High-order mutants reveal an essential requirement for peroxidases but not laccases in Casparian strip lignification

The invention of lignin has been at the heart of plants capacity to colonize land, allowing them to grow tall, transport water within their bodies and protect themselves against various stresses. Consequently, this polyphenolic polymer, that impregnates the cellulosic plant cell walls, now represents the second most abundant polymer on Earth, after cellulose itself. Yet, despite its great physiological, ecological and economical importance, our knowledge of lignin biosynthesis in vivo, especially the crucial last steps of polymerization within the cell wall, remains vague. Specifically, the respective roles and importance of the two main polymerizing enzymes classes, laccases and peroxidases have remained obscure. One reason for this lies in the very high numbers of laccases and peroxidases encoded by 17 and 73 homologous genes, respectively, in the Arabidopsis genome. Here, we have focused on a specific lignin structure, the ring-like Casparian strips (CS) within the endodermis of Arabidopsis roots. By reducing the number of possible candidate genes using cellular resolution expression and localization data and by boosting the levels of mutants that can be stacked using CRISPR/Cas9, we were able to knock-out more than half of all laccases in the Arabidopsis genome in a nonuple mutant - abolishing the vast majority of laccases with detectable endodermal-expression. Yet, we were unable to detect even slight defects in CS formation. By contrast, we were able to induce a complete absence of CS formation in a quintuple peroxidase mutant. Our findings are in stark contrast to the strong requirement of xylem vessels for laccase action and indicate that lignin in different cell types can be polymerized in very distinct ways. We speculate that cells lignify differently depending on whether they deposit lignin in a localized or ubiquitous fashion, whether they stay alive during and after lignification as well as the composition of the cell wall.