Emonet, A. | Search bioRxiv

bioRxiv2020

Spatially Restricted Immune Responses Allow for Root Meristematic Activity During Bacterial Colonisation

Plants circumscribe microbe-associated molecular pattern (MAMP)-triggered immune responses to weak points of the roots. This spatially restricted immunity was suggested to avoid constitutive responses to rhizosphere microbiota. To demonstrate its relevance, we combined cell-type specific expression of the plant flagellin receptor (FLS2) with fluorescent defence markers and mapped immune competency at cellular resolution. Our analysis distinguishes cell-autonomous and non-cell autonomous responses and reveals lignification to be tissue-independent, contrasting cell-type specific suberisation. Importantly, our analysis divides the non-responsive meristem into a central zone refractory to FLS2 expression, and a cortex that becomes highly sensitised by FLS2 expression, causing meristem collapse upon MAMP exposure. Meristematic epidermal expression generates super-competent lines that detect native bacterial flagellin and bypass the absence of response to commensals, providing a powerful tool for studying root immunity. Our precise manipulations and read-outs demonstrate incompatibility of meristematic activity and defence and the importance of cell-resolved studies of plant immunity.

plant biology↗

bioRxiv2020

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.

plant biology↗