Cell adhesion in plants is under the control of putative O-fucosyltransferases

Abstract

Cell-to-cell adhesion in plants is mediated by the cell wall and the presence of a pectin-rich middle lamella. However, we know very little about how the plant actually controls and maintains cell adhesion during growth and development and how it deals with the dynamic cell wall remodeling that takes place. Here we investigate the molecular mechanisms that control cell adhesion in plants. We carried out a genetic suppressor screen and a genetic analysis of cell adhesion-defective Arabidopsis thaliana mutants. We identified a genetic suppressor of a cell adhesion defect affecting a putative O-fucosyltransferase. Furthermore, we show that the state of cell adhesion is not directly linked with pectin content in the cell wall but instead is associated with altered pectin-related signaling. Our results suggest that cell adhesion is under the control of a feedback signal from the state of the pectin in the cell wall. Such a mechanism could be necessary for the control and maintenance of cell adhesion during growth and development.

Keywords: Arabidopsis thaliana; Cell adhesion; Cell wall integrity; O-fucosyltransferases.

Fig. 1.

qua1 and qua2 primary suppressor screen. Phenotypes of (A) Col-0, qua2-1, qua2-1/esmd1-1 and (B) Ws-4, qua1-1 and qua1-1/esmd1-2 light-grown A. thaliana seedlings on a culture medium supplemented with 3% of sucrose. Among other isolated suppressor lines, the esmd1-1 mutant was isolated as a suppressor of qua2-1 (Col-0) and esmd1-2 as a suppressor of qua1-1 (Ws-4). These growth conditions were used for an efficient primary screening owing to the high sensitivity of the qua1 and qua2 mutants to high sucrose concentrations. Scale bars: 1 mm.

Fig. 2.

qua2, frb1 and esmd1 affect cell adhesion in the same pathway. (A) z-projections of confocal stacks from representative, propidium iodide-stained, 4-day-old dark-grown hypocotyls, revealing the state of cell adhesion in the different mutant lines. (B) Length of 4-day-old dark-grown hypocotyls of the mutant lines, showing the effect of loss of cell adhesion on hypocotyl elongation. Average value with standard deviation, of three biological replicates of 20 seedlings each. Scale bars: 75 μm.

Fig. 3.

Cell wall homogalacturonan (HG) content, pectin-related signaling and potential substrates of O-fucosyltransferases. (A) Galacturonic acid content (constitutive monomer of HG) measured on HG-enriched cell wall extracts from Col-0, qua2-1, qua2-1/esmd1-1 and esmd1-1. (B) Expression levels of FADLox in Col-0, qua2-1, qua2-1/esmd1-1 and esmd1-1 seedlings. Expression is fold change relative to Col-0. (A,B) Average with s.d. of three biological replicates; *P<0.05 (t-test) versus Col-0. (C) Potential substrates of ESMD1 and FRB1 as revealed by UniProtKB/Swiss-Prot database searches. Four classes of proteins contain EGF-like domains: the wall-associated kinases (WAKs); the WAK-like; the S-domain receptor-like kinases (SRKs); and the vacuolar sorting receptors (VSRs). Only the WAKs and some of the SRKs actually have the conserved site for O-fucosylation. GUB-WAK, galacturonic acid binding domain–wall-associated kinase; PA, protease-associated domain; PAN, PAN module. The drawing of each protein type/family is intended to be an average representative structure, but variations exist within families (see Table S1). Not all the proteins considered as part of these families had conserved EGF-like domains (e.g. there are 21 WAK-like proteins, but only 19 have EGF-like domains).

Fig. 4.

Model of qua, esmd1 and frb1 impact on the control of cell adhesion. Our previous understanding was that the decreased HG content in the cell wall of the quasimodo mutants was directly responsible for the loss of cell adhesion. The current work instead shows that esmd1 restores cell adhesion in the qua2 mutant without restoring the HG content in the cell wall. We also demonstrate that esmd1 and frb1 mutants have opposite effects on cell adhesion that are independent of the HG content in the cell wall. On this basis, we propose that the pectin deficiency in the quasimodo mutants is not directly responsible for the loss of cell adhesion (dashed arrow), but instead creates a signal that activates a signaling pathway leading to the loss of cell adhesion. The frb1 and esmd1 mutations affect this pathway in a positive and negative manner, respectively, thus triggering or suppressing the cell adhesion defect through as yet unidentified cell wall modifications. The blue arrows represent the previously described (dashed) and newly identified (solid) pathways implicated in cell adhesion. The red arrows illustrate the effect of the mutation on these pathways.