Managing pectin: maintenance of cell wall integrity in the streptophyte alga Penium margaritaceum

Plants continuously monitor the integrity of their cell walls and in coordination with diverse signalling networks the cell wall will chemically and structurally modulate in response to developmental cues and environmental stress. Pectins, including homogalacturonan, have been shown to be key stress signal receptors, signal transducers and modulating polymers. In this study, we used the unicellular streptophyte alga Penium margaritaceum and its unique homogalacturonan lattice to monitor the effects of exogenous pectin esterases. The aims of this study were to experimentally investigate Penium's homogalacturonan lattice with regard to changes in its cell wall and phenotype and to examine the mechanisms of lattice regeneration and repair.

Microplate assays of pectin esterases with live cells over various time sequences were employed. Alterations to cell wall architecture and lattice regeneration and repair were analysed by immunocytochemistry, field emission scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy.

The architecture of the homogalacturonan lattice is notably malleable during incubation with exogenous pectin methylesterase, pectin acetylesterase, pectin methylesterase and epigallocatechin gallate. These also resulted in alterations to expansion dynamics, cell morphology and calcium content of the lattice and altered remnants. Upon recovery, the alga activates both bipolar and unipolar lattice regeneration, and often a distinct lattice repair mechanism.

The results of this study provide new insight into the mechanisms of stress-related cell wall integrity management of extant zygnematophytes and innovations to pectin management that may have been critical to their ancestors during terrestrialization ∼500+ million years ago.