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COBRA-like4 : a GPI-anchored protein functioning as a mediator of cellulose ultrastructure in herbaceous and woody plants McNair, Grant

Abstract

Cellulose biosynthesis is a dynamic and specialized cellular process with multiple layers of organization. This abundant, vital polymer is synthesized by cellulose synthase complexes (CSCs) localized at the plasma membrane. Cellulose chains are extruded into the apoplast, and rapidly self-assemble into microfibrils. The mechanisms controlling organization of the product, cellulose microfibrils, are still unclear. The GPI-anchored protein COBRA (COB), localized at the outer leaflet of the plasma membrane, is required for normal cellulose deposition in primary cell walls. A closely related protein, COBRA-LIKE4 (COBL4), is required for secondary cell cellulose organization. Loss-of-function, in Arabidopsis cobl4 mutants originally called irregular xylem 6 (irx6), results in reduced cellulose content, cellulose of lower crystallinity, and thinner secondary cell walls. To better understand COBL4 function, I investigated the chemical and ultrastructural properties of novel irx6-2 and irx6-3 alleles of Arabidopsis. I followed this up by demonstrating functional conservation between COBL4 in woody (Populus trichocarpa) and herbaceous (Arabidopsis) species. A fluorescently labelled poplar COBL4, PtCOB4a, was co-localized with secondary cell wall thickenings in an inducible Arabidopsis protoxylem experimental system. To further refine our understanding the molecular role of COBL4, AtCOBL4 was over-expressed in hybrid poplar, in a secondary cell wall specific manner. Increased AtCOBL4 abundance did not significantly alter cell wall derived glucose content compared to control plants; this was confirmed by the absence of a significant increase in α-cellulose. The ultra-structural characteristics of deposited cellulose, specifically cellulose DP and cellulose crystallinity, were significantly increased in a number of over expression lines relative to control trees. These findings confirm COBL4 as a protein involved in organizing cellulose biosynthesis in plants. The increased cellulose DP and subsequent proportion of crystalline cellulose suggest that COBL4, in part, affects cellulose biosynthesis efficiency. To further resolve the role that cellulose ultrastructure plays in limiting intrusive tip growth of fibre cells, we measured xylary fibre lengths of AtCOBL4 overexpression poplar lines. Overexpression lines had on average shorter fibres than wild-type trees. This demonstrates that increased DP and the overall structural organization of cellulose, mediated by AtCOBL4, may be sufficient to restrict intrusive growth of fibre cells.

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Attribution-NonCommercial-NoDerivs 2.5 Canada

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