Molecular-genetic analysis of natural variation in photoperiodic flowering of Arabidopsis thaliana

Giakountis, Antonis (2008). Molecular-genetic analysis of natural variation in photoperiodic flowering of Arabidopsis thaliana. PhD thesis, Universität zu Köln. Open Access

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Abstract

In Arabidopsis thaliana, the focus of my research, three developmental switches controlling the life cycle can be recognised. The first is germination that separates embryonic from post-embryonic development. The second signals the transition from the juvenile to the adult vegetative phase while the third, flowering, marks the initiation of the reproductive phase (Isabel Baurle and Caroline Dean, Cell 2006). All three exhibit both external (environmental) and endogenous (hormones) regulation. Natural genetic variation, namely phenotypic diversity due to genetic differences between individuals of the same species, has been reported both for germination and flowering initiation (Bentsink et al., PNAS 2006; O Neill et al., TAG 2008). Since individuals of Arabidopsis, commonly referred to as accessions, are collected from a variety of locations, it is believed that this genetic diversity reflects differences in the seasonal oscillations of environmental cues among the collection sites leading to local adaptation. Although natural genetic variation as a tool has been used in the study of flowering initiation in Arabidopsis (Alonso-Blanco and Maarten Koornneef, Trends in Plant Science 2000) a systematic survey that focuses mainly on the photoperiodic aspect of this regulation has been lacking. In order to expand the current knowledge two approaches were designed. First a survey for natural genetic variation in the flowering responses of phylogenetically distant Arabidopsis accessions under six different photoperiods was made. In parallel the transgenic equivalents of the same accessions, carrying a promoter fusion of the flowering time and circadian clock gene GIGANTEA (GI) were screened in the same photoperiods as for flowering time in order to detect for the first time trans-specific natural variation in the circadian regulation of an evening gene. Here I present evidence that natural genetic variation is present in a wide range of photoperiods both for the circadian clock and for flowering initiation per se. The flowering time responses are compared with the ones of mutants and transgenic lines of previously identified flowering time genes and I show that the affected known genes cannot fully cover the different patterns of day length discrimination that the natural accessions exhibit. Five different mapping populations were constructed by selecting interesting accessions from both screens, which led to the identification of new as well as known QTL, which alter various circadian and flowering responses between short and long days of similar duration. Generating advanced genetic material allows fine mapping and eventually cloning of some of the loci, while identification of genome-wide patterns of genetic interactions reveals additional loci that classical QTL mapping approaches cannot detect. Using RT-PCR and in situ hybridisation, I link this novel natural genetic variation between similar long day lengths with molecular variability in the temporal and spatial expression of flowering time genes FT and SOC1 thereby also demonstrating the tight dependence of the SAM floral commitment on the FT florigen. Finally I show that in nature, genetic variability in the property of enhanced photoperiod discrimination under similar long days, is enough to prevent winter flowering in a plant without any requirements for vernalization. Cologne, 2008

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Thesis (PhD thesis)

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