Ropartz, D. | Search bioRxiv

bioRxiv2023

TBL38 is an atypical homogalacturonan acetylesterase with a peculiar cell wall microdomain localization in Arabidopsis seed mucilage secretory cells

Plant cell walls are made of complex polysaccharidic/proteinaceous network whose biosynthesis and dynamics implicate several cell compartments and impact plant development. The synthesis and remodeling of homogalacturonan pectins is associated with multiple developmental processes ranging from growth to response to biotic/abiotic stress. It encompasses Golgi-localized methylation and acetylation and subsequent demethylation and deacetylation in the cell wall. In the last decade, our comprehension of plant polysaccharides acetylation has increased significantly thanks to the study of the TRICHOME BIREFRINGENCE-LIKE (TBL) protein family. TBLs are mostly described as Golgi-localized acetyltransferases specifically targeting diverse hemicelluloses or pectins. Various tbl mutants showed altered wall mechanical properties and dynamics. Here, we study TBL38 that is co-expressed with PECTIN METHYLESTERASE INHIBITOR6 (PMEI6) and PEROXIDASE 36 (PRX36) during the development of Arabidopsis seed mucilage secretory cells (MSCs). We demonstrate the atypical TBL38 cell wall localization restricted to the PMEI6/PRX36 MSC cell wall microdomain. A tbl38 mutant displays an intriguing homogalacturonan immunological phenotype in this cell wall microdomain and in a MSC surface-enriched abrasion powder. This fraction was further characterized by mass spectrometry oligosaccharide profiling revealing an increased homogalacturonan acetylation phenotype. Finally, a recombinant TBL38 is shown to display pectin acetylesterase activity in vitro. These results indicate that TBL38 is an atypical cell wall-localized TBL that displays a homogalacturonan acetylesterase activity rather than a Golgi-localized acetyltransferase activity as observed in previously studied TBLs. TBL38 function during seed development is discussed.

plant biology↗

bioRxiv2022

The maize pathogen Ustilago maydis secretes glycoside hydrolases and carbohydrate oxidases directed towards components of the fungal cell wall

Filamentous fungi are keystone microorganisms in the regulation of many processes occurring on Earth, such as plant biomass decay, pathogenesis as well as symbiotic associations. In many of these processes, fungi secrete carbohydrate-active enzymes (CAZymes) to modify and/or degrade carbohydrates. Ten years ago, while evaluating the potential of a secretome from the maize pathogen Ustilago maydis to supplement lignocellulolytic cocktails, we noticed it contained many unknown or poorly characterized CAZymes. Here, and after re-annotation of this dataset and detailed phylogenetic analyses, we observed that several CAZymes (including glycoside hydrolases and carbohydrate oxidases) are predicted to act on the fungal cell wall (FCW), notably on {beta}-1,3-glucans. We heterologously produced and biochemically characterized two new CAZymes, called UmGH16_1-A and UmAA3_2-A. We show that UmGH16_1-A displays {beta}-1,3-glucanase activity, with a preference for {beta}-1,3-glucans with short {beta}-1,6 substitutions, and UmAA3_2-A is a dehydrogenase catalyzing the oxidation of {beta}-1,3- and {beta}-1,6-gluco-oligosaccharides into the corresponding aldonic acids. Working on model {beta}-1,3-glucans, we show that the linear oligosaccharide products released by UmGH16_1-A are further oxidized by UmAA3_2-A, bringing to light a putative biocatalytic cascade. Interestingly, analysis of available transcriptomics data indicates that both UmGH16_1-A and UmAA3_2-A are co-expressed, only during early stages of U. maydis infection cycle. Altogether, our results suggest that both enzymes are connected and that additional accessory activities still need to be uncovered to fully understand the biocatalytic cascade at play and its physiological role. ImportanceFilamentous fungi play a central regulatory role on Earth, notably in the global carbon cycle. Regardless of their lifestyle, filamentous fungi need to remodel their own cell wall (mostly composed of polysaccharides) to grow and proliferate. To do so, they must secrete a large arsenal of enzymes, most notably carbohydrate-active enzymes (CAZymes). However, research on fungal CAZymes over past decades has mainly focused on finding efficient plant biomass conversion processes while CAZymes directed at the fungus itself have remained little explored. In the present study, using the maize pathogen Ustilago maydis as model, we set off to evaluate the prevalence of CAZymes directed towards the fungal cell wall during growth of the fungus on plant biomass and characterized two new CAZymes active on fungal cell wall components. Our results suggest the existence of a biocatalytic cascade that remains to be fully understood.

biochemistry↗