Lay summary

Carbohydrates are key-players in a multitude of fundamental physiological events in plants, such as development, signaling, carbon transport and storage, cell wall synthesis, and stress protection. We have primarily focused our research on one type of carbohydrate, the water-soluble raffinose family oligosaccharides (RFOs) which are α-1,6 galactosyl extensions of sucrose (Suc) with the most common species being raffinose (Suc-Gal1), stachyose (Suc-Gal2), and verbascose (Suc-Gal3). RFOs are the most abundant oligosaccharides in the plant kingdom and many RFO-producing plants are of economic importance including cucurbits, mints, legumes, olives, grapes, and pines. As primary photosynthetic products RFOs are jacks of all trades, fulfilling diverse physiological functions, such as storage and translocation of carbon and protection against environmental stresses. We have mainly worked with the Lamiaceae, common bugle (Ajuga reptans), because this frost-hardy evergreen is an RFO all-rounder. We have shown that Ajuga utilizes RFOs as the principle form of carbon storage in leaves and roots (up to 80% of the DW) and carbon transport in the phloem (up to 80% of the phloem sugars. After having cloned, characterized, and localized most key-enzymes of the RFO biosynthetic pathway in Ajuga, we will now be focusing our research mainly (i) on the role of RFOs in frost tolerance of Ajuga, comparing it to that of the Brassicaceae, Arabidopsis thaliana (thale cress or mouse-ear cress, a facultative transient RFO accumulator) and (ii) on the pathways of RFO mobilization in source and RFO utilization in sink tissues. To this end, state-of-the-art methods, such as cDNA library construction and screening, biochemical characterization of enzymes using heterologous gene expression systems, stable plant transformation, in situ enzyme activity staining, in situ hybridization, confocal microscopy, immunolocalization, HPLC carbohydrate analysis etc. will be applied. Collectively, the proposed research will illuminate how RFOs are involved in conferring stress tolerance and how the organization of the RFO catabolic pathway facilitates proper growth. Both processes are of fundamental importance and still poorly understood.