and the vegetative tissue in most higher plants. Both wild campanulas and petunias have blue/purple flowers because of the delphinidin-based anthocyanins. However, most of the flower color in the genus Campanula is mostly blue, rarely white, and a very few species are pink, while modern commercial petunias are known for their wide range of flower color in different patterns. Many kinds of research have proven that the R2R3-MYB transcription factors are responsible for regulating anthocyanin biosynthesis in different tissues. In Petunia spec., ANTHOCYANIN-2 (AN2) is considered to be the major contributor to coloration in the petal limb, while several other paralogs are involved in other tissues or stress responses. Unlike petunias, which have a convenient system for regeneration and a stable genetic plant transformation, there are no related reports for C. medium. Therefore, an efficient protocol for regeneration and Agrobacterium‑mediated transformation based on leaf disk by using GFP as the reporter gene for C. medium was established in the first part. In the second chapter, two strategies were designed to change the anthocyanin pattern in C. medium. The first is the redirection of delphinidin-based anthocyanins to pelargonidin-based anthocyanins in a blue cultivar. The second strategy was to improve the pigmentation in a pink-flowering variety of C. medium by overexpression of an R2R3-MYB transcription factor (PhAN2). In the third chapter, the same AN2 overexpression vectors used for Campanula and another flowerspecific guided AN2 constructs were also transferred into an an2- genetic petunia cultivar. The results indicated that overexpression of AN2 leads to various physiological changes in addition to increased pigmentation. The mechanism of the flower limb coloration of phenotypically "darkly-veined"petunias, and the roles of different R2R3-MYB genes were characterized in the last part. Overall, several conclusions obtained in this dissertation: (i) genetic transformation of C. medium is available; (ii) PhAN2 and PhAN4 can activate anthocyanin biosynthesis in several tissues of Campanula and Petunia and the role of PhAN2 in plant growth physiology may be diverse; (iii) the restoration of anthocyanins in the petals of commercial "dark-veined" petunias from a whiteflowered ancestor is achieved by up-regulation of AN4 as part of a long-term breeding process by humans rather than by natural selection.