Cellulose is the most abundant biopolymer on the planet, and this complex polysaccharide is synthesized at the plasma membrane by large multimeric cellulose synthase complexes (CSCs) containing multiple non-redundant Cellulose Synthase A (CESA) catalytic subunits.  Live-cell imaging has revealed that CSCs form motile puncta at the plasma membrane that move with a constant velocity along cortical microtubules, and the velocity of these puncta correlate with cellulose production rates.  While the CSC composition is rapidly being elucidated, it is relatively unclear how cellulose biosynthesis is regulated.  Large-scale phosphoproteomic surveys indicate that numerous CSC components, including the CESA subunits, are phosphorylated at multiple positions.  We synthesized peptides mimicking the unphosphorylated state for each of these phosphorylation events and used these peptides as substrates to identify protein kinases that catalyzed potential CSC-regulating phosphorylation events.  These assays identified BRASSINOSTEROID INSENSITIVE 2 (BIN2) kinase as a cognate protein kinase for CESA1.  BIN2 phosphorylated a peptide derived from the N-terminus of CESA1, and subsequent phosphorylation site mapping studies identified CESA1 T157 as the BIN2 phosphorylation site.  Furthermore, live-cell imaging studies indicated that CSC speeds were reduced in the hypermorphic bin2-1 background, suggesting that BIN2 is a negative regulator of cellulose biosynthesis.  Phosphonull mutations in the BIN2 phosphorylation site restored CSC speeds in the bin2-1 background and partially complemented the mutant hypocotyl elongation phenotype.  Additionally, CESA1 T157A mutants exhibited increased cell expansion compared to wild-type controls.  Overall, these results suggest that BIN2 is a critical negative regulator of cellulose biosynthesis and that CSCs which are insensitive to BIN2 phosphorylation produce more cellulose, suggesting that CSC phosphorylation events can be manipulated for enhanced cellulose production in biomass and forage feedstocks.

Brassinosteroids regulate cellulose biosynthesis in <i>Arabidopsis thaliana</i> by direct phosphorylation

253rd National Meeting (2017)

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Brassinosteroids regulate cellulose biosynthesis in Arabidopsis thaliana by direct phosphorylation