Understanding The Role Of SPHK1 In FB1-induced Cell Death In Arabidopsis

Sphingolipids are the key components of lipid raft in eukaryotic cell membrane.The balance between long chain base(LCB)and long chain base-phosphate(LCBP)acts as a rheostat in sphingolipid metabolism.Sphingosine kinases are the key enzymes responsible for phosphoralating LCB to LCBP and for keeping the celluar balance of sphingolipid metabolism.The fungal toxin FBI is an analogue of dihydro-sphingosine and can perturb sphingolipid hemostasis by inhibiting ceramide synthase.Furthermore,FBI is a strong inducer to trigger plant hypersensitive responses(HR)along with increased LCB and LCBP contents.However,the regulatory mechanism of FBI action and how the LCB/LCBP signaling cassette functions during these processes are still not fully understood.Here,we identified sphingosine kinase 1(SPHK1)as a key factor in FBI-induced HR by modulating the SA pathway and ROS accumulation in Arabidopsis thaliana.Overexpression of SPHK1 increases the FBI-induced accumulation of ROS and SA biosynthesis.The double mutant that simultaneously overexpresses SPHK1 and suppresses the SPPASE or DPL1,two enzymes mainly responsible for Phyto-S1P removal,showed further enhanced susceptibility to FBI killing and FBI-induced SA activation than the plants overexpress SPHK1 alone.Exogenous S1P can modulate the transcription of the SA-responsive marker gene PR1 in a concentration-dependent biphasic manner,whereas suppression of SPHK1 decreases SA production but promotes JA biosynthesis in response to FB1 applications.Our findings indicate that SPHK1 has a key role in modulating FBI-triggered cell death via SA and JA interactions.Intriguingly,suppressing the function of SPHK1 caused enhanced sensitive to salinity and oxidative stresses whereas overexpression of SPHK1 showed no impact on plant resistance to either stresses.