RICARDI Martiniano Maria
Low sugar is not always good: Impact of specific O-glycan defects on tip growth in Arabidopsis
SILVIA M VELASQUEZ; ELIANA MARZOL; CECILIA BORASSI; LAERCIO POL-FACHIN; MARTINIANO MARIA RICARDI; SILVINA MANGANO; SILVINA PAOLA DENITA JUAREZ; JUAN DAVID SALGADO SALTER; JAVIER ANSELMO GLOAZZO DOROSZ; SUSAN E MARCUS; J P KNOX; JOSE R DINNENY; NORBERTO D IUSEM; HUGO VERLI; JOSE MANUEL ESTEVEZ
AMER SOC PLANT BIOLOGISTS
The hydroxyproline-rich O-glycoproteins (HRGPs) comprise several groups of O-glycoproteins, including extensins (EXTs), ultimately secreted into plant cell walls. The latter are shaped by several posttranslational modifications, mainly hydroxylation of Pro residues into Hyp and further O-glycosylation on Hyp and Ser (Supplemental Fig. S1A). EXTs contain several Ser-(Hyp)4 repeats, usually O-glycosylated with chains of up to four or five linear arabinosyl units on each Hyp (Velasquez et al., 2011; Ogawa-Ohnishi et al., 2013) and monogalactosylated on Ser residues (Saito et al., 2014). O-Glycosylated Ser-(Hyp)4 repeats are not only present in EXTs but can potentially decorate several other EXT-like chimeras and hybrid EXT glycoproteins that contain other domains, such as arabinogalactan protein-EXTs, Pro-rich protein-EXTs, Leu-rich repeat-EXTs, Pro-rich kinases, and formins with an extracellular EXT domain, etc. In addition, Hyp-O-arabinosylation also occurs in single Hyp units in the small secreted glycopeptide hormones (e.g. CLAVATA3 [CLV3]) with up to three arabinosyl units (Ohyama et al., 2009; Matsubayashi, 2010; Shinohara and Matsubayashi, 2013). In this context, three groups of arabinosyltransferases (AraTs), hydroxyproline O-arabinosyltransferase1 (HPAT1) to HPAT3 (classified as GT8 in the Carbohydrate Active Enzymes database), Reduced Residual Arabinose1 (RRA1) to RRA3, and Xyloglucanase113 (XEG113; GT77 family), have recently been implicated in the sequential addition of the innermost three l-arabinosyl residues (Egelund et al., 2007; Ogawa-Ohnishi et al., 2013; Supplemental Table S1). In addition, one novel peptidyl-Ser galactosyltransferase named SERGT1 has been reported to add a single α-galactopyranose residue to each Ser residue in Ser-(Hyp)4 motifs of EXTs, thus belonging to the GT96 family within the Carbohydrate Active Enzymes database (Supplemental Table S1). Finally, glycosylated EXTs are possibly cross-linked by putative type III peroxidases 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 hair, and several EXT and EXT-related mutants (e.g. ext6-7, ext10-12, and Leu-rich repeat extensin1, 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 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 types of HRGP like arabinogalactan proteins, 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.