Glycosylinositol phosphorylceramides (GIPCs) are a class of glycosylated sphingolipids found in plants, fungi and protozoa. They are extremely abundant in the plant plasma membrane, estimated to form ~25-40 % of total lipids, but almost nothing is known about their function. GIPCs consist of a ceramide attached to a glycan headgroup via a phosphate group. Recently we have identified the first two proteins involved in the headgroup biosynthesis - IPUT1 (a glucuronic acid glycosyltransferase) and GONST1 (a GDP-Man transporter). Plants lacking functional copies of these proteins are either pollen lethal (iput1) or have extreme developmental defects (gonst1), despite the lipid portion of the GIPC being unaffected. This implies a critical function for the GIPC glycan headgroup in membrane function. Here, we describe a new Golgi-localized protein involved in GIPC headgroup biosynthesis - GMT1 (GIPC MANNOSYLTRANSFERASE1). Plants lacking GMT1 show a phenotype similar to gonst1, including small stature, severe early senescence and poor seed set, as well as a constitutive defense response which is visible from the seedling stage. Heterologous expression of GMT1 in tobacco BY2 cells and yeast resulted in the production of mannosylated "Arabidopsis-like" GIPCs. Further investigation into the cause of the developmental defects showed that whilst the gmt1 cell wall has wild type-like matrix polysaccharides, it has a large reduction in crystalline cellulose, accompanied by an increase in lignin. We will present some possible hypotheses regarding the roles of these mysterious lipids in cellulose biosynthesis and membrane organization.

Sphingolipid glycosylation in plants: Some implications for cellulose biosynthesis

253rd National Meeting (2017)

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