Isolation and Functional Characterisation of the Genes Encoding Δ~8-Sphingolipid Desaturase from Brassica rapa

8-Sphingolipid desaturase is the key enzyme that catalyses desaturation at the C8 position of the long-chain base of sphingolipids in higher plants. There have been no previous studies on the genes encoding AS-sphingolipid desaturases in Brassica rapa. In this study, four genes encoding AS-sphingolipid desaturases from B. rapa were isolated and characterised. Phylogenetic analyses indicated that these genes could be divided into two groups： BrD8A, BrD8C and BrD8D in group I, and BrD8B in group II. The two groups of genes diverged before the separation of Arabidopsis and Brassica. Though the four genes shared a high sequence similarity, and their coding desaturases all located in endoplasmic reticulum, they exhibited distinct expression patterns. Heterologous expression in Saccharomyces cerevisiae revealed that BrD8A/B/C/D were functionally diverse AS-sphingolipid desaturases that catalyse different ratios of the two products 8（Z）- and 8（E）-C18-phytosphingenine. The aluminium tolerance of transgenic yeasts expressing BrD8A/B/C/D was enhanced compared with that of control cells. Expression of BrD8A in Arabidopsis changed the ratio of 8（Z）：8（E）-C 18-phytosphingenine in transgenic plants. The information reported here provides new insights into the biochemical functional diversity and evolutionary relationship of AS-sphingolipid desaturase in plants and lays a foundation for further investigation of the mechanism of 8（Z）- and 8（E）-C18- phytosphingenine biosynthesis.

Emerging Roles of Sphingolipid Signaling in Plant Response to Biotic and Abiotic Stresses

Plant sphingolipids are not only structural components of the plasma membrane and other endomembrane systems but also act as signaling molecules during biotic and abiotic stresses.However,the roles of sphingolipids in plant signal transduction in response to environmental cues are yet to be investigated in detail. In this review,we discuss the signaling roles of sphingolipid metabolites with a focus on plant sphingolipids.We also mention some microbial sphingolipids that initiate signals during their interaction with plants, because of the limited literatures on their plant analogs.The equilibrium of nonphosphorylated and phosphorylated sphingolipid species determine the destiny of plant cells,whereas molecular connections among the enzymes responsible for this equilibrium in a coordinated signaling network are poorly understood.A mechanistic link between the phytohormone-sphingolipid interplay has also not yet been fully understood and many key participants involved in this complex interaction operating under stress conditions await to be identified.Future research is needed to fill these gaps and to better understand the signal pathways of plant sphingolipids and their interplay with other signals in response to environmental stresses.