Microcystins modify cell wall composition and pectin methylesterase activity in plant cells

Microcystins (MCs) inhibit protein phosphatases 1 and 2A, resulting in disruption of the cytoskeleton (i.e. microtubules and F-actin). Since microtubules play a role in plant cell wall deposition and structure, MCs could affect cell wall as well, which is among the less studied putative targets of MCs. This study focused on cell wall alterations in differentiated Oryza sativa (rice) root cells, induced by either pure microcystin-LR (MC-LR) or the extracts of MC- and non-MC-producing Microcystis strains. Immunolabeling of various cell wall components, including pectins, arabinogalactan-proteins (AGPs) and hemicelluloses, was performed on semi-thin transverse sections of LR-White embedded roots (control and affected), observed by fluorescence microscopy. The distribution patterns of pectic epitopes, such as homogalacturonans (HGs) and arabinans, were altered in affected roots, while pectin methylesterase (PME) activity assays revealed that PMEs were also affected, by both pure MC-LR and cyanobacterial extracts, in a time-dependent manner. Elevated intracellular calcium levels, along with increased callose deposition, were induced after treatments. This is the first report of cyanobacterial metabolites affecting PME activity and pectin distribution in plant cell walls. The exact mechanism of toxicity (direct effect on PMEs or indirectly, possibly via the production of reactive oxygen species) is to be further investigated.