Polygalacturonase activity promotes aberrant cell separation in the quasimodo2 mutant of Arabidopsis thaliana

In plants, cell adhesion relies on balancing the integrity of the pectin-rich middle lamella with wall loosening during tissue expansion. Mutation of QUASIMODO2 (QUA2), a pectin methyltransferase, causes defective hypocotyl elongation and cell adhesion in Arabidopsis thaliana hypocotyls. However, the molecular function of QUA2 in cell adhesion is obscured by complex genetic and environmental interactions. To dissect the role of QUA2 in cell adhesion, we investigated a qua2 loss-of-function mutant and a suppressor mutant with restored cell adhesion, qua2 esmeralda1, using a combination of imaging and biochemical techniques. We found that qua2 hypocotyls have reductions in middle lamellae integrity, pectin methyl-esterase (PME) activity, pectin content and molecular mass, and immunodetected Ca(2+)-crosslinking at cell corners, but increased methyl-esterification and polygalacturonase (PG) activity, with qua2 esmd1 having wild type-like or intermediate phenotypes. Our findings suggest that excessive pectin degradation prevents pectin accumulation and the formation of a sufficiently Ca(2+)-crosslinked network to maintain cell adhesion in qua2 mutants. We propose that PME and PG activities balance tissue-level expansion and cell separation. Together, these data provide insight into the cause of cell adhesion defects in qua2 mutants and highlight the importance of harmonizing pectin modification and degradation during plant growth and development.