Aberrant growth and expansion in <i>Penium margaritaceum</i> triggered by disruption of microtubules and the cell wall

Penium margaritaceum, a unicellular zygnematophyte (Streptophyta), was employed to elucidate changes in cell expansion when cells were challenged with the fungal pectinolytic enzyme, pectate lyase, and/or the microtubule-disrupting agent, amiprophos-methyl (APM). Microtubule disruption by APM resulted in significant swelling at expansion zones. These swollen zones provided an easy marker for the location of expansion zones, particularly in cells with altered cell wall pectin. Short-term treatment with pectate lyase showed pectin degradation primarily at the isthmus expansion zone and two satellite bands, corresponding to the location of future expansion in daughter cells. When the homogalacturonan lattice of the cell wall was removed by treatment with pectate lyase during long treatments, cell division was maintained, but daughter cell products were considerably smaller. Treatment of cells with a mixture of both pectate lyase and APM resulted in a distinct phenotype, consisting of 'dumbbell'-shaped cells, as APM-induced swelling occurs at the novel expansion centers exposed by pectate lyase treatment. These cells also presented other curious alterations, including an extensive, chloroplast-free cytoplasmic zone at the center of the cell, a septum containing β-glycan, arabinogalactan and homogalacturonan epitopes, unique stacks of endoplasmic reticulum, displaced Golgi bodies, and an extensive network of vacuoles. These results provide insight into the importance of cell wall integrity in defining the location of cell growth and division in P. margaritaceum. Understanding these processes in a unicellular zygnematophyte may provide insights into steps involved in the evolution of land plants.