Once cells become committed to a specific developmental program, they start to differentiate and become visibly different from each other; which involves the specific activation of several 1000's of cell- and tissue-specific genes. To study how cell differentiation and pattern formation takes place, resulting ultimately in the activation of defined sets of genes, we focus on genes that are involved in anthocyanin synthesis. These pigments are accumulated in specialized cells (epidermis) on the surface of flower petals. Any mutation affecting the genes involved in their synthesis or in the characteristics of the cells accumulating them, results in an easy to spot change of colour. In this way we have identified the genes controlling the synthesis deposition of anthocyanins in specific organs and at specific moments. A group of regulatory genes encodes for transcription factors, forming a transcription complex required to “switch on” the genes for the enzymes directing the biosynthesis of the pigments. Such complex, called WMBW complex (from the names of the transcription factors taking part to it), is specifically expressed only in the cells of the petal epidermis.
Recently, the characterization of blue flower colour mutants brought us to discover a novel mechanism of acidification of the lumen of cellular compartments. The colour of petals is determined by the pH of the compartment in which the pigments are stored. If the pH becomes higher, the colour shifts towards blue, while a rather acidic environment results in reddish colours. These are necessary to attract pollinators and guarantee a numerous progeny to the plant. Thanks to the combined action of two P-ATPases (PH1 and PH5) petal epidermal cells can build a large pH gradient between the cytoplasm and the vacuolar lumen, allowing the flower to display a brilliant reddish colour. The expression of PH1 and PH5 is controlled by the WMBW complex. This hyperacidification mechanism was until now unknown also due to the difficulty to isolate mutants in other systems.
Also the stabilization of the pigments in the vacuole is controlled by the WMBW complex, as shown by the “fading” of the colour in the petal of mutants for some of the transcription factors within the complex. We use these mutants to identify the factors involved in the stabilization of the vacuolar content, with an eye to a large range of possible applications.
Among the peculiarities of the petal epidermal cells is a novel organelle, resembling a small vacuole and serving as “intermediate station” for proteins on their way to the central vacuole. This compartment, which we called vacuolino, is absent in cells that do not express the WMBW complex. As mutants, lacking organelles are scarce (due to their lethality in most systems), these plants offer the unique possibility to identify genes involved in the genesis of vacuolar compartments.
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