One of the most important determinates of yield is the correct timing of reproductive growth. A lot is known about how plants interpret external signals like photoperiod to regulate the floral transition and tuberization. Far less is known about how internal metabolic signals regulate the timing and the intensity of flower and tuber initiation. My aim is to understand how the nitrogen (N) supply regulates flowering and tuberization in the important crop potato (Solanum tuberosum). Excessive application of chemical fertilizers leads to increased emission into the atmosphere, pollution of ground water resources and leakage into soil patches causing environmental and human health problems as well as a decrease in biodiversity. Minimizing the use of synthetic fertilizers while still producing enough food to feed an ever growing world population is therefore one of the major challenges humanity currently faces. Optimizing a plant’s nitrogen use efficiency (NUE) is among the approaches taken.

I will make use of an almost natural, soil-based N-limited growth system that allows analysis of plants that have adapted to a small but sustained reduction of N. In this system plants are allowed to maintain a balance between the available N in the soil and constant growth, which is of advantage, since stress symptoms such as accelerated senescence, changes in protein and amino acid contents or large increase of secondary metabolites are avoided under N-limited conditions. The environmental control of stolon-to-tuber development and floral transition share many common features, e.g. the florigen identified in Arabidopsis thaliana is homologous to the tuberigen in potato, where it controls short day-dependent tuberization. This highlights how the identification of components of the flowering pathway in model species provides a basis for the identification of new factors with a function in crop species. We have studied nitrogen dependent flowering in Arabidopsis and have found two new N signal perception sites within the flowering network: (i) a positive interaction via nitrate response elements (NRE) in the regulatory region of important flowering genes and (ii) a negative interaction via a transcription factor complex at the shoot apical meristem (SAM) which represses flowering under limited N. Given the similarity between Arabidopsis floral induction and potato tuberization, I believe the knowledge we gained is transferable to studying N-dependent tuberization in potato. Here I propose approaches, which eventually aim at developing marker-assisted breeding tools for optimized yield under N-limited conditions. Correlation analysis of the resulting data at each level will help to obtain first predictive traits, e.g. from metabolite and gene expression levels, initially from a small number of cultivars. Apart from ‘proof of concept’ experiments, such as the functional validation of the results gained, the focus of the second part of the funding period will lie in the validation of specific differences of more cultivars and the development of tools that will aid future breeders to generate potato plants with stable and increased yield under limited N conditions. Future analyses of naturally occurring potato species using the tools developed within this approach can serve as a source for new traits for breeding.