Complex Glycan-Less1 (CGL1, in Arabidopsis also N-acetylglucosaminyltransferase I, GNTI), a membrane-bound enzyme that initiates complex-type N-glycan formation on secreted and membrane-bound glycoproteins in the Golgi apparatus, was already discovered more than 20 years ago (von Schaewen et al., 1993). However to date, the exact role and function of complex-type N-glycans in plants is still not known. Proteome analyses of cgl1-2 seedlings (unpublished data) revealed a strong deregulation of the plasma membrane-localized receptor kinase Brassinosteroid (BR) Insensitive1 (BRI1), which contains several N-glycosylation sites in its ectodomain. Therefore responses to brassinolide (BL, the most active BR) and/or its antagonist abscisic acid (ABA) were analyzed during seed germination and seedling establishment by comparing dormancy release, germination capacity and hypocotyl elongation of wild-type (WT) versus cgl1-2 mutant plants under different conditions and treatments. We found strong evidence, that cgl1-2 is impaired in proper BR-signal transduction, showing hypo-sensitivity towards exogenous BL and hyper-sensitivity towards exogenous as well as endogenous ABA. Compared to wild-type plants, cgl1-2 mutants displayed dose-dependent defects in seedling establishment after exogenous ABA application, while germination sensu stricto (radicle protrusion) was not significantly affected (Figure 1 a,b,f). It is known that ABA-inhibited seed germination can be rescued by BL application. Indeed, low epi-BL concentrations failed to rescue mutant seedling establishment, while WT seedling establishment was restored (Figure 1 c). Also, seedling establishment of cgl1-2 was more severely inhibited by the BR synthesis inhibitor Brassinazole (BRZ; Figure 1 e). These results demonstrate an important role of complex-type N-glycan decoration for BRI1, which should be analysed further.
Hannah Elisa Krawczyk conducted this work at the Institute for Biology and Biotechnology of Plants (IBBP), Molecular Physiology of Plants, Westfälische Wilhelms-Universität Münster in the group of Professor Dr. Dr. Antje von Schaewen. She now completes her studies at the Albrecht-von-Haller-Institute for plant sciences at Georg-August-Universität Göttingen.