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

Abstract

Lignin has enabled plants to colonize land, grow tall, transport water within their bodies, and protect themselves against various stresses. Consequently, this polyphenolic polymer, impregnating cellulosic plant cell walls, is the second most abundant polymer on Earth. Yet, despite its great physiological, ecological, and economical importance, our knowledge of lignin biosynthesis in vivo, especially the polymerization steps within the cell wall, remains vague—specifically, the respective roles of the two polymerizing enzymes classes, laccases and peroxidases. One reason for this lies in the very high numbers of laccases and peroxidases encoded by 17 and 73 homologous genes, respectively, in Arabidopsis. Here, we have focused on a specific lignin structure, the ring-like Casparian strips (CSs) within the root endodermis. By reducing candidate numbers using cellular resolution expression and localization data and by boosting stacking of mutants using CRISPR-Cas9, we mutated the majority of laccases in Arabidopsis in a nonuple mutant—essentially abolishing 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 lignin is localized or ubiquitous and whether cells stay alive during and after lignification, as well as the composition of the cell wall.

Two chemically distinct root lignin barriers control solute and water balance

“…The lack of compensatory cell-corner lignification in sgn3-3 myb36-2 could explain the complete permeability of the roots in the double mutant. This finding supports recent observations assigning a role as an apoplastic barrier to SGN3-dependent cell-corner lignification 14 . In addition, constitutive activation of the Schengen-pathway is also known to trigger an enhanced suberisation in certain CS mutants, including myb36 17 .…”

“…CS lignin encircles each endodermal cell, forming a bridge between them. This precise lignin deposition is defined by the presence of the transmembrane Casparian strip membrane domain proteins (CASPs) 12 , peroxidases 13 , 14 and the dirigent-like protein ESB1 15 . The expression of this lignin polymerization machinery is tightly controlled by the transcription factor MYB36 16 , 17 .…”

Phenoloxidases in Plants—How Structural Diversity Enables Functional Specificity

“…One of the main proposed roles of PRXs is during lignin formation to oxidise secreted lignin phenolic monomers in specific cell wall layers of distinct cell types (Herrero et al, 2013;Shigeto et al, 2013). PRXs are the main phenoloxidase responsible of the lignification of the casparian strip in endodermal cells of A. thaliana (Lee et al, 2013;Rojas-Murcia et al, 2020). Ectopic lignin formation in the cell walls of flax bast fibres (Chantreau et al, 2014) and in the extracellular medium of Norway spruce cell cultures (Laitinen et al, 2017) also depend on PRXs.…”

“…At the whole plant level, LACs are mainly expressed in the different lignified tissues. In A. thaliana, AtLAC4, 5, 10, 12, and 17 are expressed in vascular bundles (Turlapati et al, 2011) and co-regulated with secondary cell wall formation in tracheary elements (Derbyshire et al, 2015) whereas AtLAC1, 3, 5, and 13 are expressed in endodermal cells (Rojas-Murcia et al, 2020). AtLAC5 (Yonekura-Sakakibara et al, 2020) and AtLAC15 (Turlapati et al, 2011) which catalyse the formation of neolignans and proanthocyanidins respectively are strongly expressed in seed coats.…”

Proteomic Analysis: Explosive Salt Accumulation in Leaves of Morus alba L. under Salt Stress

“…After binding with SGN3, it phosphorylates SGN1, and then activates RBOHF to induce the production of local reactive oxygen species. The oxidative cross-linking of the lignin monomer is mediated by peroxidase [71][72][73]. In this study, the expression patterns of PERs were different, which may be due to the tissue-specific lignification under salt stress.…”