In higher plants, the most abundant
sterol derivatives are steryl
glycosides (SGs) and
acyl SGs. Arabidopsis (
Arabidopsis thaliana) contains two genes, UGT80A2 and UGT80B1, that encode UDP-Glc:
sterol glycosyltransferases,
enzymes that
catalyze the synthesis of SGs. Lines having
mutations in UGT80A2, UGT80B1, or both UGT80A2 and UGT8B1 were identified and characterized. The ugt80A2 lines were viable and exhibited relatively minor effects on plant growth. Conversely, ugt80B1
mutants displayed an array of
phenotypes that were pronounced in the
embryo and seed. Most notable was the finding that ugt80B1 was
allelic to transparent testa15 and displayed a transparent testa
phenotype and a reduction in seed size. In addition to the role of UGT80B1 in the deposition of
flavanoids, a loss of suberization of the seed was apparent in ugt80B1 by the lack of
autofluorescence at the hilum region. Moreover, in ugt80B1, scanning and
transmission electron microscopy reveals that the outer integument of the seed coat lost the electron-dense
cuticle layer at its surface and displayed altered cell morphology.
Gas chromatography coupled with
mass spectrometry of
lipid polyester monomers confirmed a drastic decrease in
aliphatic suberin and cutin-like
polymers that was associated with an inability to limit tetrazolium
salt uptake. The findings suggest a membrane function for SGs and
acyl SGs in trafficking of
lipid polyester precursors. An ancillary observation was that
cellulose biosynthesis was unaffected in the double
mutant, inconsistent with a predicted role for SGs in priming
cellulose synthesis.
