A Higher Plant Δ8 Sphingolipid Desaturase with a Preference for (Z)-Isomer Formation Confers Aluminum Tolerance to Yeast and Plants

Three plant cDNA libraries were expressed in yeast (Saccharomyces cerevisiae) and screened on agar plates containing toxic concentrations of aluminum. Nine cDNAs were isolated that enhanced the aluminum tolerance of yeast. These cDNAs were constitutively expressed in Arabidopsis (Arabidopsis thaliana) and one cDNA from the roots of Stylosanthes hamata, designated S851, conferred greater aluminum tolerance to the transgenic seedlings. The protein predicted to be encoded by S851 showed an equally high similarity to Δ6 fatty acyl lipid desaturases and Δ8 sphingolipid desaturases. We expressed other known Δ6 desaturase and Δ8 desaturase genes in yeast and showed that a Δ6 fatty acyl desaturase from Echium plantagineum did not confer aluminum tolerance, whereas a Δ8 sphingobase desaturase from Arabidopsis did confer aluminum tolerance. Analysis of the fatty acids and sphingobases of the transgenic yeast and plant cells demonstrated that S851 encodes a Δ8 sphingobase desaturase, which leads to the accumulation of 8(Z/E)-C18-phytosphingenine and 8(Z/E)-C20-phytopshingenine in yeast and to the accumulation of 8(Z/E)-C18-phytosphingenine in the leaves and roots of Arabidopsis plants. The newly formed 8(Z/E)-C18-phytosphingenine in transgenic yeast accounted for 3 mol% of the total sphingobases with a 8(Z):8(E)-isomer ratio of approximately 4:1. The accumulation of 8(Z)-C18-phytosphingenine in transgenic Arabidopsis shifted the ratio of the 8(Z):8(E) isomers from 1:4 in wild-type plants to 1:1 in transgenic plants. These results indicate that S851 encodes the first Δ8 sphingolipid desaturase to be identified in higher plants with a preference for the 8(Z)-isomer. They further demonstrate that changes in the sphingolipid composition of cell membranes can protect plants from aluminum stress.