GhCASPL1 regulates secondary cell wall thickening in cotton fibers by stabilizing the cellulose synthase complex on the plasma membrane

Cotton(Gossypium hirsutum)fibers are elongated single cells that rapidly accumulate cellulose during secondary cell wall(SCW)thickening,which requires cellulose synthase complex(CSC)activity.Here,we describe the CSC-interacting factor CASPARIAN STRIP MEMBRANE DOMAIN-LIKE1(GhCASPL1),which contributes to SCW thickening by influencing CSC stability on the plasma membrane.GhCASPL1 is preferentially expressed in fiber cells during SCW biosynthesis and encodes a MARVEL domain protein.The ghcaspl1 ghcaspl2 mutant exhibited reduced plant height and produced mature fibers with fewer natural twists,lower tensile strength,and a thinner SCW compared to the wild type.Similarly,the Arabidopsis(Arabidopsis thaliana)caspl1 caspl2 double mutant showed a lower cellulose content and thinner cell walls in the stem vasculature than the wild type but normal plant morphology.Introducing the cotton gene GhCASPL1 successfully restored the reduced cellulose content of the Arabidopsis caspl1 caspl2 mutant.Detergent treatments,ultracentrifugation assays,and enzymatic assays showed that the CSC in the ghcaspl1 ghcaspl2 double mutant showed reduced membrane binding and decreased enzyme activity compared to the wild type.GhCASPL1 binds strongly to phosphatidic acid(PA),which is present in much higher amounts in thickening fiber cells compared to ovules and leaves.Mutating the PA-binding site in GhCASPL1 resulted in the loss of its colocalization with GhCesA8,and it failed to localize to the plasma membrane.PA may alter membrane structure to facilitate protein–protein interactions,suggesting that GhCASPL1 and PA collaboratively stabilize the CSC.Our findings shed light on CASPL functions and the molecular machinery behind SCW biosynthesis in cotton fibers.

The trafficking and behavior of cellulose synthase and a glimpse of potential cellulose synthesis regulators

Cellulose biosynthesis is a topic of intensive research not only due to the significance of cellulose in the integrity of plant cell walls,but also due to the potential of using cellulose,a natural carbon source,in the production of biofuels.Characterization of the composition,regulation,and trafficking of cellulose synthase complexes(CSCs)is critical to an understanding of cellulose biosynthesis as well as the characterization of additional proteins that contribute to the production of cellulose either through direct interactions with CSCs or through indirect mechanisms.In this review,a highlight of a few proteins that appear to affect cellulose biosynthesis,which includes:KORRIGAN(KOR),Cellulose Synthase-Interactive Protein 1(CSI1),and the poplar microtubule-associated protein,PttMAP20,will accompany a description of cellulose synthase(CESA)behavior and a discussion of CESA trafficking compartments that might act in the regulation of cellulose biosynthesis.