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

SILVIA M. VELASQUEZ, ELIANA MARZOL, CECILIA BORASSI, MARTINIANO M. RICARDI, SILVINA MANGANO, SILVINA PAOLA DENITA JUAREZ, JUAN D. SALGADO SALTER, JAVIER GLOAZZO DOROSZ, JOSÉ M. ESTEVEZ

PLANT PHYSIOLOGY.

AMER SOC PLANT BIOLOGISTS

Lugar: Rockville; Año: 2015

0032-0889

Hydroxyproline-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 (Ara) on each Hyp (Velasquez et al., 2011; Ogawa-Ohnishi et al., 2013) and mono-galactosylated on Ser residues (Saito et al., 2014). O-glycosylated Ser-(Hyp)4 repeats are not only present in EXTs but they can potentially be decorating several other EXT-like chimeras and hybrid-EXT glycoproteins that contain other domains such as AGP (ArabinoGalactan Protein)-EXTs, Proline Rich Proteins (PRP)-EXTs, Leucine Rich Repeats (LRR)-EXTs, Proline-Rich Kinases (PERKs) and Formins with an extracellular EXTs domain, etc. In addition, Hyp-O-arabinosylation also occurs in single Hyp units in the small secreted glycopeptide hormones (e.g. CLAVATA 3, CLV3) with up to 3 Ara units (Ohyama et al., 2009; Matsubayashi, 2010; Shinohara and Matsubayashi, 2013). 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) (Table S1). In addition, one novel peptidyl-Ser galactosyltransferase named SERGT1 has been reported to add a single a-Galp (Galactopyranose) residue to each Ser residue in Ser-(Hyp)4 motifs of EXTs, thus belonging to GT96 family within CAZy (Table S1). Finally, glycosylated EXTs are possibly crosslinked by putative type-III peroxidases (PERs) at the Tyr residues 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). EXT assembly into a putative glycoprotein network seems to be crucial for cell expansion of root hairs and several EXT and EXT-related mutants (e.g. ext6-7, ext10-12, lrx1, etc.) were previously isolated with abnormal root hair cell expansion phenotypes (Ringli, 2010; Velasquez et al., 2011). Here, by using mutants of several known enzymes of the O-glycosylation pathway of HRGPs, we addressed to what extent each specific defect on the O-glycosylation machinery impacts on root hair tip growth. In addition, we refer only to Hyp-O-arabinosylation and Ser-O-galactosylation modifications of EXT and EXT-related proteins while we have excluded Hyp O-(arabino)galactosylation, commonly present in other type of HRGP like AGPs, from our analysis. Finally, by molecular dynamic simulations, we propose a possible model to explore how these two specific types of O-glycan defects would affect EXT self-assembly and, ultimately, their impact on the polarized cell expansion. We use a classical EXT repetitive sequence to begin to explore how O-glycosylation might affect glycoprotein conformation and possible self-interactions in the context of polarized growth but we are aware of the complexity and diversity of EXT and EXT-related proteins that offers several other possible scenarios.