FLOWER DEVELOPMENT IN GERBERA HYBRIDA (ASTERACEAE)

Abstract

As a genetically determined structure, flower is an attractive object for developmental studies in plants. Flower development provides a good system for understanding cell differentiation and genetic mechanisms needed for organogenesis. The current molecular view on flower development has been based on studies on relatively few model species, like Arabidopsis and Antirrhinum. Research in model species, for which classical genetics is available, remains to be an important approach although it is evident that a large proportion of biological phenomena is missing from the range of variation in these species. Therefore it is important to study molecular processes behind the flower development in other species as well. Our approach for studying the central developmental phenomena is to use Gerbera hybrida as an experimental system. Gerbera is a member of plant family Asteraceae, which is one of the largest families in angiosperms, with over 20 000 species world wide. The Asteraceae is characterized by composite inflorescence, the capitulum, that is structurally highly adapted to insect mediated pollination. One of the major advantages of using gerbera as a model for Asteraceae is the ability to genetically transform this species. In this study, functional analysis of the general regulation of various aspects of flower development and organ differentation has been presented. By using the regulatory genes from the MADS box family, it has been shown that the basic features of flower organ determination deduced from the model species are also present in gerbera. The characterization of genes which participate in B and C functions allowed us to use them as instruments to study the flower characteristics that are typical to Asteraceae. By using this approach it could be concluded that pappus bristles are true sepals, that B and C function genes are not participating in feminization of marginal flowers and that abortion of stamens in the marginal female flowers depends on their identity. Furthermore, genetic down regulation of expression of GRCD1, a member of a large AGL2-like MADS box gene family, revealed that it participates in determination of stamen identity during flower development in gerbera. The development of petals, like other floral organs, is determined by genetic factors. The simple internal structure of petals and their relatively large size in gerbera ray flowers, make them a good model for studying plant organogenesis. In this study, two different strategies were chosen. First we isolated and analysed genes that were abundantly expressed in petals and secondly we attempted to to isolate genes with differential expression patterns within the petal. In addition to petals, all the clones analysed were expressed also in other floral organ. The most prominent class of genes was the one that was expressed abundantly during the development of petals and carpels. Detailed analysis of expression patterns of seven genes within the petal showed that their expression followed a basipetal pattern, thus first signal can be seen in the distal region of the petal with expression proceeding towards the proximal part of the petal. One of the most abundantly expressed genes during petal development is a gerbera lipid transfer protein (GLTP1). It’s expression is petal and carpel specific and proceeds basipetally during petal development. Another abundantly expressed gene during petal development is gerbera homolog of GAST1, GEG. GEG expression was detected in petals and carpels, with expression temporally correlating with the cessation of longitudinal cell expansion. In plants constitutively expressing GEG, reduced petal lengths and carpels with shortened and radially expanded stylar parts were found. Epidermal cells of both corolla and carpel are reduced in length. Radial expansion of the epidermal cells of carpel was also observed. Taken together these observations indicate that GEG participate in the regulation of cell and organ shape during petal and carpel development in gerbera. abstract