Investigating the genetic control of flowering time and seed dormancy in a model for temperate cereals

Flowering and seed germination are two major events in the life cycle of a plant where a decision is made to switch from a comparatively more robust state to a more vulnerable one. Both events are inextricably linked with either process determining the timing of the other. The timing is also strongly regulated by the environment with variables such as temperature, water availability and light having the greatest impacts. Many conditions need to be met for either process to occur to ensure the phase transition occurs at the most favourable moment and to ensure maximum reproductive success. Many temperate species require a prolonged period of cold or vernalisation to flower as this acts as a signal of seasonal change to time flowering with the favourable conditions of spring. Levels of seed dormancy i.e., the inability of a seed to germinate despite favourable environmental conditions upon imbibition, are also strongly regulated by temperature. Cool temperatures during seed development can increase dormancy levels while cool temperatures upon imbibition can promote germination in some species.

Not only are these traits important from a biological perspective, flowering time and seed dormancy have been selected by humans to expand the cultivation zone of important crops. Through modifications in flowering time, temperate crops like wheat are now grown worldwide in a variety of different climates. Low seed dormancy has been selected for homogenous seed germination to boost efficiency in agricultural or industrial practices. Knowledge obtained on these processes can allow us to further fine tune plant development and boost crop productivity or assist with generating new crop varieties better adapted to new or future climate conditions.

This thesis describes how flowering and seed dormancy are environmentally and genetically regulated in the model temperate grass Brachypodium distachyon (Brachypodium). In Chapter 2, a comprehensive literature review of all references to FLC homologs in cereals was conducted. This chapter describes the functions these genes play in flowering time regulation while explicitly acknowledging their relationship to the FLC clade in Arabidopsis thaliana, as this has generally been missing from the literature. New insights are also discussed highlighting that FLC homologs have been linked to other processes including seed development and potentially stress response pathways.

In Chapter 3, the function of BdVRT2 in regulating flowering time is described. The devernalisation response of wild type Brachypodium Bd21-3 is also described for the first time.

In Chapter 4, the function of the FLC homolog BdMADS37 was investigated. Some evidence was gathered to support the role of BdMADS37 as a negative regulator of flowering time however further work needs to be done to validate this.

In Chapter 5, the influence of maternal and imbibition temperatures on seed dormancy of Brachypodium was investigated. Evidence was gathered to show that temperatures experienced at several stages of development have independent influences on dormancy levels. A new role was also discovered for the FLC homolog BdOS2 in dormancy regulation. Furthermore, several new mutants were generated via EMS-induced mutagenesis which will be used to identify novel regulators of seed dormancy.

In conclusion, this thesis highlighted the various ways temperature can influence important events in the life cycle of the plant. Novel functions for BdVRT2 and BdOS2 were also described. The plant science community was updated on the status of FLC research in cereals. Several new mutants in flowering time and dormancy genes have been generated to continue to investigations in future.