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Flower development of Lilium longiflorum - The Lilium information ...

Flower development of Lilium longiflorum - The Lilium information ...

The

The potential of VIGS in functional genetics of transcription factors In monocot species, despite many cases of significant differences in floral structure, the ABCDE model appears to be applicable, but with reservations (Nagasawa et al., 2003). Monocot genes showing homologous ABCDE functions have already been identified (reviewed by Fornara et al., 2003). Rice and maize are model species for flower development in grasses and the expression pattern of ABCDE genes is helping to clarify the organ homology between flowers from grasses and other species, considering that this topic has been a matter of discussion for a long time. In bulbous species, such as hyacinth (Hyacinthus orientalis), lily (Lilium spp.), narcissus (Narcissus pseudonarcissus), and orchids (Oncidium sp. and Dendrobium sp.), MADS-box genes involved in flower development are also being isolated and functionally characterized (Li et al., 2002; Tzeng and Yang, 2001; Tzeng et al., 2002; Hsu and Yang, 2002; Yu and Goh, 2000). Liliaceae species show an interesting floral aspect with the perianth organs (sepals and petals) evolving into petal-like structures, receiving the designation of “tepals”. This feature led to the proposition of a modified ABCDE model for the Liliaceae family members, with its B function being extended to the first whorl, in order to cope with petal characteristics of this organs (van Tunen et al., 1993). Perspectives for reverse genetics of floral transcription factors based on VIGS Overexpression of genes transcribing hpRNA of several genes involved in flower developmental pathways leads to specific and heritable silencing of the endogenous genes, resulting in null phenotypes in Arabidopsis (Chuang and Meyerowitz, 2000). VIGS of a regulatory flower developmental gene was first accomplished in N. benthamiana with its LEAFY/FLORICAULA orthologue, NFL (Ratcliff et al., 2001), opening avenues for using this system for efficient characterization of floral gene functions. Homeotic transcription factors, including the MADS-box genes involved in the ABCDE model, are perfectly suitable for assessing their function via VIGS, since they can instigate clear null phenotypes. They present a sequence structure conserved throughout evolution, that may permit studies of reverse genetics in heterologous systems, especially in model species in which VIGS is easily manipulated, such as N. benthamiana and, perhaps, rice. Our hypothesis is that a VIGS vector carrying a fragment of a homeotic transcription factor would trigger transient silencing of its endogenous homologue and 110

Chapter 7 induce phenotypes with homeotic transformations, if there is enough sequence identity between the genes involved. Importantly, the sequence structure within members from a gene family can interfere with the proper specificity of silencing, since gene boxes with highly conserved sequences can provoke functional suppression not only of the target gene but also of other members of this family (Ratcliff et al., 2001), as confirmed by Immink et al. (1999) with a crossed cosuppression event between two MADS-box genes in petunia. For optimal results leading to a specific, effective and reliable gene silencing in a heterologous system, the establishment of the level of sequence homology required is pivotal information. It is generally assumed that 85% nucleotide identity would be the lowest limit for triggering the silencing mechanism, however, experimental evidence is being awaited. Although some studies are still necessary to enable the use of VIGS in the functional identification of MADS-box genes in homo- or heterologous systems, the future seems promising and possibly in the near future this field will be open for speedy and easier functional characterization of the many genes from new species that await their function to be discovered or confirmed. Acknowledgements The authors are greatly thankful to Koen Pelgrom, Martin Verbeek and Suzan Gabriels for significant contributions during the course of this work. Anne-Marie Wolters is especially acknowledged for critical reading of the manuscript. This work was supported by the CNPq, an Organ for Science and Technology Development of the Brazilian Government (200851/98-5), and by the Dutch Ministry of Agriculture, Nature Management and Fisheries (DWK 364). REFERENCES Al-Kaff NS, Covey SN, Kreike MM, Page AM, Pinder R, Dale PJ (1998) Transcriptional and post-transcriptional plant gene silencing in response to a pathogen. Science 279: 2113- 2115 Angenent GC, Linthorst HJM, van Belkum AF, Cornelissen BJC, Bol JF (1986) RNA2 of tobacco rattle virus strain TCM encodes an unexpected gene. Nucleic Acids Res 14: 4673- 4682 111

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