Golgi-Localized Mannanases Sustain Hemicellulose Biosynthesis
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Mannans with β-1,4-linked backbones are common cell wall components of algae and land plants. Prior challenges to enhance β-mannan content in plants point to unclear metabolic bottlenecks and the potential for hidden biosynthetic players.
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Endo-β-MANNANASEs (MANs) are the main glycosyl hydrolases that mobilize extracellular β-mannans during seed germination. However, we found that Arabidopsis man2 man5 seeds resemble β-mannan biosynthetic mutants.
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CELLULOSE SYNTHASE-LIKE A (CSLA) overexpression restored β-mannan synthesis in the man double mutant and increased the distribution of crystalline polymers, but impaired the release of other mucilaginous polysaccharides.
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Using yeast synthetic biology, we dissected the functional interplay of MAN enzymes with CSLAs. Intracellular MAN2 and MAN5 reduced the quantity of insoluble β-mannan but elevated the content of water-soluble carbohydrates.
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We propose that Arabidopsis MAN2/5, and orthologous crop enzymes with a transmembrane domain, sustain hemicellulose production in the Golgi apparatus by cleaving insoluble β-mannan polymers into hydrophilic counterparts.
