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Institute
Sphingolipid long-chain bases (LCBs) are the building blocks of the biosynthesis of sphingolipids. They
are defined as structural elements of the plant cell membrane and play an important role
determining the fate of the cells. Complex ceramides represent a substantial fraction of total
sphingolipids which form a major part of eukaryotic membranes. At the same time, LCBs are well
known signaling molecules of cellular processes in eukaryotes and are involved in signal transduction
pathways in plants. High levels of LCBS have been shown to be associated with the induction of
programmed cell death as well as pathogen-derived toxin-induced cell death. Indeed, several studies
confirmed the regulatory function of sphingobases in plant programmed cell death (PCD):
(i) Spontaneous PCD and altered cell death reaction caused by mutated related genes of sphingobase
metabolism. (ii) Cell death conditions increases levels of LCBs. (iii) PCD due to interfered sphingolipid
metabolism provoked by toxins produced from necrotrophic pathogens, such as Fumonisin B1 (FB1).
Therefore, to prevent cell death and control cell death reaction, the regulation of levels of free LCBs
can be crucial.
The results of the present study challenged the comprehension of sphingobases and sphingolipid
levels during PCD. We provided detailed analysis of sphingolipids levels that revealed correlations of
certain sphingolipid species with cell death. Moreover, the investigation of sphingolipid biosynthesis
allowed us to understand the flux after the accumulation of high LCB levels. However, further
analysis of degradation products or sphingolipid mutant lines, would be required to fully understand
how high levels of sphingobases are being treated by the plant.
Sphingolipid long-chain bases (LCBs) are building blocks for membrane-localized sphingolipids, and are involved in signal transduction pathways in plants. Elevated LCB levels are associated with the induction of programmed cell death and pathogen-derived toxin-induced cell death. Therefore, levels of free LCBs can determine survival of plant cells. To elucidate the contribution of metabolic pathways regulating high LCB levels, we applied the deuterium-labeled LCB D-erythro-sphinganine-d7 (D7-d18:0), the first LCB in sphingolipid biosynthesis, to Arabidopsis leaves and quantified labeled LCBs, LCB phosphates (LCB-Ps), and 14 abundant ceramide (Cer) species over time. We show that LCB D7-d18:0 is rapidly converted into the LCBs d18:0P, t18:0, and t18:0P. Deuterium-labeled ceramides were less abundant, but increased over time, with the highest levels detected for Cer(d18:0/16:0), Cer(d18:0/24:0), Cer(t18:0/16:0), and Cer(t18:0/22:0). A more than 50-fold increase of LCB-P levels after leaf incubation in LCB D7-d18:0 indicated that degradation of LCBs via LCB-Ps is important, and we hypothesized that LCB-P degradation could be a rate-limiting step to reduce high levels of LCBs. To functionally test this hypothesis, we constructed a transgenic line with dihydrosphingosine-1-phosphate lyase 1 (DPL1) under control of an inducible promotor. Higher expression of DPL1 significantly reduced elevated LCB-P and LCB levels induced by Fumonisin B1, and rendered plants more resistant against this fungal toxin. Taken together, we provide quantitative data on the contribution of major enzymatic pathways to reduce high LCB levels, which can trigger cell death. Specifically, we provide functional evidence that DPL1 can be a rate-limiting step in regulating high LCB levels.