Overexpression of Arabidopsis Ceramide Synthases Differentially Affects Growth, Sphingolipid Metabolism, Programmed Cell Death, and Mycotoxin Resistance

Ceramide synthases catalyze anN-acyltransferase reaction using fatty acyl-coenzyme A (CoA) and long-chain base (LCB) substrates to form the sphingolipid ceramide backbone and are targets for inhibition by the mycotoxin fumonisin B₁ (FB₁). Arabidopsis (Arabidopsis thaliana) contains three genes encoding ceramide synthases with distinct substrate specificities:LONGEVITY ASSURANCE GENE ONE HOMOLOG1(LOH1; At3g25540)- andLOH3(At1g19260)-encoded ceramide synthases use very-long-chain fatty acyl-CoA and trihydroxy LCB substrates, andLOH2(At3g19260)-encoded ceramide synthase uses palmitoyl-CoA and dihydroxy LCB substrates. In this study, complementary DNAs for each genewere overexpressed to determine the role of individual isoforms in physiology and sphingolipid metabolism. Differences were observed in growth resulting fromLOH1andLOH3overexpression compared withLOH2overexpression.LOH1- andLOH3-overexpressing plants had enhanced biomass relative to wild-type plants, due in part to increased cell division, suggesting that enhanced synthesis of very-long-chain fatty acid/trihydroxy LCB ceramides promotes cell division and growth. Conversely,LOH2overexpression resulted in dwarfing.LOH2overexpression also resulted in the accumulation of sphingolipids with C16 fatty acid/dihydroxy LCB ceramides, constitutive induction of programmed cell death, and accumulation of salicylic acid, closely mimicking phenotypes observed previously in LCB C-4 hydroxylase mutants defective in trihydroxy LCB synthesis. In addition,LOH2- andLOH3-overexpressing plants acquired increased resistance to FB₁, whereasLOH1-overexpressing plants showed no increase in FB₁ resistance, compared with wild-type plants, indicating thatLOH1ceramide synthase is most strongly inhibited by FB₁. Overall, the findings described here demonstrate that overexpression of Arabidopsis ceramide synthases results in strongly divergent physiological and metabolic phenotypes, some of which have significance for improved plant performance.