Recent Advances in Understanding the Roles of Pectin as an Active Participant in Plant Signaling Networks

Abstract

Pectin is an abundant cell wall polysaccharide with essential roles in various biological processes. The structural diversity of pectins, along with the numerous combinations of the enzymes responsible for pectin biosynthesis and modification, plays key roles in ensuring the specificity and plasticity of cell wall remodeling in different cell types and under different environmental conditions. This review focuses on recent progress in understanding various aspects of pectin, from its biosynthetic and modification processes to its biological roles in different cell types. In particular, we describe recent findings that cell wall modifications serve not only as final outputs of internally determined pathways, but also as key components of intercellular communication, with pectin as a major contributor to this process. The comprehensive view of the diverse roles of pectin presented here provides an important basis for understanding how cell wall-enclosed plant cells develop, differentiate, and interact.

Keywords: cell wall remodeling; outside-in signaling; pectin.

Figure 1

An overview of the synthesis, degradation, and modification of pectin in the plant cell wall. (A) Pectins—structurally complex polysaccharides including homogalacturonan (HG), rhamnogalacturonan I (RG-I), and rhamnogalacturonan II (RG-II)—are synthesized in the Golgi apparatus. HG is synthesized by the glucuronosyltransferase (GAUT) family of enzymes and modified by methyltransferase (MT) and acetyltransferase (AT). (B) The highly methylesterified pectin is delivered to the cell wall surface via vesicle-mediated transport and incorporated into the cell wall. HG is selectively demethylated via the wall-bound pectin methylesterases (PMEs) and deacetylated via pectin acetylesterases (PAEs). PME activity is controlled by the proteinaceous pectin methylesterase inhibitor (PMEI). Pectinolytic enzymes such as polygalacturonase (PG) and pectate lyase-like (PLL) are important for HG depolymerization, which results in the formation of oligogalacturonides (OGs). (C) Antiparallel polyuronate chains of HG form egg-box dimers with Ca²⁺, and HG backbones of the RG-II polysaccharides are covalently linked via borate diester crosslinks.

Figure 2

The roles of pectin in signaling processes during interactions with the environment. Salt stress affects the cell wall because sodium ions disrupt pectin crosslinking. The cell wall sensor FERONIA (FER), in combination with HERKULES1 (HERK1) and THESEUS1 (THE1), senses salinity through its pectin-binding extracellular domain, leading to an increase in cytosolic [Ca²⁺] and mitogen-activated protein kinase 6 (MPK6) function, which are required for salt-induced gene expression. Cold stress induces the expression of pectin methylesterase (PME) and modulates the rigidity of the cell wall via brassinosteroid (BR) signaling. BR molecules are perceived by BRI1, triggering the formation of BRI1-BAK1, which initiates an intracellular phosphorylation relay cascade and activates BZR1. Heat stress in combination with Ca²⁺ increases the rate of demethylesterification via PME activity. PME34 is involved in regulating stomatal aperture during heat stress. During pathogen infection, pectin is degraded to produce oligogalacturonides (OGs), which are detected by wall-associated kinase 1 (WAK1), leading to a strong immune response. The pathogen secretes polygalacturonase (PG), pectin lyase (PNL) and pectate lyase-like (PLL) to degrade pectin and attempt infection. It also regulates IDL6 expression and the downstream HAE/HSL2 pathways to produce pectin-degrading enzymes from the host. Question marks represent an unknown receptor, MPK, or transcription factor.