MADS AND GROWTH · Regulation of plant growth by MADS box organ identity genes

Regulation of plant growth by MADS box organ identity genes

Project context and objectives The overall aim of the project was to investigate the molecular regulation underlying floral organ-size specification at late stages of flower development. Flowers are at the basis of fruit and seed formation, which give rise to the most important food and feed sources. Hence, knowledge about the mechanisms controlling organ growth will be directly applicable to optimising the yield potential of economically important food and feed crops. Strong evidence exists that the specification of floral organ identity by members of the so-called MADS box transcription factor family is tightly linked to floral organ-size determination. Previous pilot experiments in our laboratory suggested an important role for TCP class transcription factors in this process, because TCP genes were identified as direct targets of MADS domain transcription factor proteins and the encoded TCP proteins appeared to have the capacity to interact physically with MADS proteins. Furthermore, analyses of plants from various species, in which TCP activity is modified, revealed the role of TCP proteins in organ growth. The objectives of this project were to analyse the functions of selected MADS and TCP transcription factors in the control of floral organ growth and to decipher the underlying molecular mechanisms. Work performed In order to get insight into the timing and patterning of TCP transcription factor activity during Arabidopsis flower development, green fluorescent protein (GFP) tagging was applied to a selected set of TCP protein encoding genes (TCP4, TCP5, TCP9, TCP10, TCP13, TCP19, TCP20, and TCP21). Subsequently, the expression patterns were determined by confocal microscopy. Remarkably, some of the TCP genes, such as TCP20, were broadly expressed in all floral organs and at various time points during development. In contrast, others, e.g. TCP19, were only expressed in a limited number of cells at defined developmental stages, suggesting a broad functional diversification within the family. For a few selected TCP-GFP fusions, the expression was analysed in homeotic mutant backgrounds in order to link expression patterns with floral organ identity and growth. To study the function of TCP and MADS domain transcription factors we generated specific transgenic lines and investigated the effects of temporal and spatial over-expression, as well as the knock-down of genes of interest. For this purpose tissue specific promoters, such as the L1-layer specific ATML1 promoter, and floral whorl specific promoters, e.g. the AP3 promoter, have been used. Based on these experiments we could uncouple the floral organ identity specifying function from the growth regulating function for selected MADS domain transcription factors. Furthermore, a clear effect on organ size could be obtained upon ectopic expression or knock-down of selected TCP genes in the L1 layer exclusively. This result attributes an important function to the L1 layer and to specific TCP transcription factor genes in the control of floral organ sizes. In addition, we have identified a role for the class I TCP9 and TCP20 genes in repressing organ senescence, a function that is counteracted by the JAW TCP genes. Main results In summary, the results obtained in this project provide a solid basis for further analysis of the molecular mechanisms underlying floral organ growth and size control. Growth regulation is complex and a detailed knowledge is needed to improve crop yield in a sustainable manner. The outcome of this project provides a good starting point for discussions with breeding companies, to translate this knowledge to crop species and to initiate future applications. The results have shown that it will be necessary to select alleles of genes so as to achieve specifically temporal up-regulation or down-regulation of the encoded protein. Moreover, it also implies that it should be possible to improve yields without the use of genetic modification, a highly desirable approach by the majority of Europeans. More information about this project and follow up of the work can be obtained from: Prof. Dr Richard G.H. Immink, Wageningen University and Research centre, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands; e-mail: Richard.Immink@wur.nl.

Data: CORDIS, © European Union