The function of cytokinin in regulating flowering time

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The promotive activity of cytokinin on flowering was already described soon after the discovery of this plant hormone almost fifty years ago. However, the molecular mechanisms underlying this activity remain elusive despite great progress in understanding the metabolism and signalling of the hormone. The objective of this research proposal is to study the extent and the molecular pathways by which cytokinin regulates flowering time. Most of this work will be done in Arabidopsis thaliana but the transferability to oilseed rape (Brassica napus L.), where flowering time is an important breeding target, will be studied as well. In a first part, a detailed study of the consequences of loss-of-function of most of the ~60 cytokinin metabolism and signalling genes on flowering time will be performed to identify the functionally relevant genes. Our preliminary work has identified the cytokinin receptors AHK2 and AHK3 as well as the transcription factors ARR10 and ARR12 to play central roles in mediating cytokinin action. This analysis will be complemented by a study on the relevance of the cytokinin status of different tissues using transgenic plants with a tissue-specifically altered cytokinin status, focussing on the shoot apical meristem, the leaves, the phloem and the root. The investigation includes a comparison of the transcript levels of known flowering pathway genes under different day lengths and after a short day-to-long day shift. The influence of different environmental cues (light, nutrition) on cytokinin action in regulating flowering time will be tested to understand under which circumstances its role is particularly relevant. Mining of transcriptomic data has yielded several hypotheses about a modulatory role of cytokinin in known flowering pathways, including regulation of the flower repressor gene ATC, miR156, miR172 and interactions with the gibberellin and trehalose-6-phosphate pathways, which will be explored further by molecular and genetic approaches. These analyses and the identification of downstream targets of ARR10 and ARR12 aim to link the action of cytokinin to known components of flowering pathways. Furthermore, a genetic approach is chosen to obtain access to novel components regulating the action of cytokinin. Strong cytokinin deficiency causes lack of flowering under short days, which can be reversed by dominant gain-of-function mutants isolated from a suppressor screen. Novel mutants specifically affecting the flowering pathway should be identified and the respective genes cloned by a map-based approach. Finally, the role of cytokinin in regulating flowering in Brassica napus will be explored using transgenic lines with a genetically engineered cytokinin status in different tissues. This study should reveal whether mechanisms regulating flowering time have been conserved in this important crop plant and may be used as a breeding target to modulate its flowering behaviour.