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

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

Chapter 4 Introduction

Chapter 4 Introduction In plants, the species Arabidopsis thaliana was chosen as the contemporary, most important model for genetic and developmental studies. Its short life cycle (around 6 weeks from germination to production of mature seeds), small size (15 to 20 cm height), natural self-fertilisation and large progeny (about 5,000 seeds per plant), small nuclear genome size (125 Mbp) distributed in only 5 chromosomes, and an easy transformation system made this species ideal to the quest for unravelling many plant biology aspects (Meinke et al., 1998; Meyerowitz and Somerville, 1994). As a comparison, trumpet lily (Lilium longiflorum) can take up to two years to flower, has the largest genome in the plant kingdom, estimated to be 90 Gbp (more than 700 times larger than the Arabidopsis genome), located in 12 chromosomes frequently in multiple copies, and offers great recalcitrance to transformation. Notwithstanding the fascination for the beauty of their flowers, molecular biology studies in lily species have time and methodological constraints. A way to circumvent these problems is using model species such as Arabidopsis as a heterologous system, in order to assess gene function and other biological aspects of lily. Studies on Arabidopsis started with Mendelian genetics (Laibach, 1943; Rédei, 1970; Koornneef et al., 1983) and is now culminating in the “-omics” era, in which its entire genome is already unveiled (Arabidopsis Genome Initiative, 2000; Meyerowitz, 2001; Somerville and Koornneef, 2002). The “-omics” approaches are capable to generate data in a high quantity and speed, but usually these data require further studies to validate the hypotheses generated with this information, especially concerning gene functions. Good indications on gene functionality can be obtained by in vivo methods such as protein interactions in yeast hybrid systems (Causier and Davies, 2002; Immink and Angenent, 2002; Moon et al., 1999; Pelaz et al., 2001), fluorescence resonance energy transfer (FRET, Immink et al., 2002), gene overexpression and knock out (Hirschi, 2003) in homologous or heterologous species, nonetheless the final confirmation can only be accomplished by the introduction of a genetic function into a related defective genotype in order to recover a functional phenotype for the gene in study. Complementation testing is an elegant and, in principle, simple way of assessing gene functions. Conceptually, it means that when two mutant genotypes with mutations in different loci are crossed, they will restore the wild-type phenotype in the F1 siblings by complementing the defective allele of each other with the wild-type allele from the other parent (Figure 1). 52

+ = aaBB AAbb AaBb (wt) if aa + Z = Ascribing genetic function from lily using heterologous system A+ aa + A = so A and Z present the same function Figure 1. Diagram representing the genetic complementation test. Vertical bars denote homolog chromosomes in a diploid individual. Circles and letters in lowercase represent mutant recessive alleles whereas the absence of circles and the letters in uppercase represent the wild-type allele in a given locus. The traditional complementation test is shown in the left box on the top: crosses of two plants with defective mutations can restore the wild-type phenotype if the mutations are in different loci. The right box represent the alternative approach using a transformation system to introgress a functional gene in the defective genome. The squares correspond to the transgene whereas the signal + represents overexpression. In the lower box, the approach for characterising functional genetic homology is represented. If the integration of a given gene can restore the same phenotype as a confirmed functional gene does, one can say that both share the same function and are, therefore, functional homologues. The advent of transformation systems brought the possibility of performing complementation tests without crossings between the defective genotypes, but instead it can be done with one defective genotype being complemented by an introduced functional gene via any transformation method available. Additionally, it allowed performing complementation tests in one species with genes originating from other species, in order to confirm functional orthologies. A+ 53

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