The Journey from Source to Sink in Setaria viridis: The Role of Sugar Exporters SWEETs

Abstract

Understanding how photoassimilates are exported out of source leaves to the sink for utilisation is of major interest to both researchers and the agriculture industry. Currently this process is not well characterised in C4 monocots. Leaf sugar export is complicated in C4 species because photosynthesis occurs in two cell types, the mesophyll and bundle sheath cells, so sugars must be moved out of both these cells for it to be loaded into the phloem. Phloem loading can occur either through the symplasmic pathway via plasmodesmata or through the apoplasmic pathway with the aid of transporters. Phloem unloading on the other hand predominantly follows a symplasmic pathway in many sinks but often also includes an apoplasmic step. Here in this thesis the role of the sugar exporters SWEETs (Sugars Will Eventually be Exported Transporters) is elucidated in the apoplasmic pathways of phloem loading, unloading and post-phloem transport in source and sink tissue. The C4 model monocot species Setaria viridis, the genome of which contains a total of 24 SWEETs, is used as a tool to study these processes. To investigate the role of SWEETs, a comprehensive expression analysis of genes encoding proteins involved in carbohydrate metabolism and transport was performed using RNA deep sequencing data. Carbohydrate profiling was also performed on source and sink tissues to investigate what substrates SWEETs could be transporting in planta. Antibodies were used to perform immunolocalisation experiments on source and sink tissue to determine in which plasma membrane cell types SWEETs were found on. Lastly, some Setaria SWEETs were functionally characterised to determine whether they could transport glucose and sucrose, the two most abundant forms of sugars in plants. Specific attention was directed toward the highly expressed SvSWEET4a, SvSWEET13a and SvSWEET13b which can be found in either source or sink tissue.