Low sugar is not always good: Impact of specific o-glycan defects on tip growth in arabidopsis

Abstract

Hydroxyproline (Hyp)-rich O-glycoproteins (HRGPs) comprises several groups of O-glycoproteins including extensins (EXTs), ultimately secreted into plant cell walls. The latter are shaped by several posttranslational modifications (PTMs), mainly hydroxylation of proline residues into hydroxyproline (Hyp) and further O-glycosylation on Hyp and Serine (Ser) (Fig. S1A). EXTs contain several Ser-(Hyp)4 repeats usually O-glycosylated with chains of up to 4-5 linear arabinosyl units on each Hyp (Velasquez et al., 2011; Ogawa-Ohnishi et al., 2013) and mono-galactosylated on Ser residues (Saito et al., 2014). In this context, three groups of arabinosyltransferases (AraTs), HPAT1-HPAT3 (classified as GT8 in the Carbohydrate Active enZymes database [CAZy]), RRA1-RRA3 and XEG113 (GT77 family) have recently been implicated in the sequential addition of the innermost three L-Ara residues (Egelund et al., 2007; Ogawa-Ohnishi et al., 2013). In addition, one novel peptidyl-Ser galactosyltransferase named SERGT1 has been reported to add a single -Galp residue to each Ser residue in Ser-(Hyp)4 motifs of EXTs, thus belonging to a new family within CAZy (Table S1). Finally, glycosylated EXTs are possibly crosslinked by putative type-III peroxidases (PERs) at the Tyr residues by forming EXT linkages (Cannon et al., 2008) able to build a three-dimensional network likely to interact with other cell wall components like pectins (Cannon et al., 2008). Here, by using appropriate mutants of several known enzymes of the O-glycosylation pathway of HRGPs, we addressed to what extent each single defect on the O-glycosylation machinery impacts on root hair tip growth.