The mechanics of plant cell walls depend on their composition and help determine cell shape and size. Pectins make up about a third of growing cell walls in eudicots and maintain cell wall integrity and adhesion between cells. Modifications to pectins by enzymes such as pectin methylesterases and polygalacturonases (PGs) can change the stiffness of the cell wall and/or its expansibility. PGs are glycosyl hydrolases that cleave the backbone of demethyl-esterified pectins. This is thought to mainly cause wall softening and separation, such as in fruit ripening and abscission. PGs comprise a widespread gene family that includes members in bacteria, animals, and fungi in addition to plants. In Arabidopsis thaliana there are 69 putative polygalacturonases, but the specificity and function of many of these genes are unknown. Based on prior phylogenetic analysis of PGs across several plant species, we identified PG genes of interest that show strong evolutionary conservation. In a CRISPR/Cas9 knockout of one of these genes, we observed unique flower phenotypes. Petals are reduced or absent, sepals have altered growth and are variable in number, and many flowers have fused organs. In addition, the plants grow ectopic flowers, with the most common origin being another flower. This is reminiscent of indeterminate growth, since an entire floral meristem can arise from a single flower, suggesting potential de-differentiation of cells in maturing flowers. We are examining the genetics of this mutant to explore how related genes might be affected. In addition to the flower phenotype, preliminary data shows that this knockout mutant has higher levels of PG activity. One explanation for this phenomenon could be compensation by other PGs, suggesting the existence of a signaling pathway between cell wall mechanics and transcriptional or post-transcriptional regulation of wall-modifying genes or their products.
Coauthors: Thomas McCarthy – The Pennsylvania State University;Igor Andrzhievsky – The Pennsylvania State University;Charles Anderson – The Pennsylvania State University