Pectin Metabolism and Assembly in the Cell Wall of the Charophyte Green Alga Penium margaritaceum

Abstract

The pectin polymer homogalacturonan (HG) is a major component of land plant cell walls and is especially abundant in the middle lamella. Current models suggest that HG is deposited into the wall as a highly methylesterified polymer, demethylesterified by pectin methylesterase enzymes and cross-linked by calcium ions to form a gel. However, this idea is based largely on indirect evidence and in vitro studies. We took advantage of the wall architecture of the unicellular alga Penium margaritaceum, which forms an elaborate calcium cross-linked HG-rich lattice on its cell surface, to test this model and other aspects of pectin dynamics. Studies of live cells and microscopic imaging of wall domains confirmed that the degree of methylesterification and sufficient levels of calcium are critical for lattice formation in vivo. Pectinase treatments of live cells and immunological studies suggested the presence of another class of pectin polymer, rhamnogalacturonan I, and indicated its colocalization and structural association with HG. Carbohydrate microarray analysis of the walls of P. margaritaceum, Physcomitrella patens, and Arabidopsis (Arabidopsis thaliana) further suggested the conservation of pectin organization and interpolymer associations in the walls of green plants. The individual constituent HG polymers also have a similar size and branched structure to those of embryophytes. The HG-rich lattice of P. margaritaceum, a member of the charophyte green algae, the immediate ancestors of land plants, was shown to be important for cell adhesion. Therefore, the calcium-HG gel at the cell surface may represent an early evolutionary innovation that paved the way for an adhesive middle lamella in multicellular land plants.

The middle lamella—more than a glue

“…Ca 2+ based cross-bridging of low esterified HG in the ML is well documented in the literature, and considered as the main mediator of its strength [19,83,100]. However, the nature of pectin-pectin interaction in the ML might be different from that at the PW-ML interface, since RG-II is absent in the ML, whereas it is present in the PW pectin [101][102][103].…”

“…The biological role of the ML has been described as early as 100 years ago [37], intercellular adhesion and separation have been studied extensively [19,[38][39][40] and excellent reviews summarize the biological aspects of this structure [7,16,18,41,42]. However, the fact that the ML represents a distinct physical layer with distinct material properties has rarely been covered in detail.…”

A family of ROP proteins that suppress actin dynamics and are essential for polarized growth and cell adhesion

“…Curiously, the pectin polymer that bears Ca 2+ cross-bridges, homo-galacturonan, is recalcitrant to extraction through Ca 2+ chelation in the gametophores of P. patens (Domozych et al, 2014) and of Selaginella moellendorffii (Harholt et al, 2012) but apparently not in P. Patens protonemata (Moller et al, 2007). When stained with fluorophores specific for cellulose, the intensity in ROP RNAi cell walls was low compared with that of control or MyoXI RNAi plants, suggesting that cellulose is present in reduced amounts in the absence of ROP proteins.…”

Microbial communities mediating algal detritus turnover under anaerobic conditions

“…2–5, Table 3). While Chlorella and Chara cell walls are quite distinct, they are both similar in being rich in fibrous cellulose and/or hemicellulose with amorphous middle layers composed mainly of pectin (homogalacturonic and rhamnogalacturonic acids polymers) in Charophyta , or sulfated polysaccharides in Chlorophyceae (Domozych, 2001; Domozych et al, 2014; Youssef et al, 2015). We reason that the relative similarity of the communities enriched on both types of algae, as well as the enrichment for multiple, rather than a single group of microbial lineages (VadinBC27, Spirochaeta , Lachnospiraceae , Buttauxiella , and Pantoea ) is a reflection of the relative ubiquity of microbial lineages capable of the anaerobic degradation of cellulose, hemicellulose, and pectin in the algal cell walls, hence allowing ready access to the intracellular substrates within the algal cells.…”