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Background  Arabinogalactan-proteins (AGPs) are ubiquitous components of cell walls throughout the plant kingdom and are extensively post translationally modified by conversion of proline to hydroxyproline (Hyp) and by addition of arabinogalactan polysaccharides (AG) to Hyp residues. AGPs are implicated to function in various aspects of plant growth and development, but the functional contributions of AGP glycans remain to be elucidated. Hyp glycosylation is initiated by the action of a set of Hyp- O -galactosyltransferase (Hyp- O -GALT) enzymes that remain to be fully characterized.    Results  Three members of the GT31 family ( GALT3 - At3g06440 ,  GALT4 - At1g27120 , and  GALT6 - At5g62620 ) were identified as  Hyp-O-GALT  genes by heterologous expression in tobacco leaf epidermal cells and examined along with two previously characterized  Hyp-O-GALT  genes,  GALT2  and  GALT5 . Transcript profiling by real-time PCR of these five  Hyp-O-GALTs  revealed overlapping but distinct expression patterns. Transiently expressed GALT3, GALT4 and GALT6 fluorescent protein fusions were localized within Golgi vesicles. Biochemical analysis of knock-out mutants for the five  Hyp-O-GALT  genes revealed significant reductions in both AGP-specific Hyp- O -GALT activity and β-Gal-Yariv precipitable AGPs. Further phenotypic analysis of these mutants demonstrated reduced root hair growth, reduced seed coat mucilage, reduced seed set, and accelerated leaf senescence. The mutants also displayed several conditional phenotypes, including impaired root growth, and defective anisotropic growth of root tips under salt stress, as well as less sensitivity to the growth inhibitory effects of β-Gal-Yariv reagent in roots and pollen tubes.    Conclusions  This study provides evidence that all five  Hyp-O-GALT  genes encode enzymes that catalyze the initial steps of AGP galactosylation and that AGP glycans play essential roles in both vegetative and reproductive plant growth.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-015-0670-7) contains supplementary material, which is available to authorized users.

bmc plant biology

A small multigene hydroxyproline-O-galactosyltransferase family functions in arabinogalactan-protein glycosylation, growth and development in Arabidopsis