Pectin signaling in responses to heavy metals and pathogens

Cell walls of growing plant organs have the remarkable property to combine resistance to extreme tensional stresses as a result of the turgor pressure in the cell, with the ability to extend during cell expansion. This property must be maintained in widely varying abiotic and biotic environments imposed by the sessile lifestyle of plants. The pectic polysaccharide homogalacturonan (HG) is a major component of the primary cell wall representing up to 20% of the cell wall dry weight. HG is deposited in the wall in a highly methylesterified form. The exposure of free carboxyl groups upon selective de- methylesterification dramatically changes the rheological properties of the cell wall, in part through the formation of Ca2+-pectate crosslinks, but also leads to the production of signaling molecules. Recent results in the laboratories of the 3 partners in Austria and France converge into a common working hypothesis in which pectate levels in the cell wall are precisely monitored and adjusted during cell expansion, in the presence of heavy metals that interfere with Ca2+-pectate crosslink formation and at early infection stages of cell wall-degrading pathogens. Our evidence suggests that pectate monitoring involves at least three wall integrity signaling pathways: (1) a brassinosteroid receptor (BRI1)-mediated pathway, (2) a wall-associated kinase (WAK)-mediated pathway and (3) a pectate-binding THESEUS1 receptor kinase signaling pathway. The adjustment of pectate levels involves large families of pectin methyl esterases (PME), PME-inhibitors (PMEI), pectin acetyl esterases (PAE), and pectate hydrolyzing enzymes such as pectate lyases (PL), and polygalacturonases (PG). Expression patterns and, most likely, enzymatic properties differ for different family members, which have evolved to cope with distinct developmental and environmental constraints. The project PECTOSIGN aims at understanding changes in pectin metabolism in growing cells upon exposure to heavy metals and during early stages of pathogen infection, and how these perturbations are detected. Therefore (1) tools will be established for the analyses and manipulation of HG metabolism and (2) the role and identity of pectin and its metabolites in binding and signaling heavy metal responses and (3) pathogen attack will be determined. (4) Finally the specificity and interaction of the signaling pathways, their feedback regulation and downstream components will be revealed. PECTOSIGN will not only provide fundamental insights into the emerging research area of cell wall integrity signaling and its role in growth control in varying environments, but may also lead to new strategies for phytoremediation and the protection of crops against abiotic stresses and pathogens.

The plant cell wall as a signaling platform for metal ion triggered growth control Plant cells are surrounded by cell walls consisting of polysaccharides, phenolic compounds and proteins. The walls of expanding cells are strong to resist turgor but also dynamic since they are constantly remodeled to allow growth. The pectic polysaccharide homogalacturonan (HG) is a major component of the primary cell wall representing up to 20% of the cell wall dry weight. Homogalacturonan (HG) is deposited in the wall in a highly methylated and uncharged form. Pectin modifying enzymes such as pectin methylesterases (PME) selectively remove methyl groups, thus exposing negative charges, which dramatically alter the rheological properties of the polymer. PECTOSIGN set out to investigate how pectin metabolism is connected to the adaptation of plants to heavy metals stresses and whether cell wall integrity sensors are involved. The PECTOSIGN project used molecular genetics and CRISPR/Cas9 gene editing with the model plant Arabidopsis thaliana, performed in vivo manipulation of pectin composition, characterized these with a novel spectrometry-based method and characterized growth responses with kinematic analysis. Using these methods, the PECTOSIGN project sustained that the cell wall acts as a signaling platform that senses the environment in form of excess of heavy metal ions and transmits signals to the cell's interior, triggering adaptive growth responses. In the PECTOSIGN project, we identified several members of a family of receptor-like kinases to be involved in the signal transmission and tolerance to certain heavy metals. We also found that this signaling depends on the metabolism of charged unmethylated homogalacturonan (HG) and revealed that heavy metal ions activate pectin modifying enzymes leading to increased pectin demethylation and a higher tolerance towards different heavy metals. PECTOSIGN has not only provide fundamental insights into the emerging research area of cell wall integrity signaling and its role in growth control in varying environments. The results have the also potential leading to new strategies for selecting crops tolerant to heavy metals.