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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.
In this study poplar trees have been examined under different stress conditions. Apart from the detailed descriptions above two main conclusions might be drawn: i) A small plant like Arabidopsis thaliana is highly susceptible to stress situations that might become life-threatening compared to a tree that has extremely more biomass at its disposal. Such an organism might be able to compensate severe stress much longer than a smaller one. It seems therefore reasonable that a crop like Arabidopsis reacts earlier and faster to a massive threat. ii) In poplar both tested stress responses seemed to be regulated by hormones. The reactions to abiotic salt stress are mainly controlled by ABA, which also has a strong impact upon cold and drought stress situations. The term commonly used for ABA is “stress hormone” and is at least applicable to all abiotic stresses. In case of herbivory (biotic stress), jasmonic acid appears to be the key-player that coordinates the defence mechanism underlying extrafloral nectary and nectar production. Thus the presented work has gained a few more insights into the complex network of general stress induced processes of poplar trees. Future studies will help to understand the particular role of the intriguing indirect defence system of the extrafloral nectaries in more detail.