Complexes with Mixed Primary and Secondary Cellulose Synthases Are Functional in Arabidopsis Plants    

Abstract

In higher plants, cellulose is synthesized by so-called rosette protein complexes with cellulose synthases (CESAs) as catalytic subunits of the complex. The CESAs are divided into two distinct families, three of which are thought to be specialized for the primary cell wall and three for the secondary cell wall. In this article, the potential of primary and secondary CESAs forming a functional rosette complex has been investigated. The membrane-based yeast two-hybrid and biomolecular fluorescence systems were used to assess the interactions between three primary (CESA1, CESA3, CESA6), and three secondary (CESA4, CESA7, CESA8) Arabidopsis (Arabidopsis thaliana) CESAs. The results showed that all primary CESAs can physically interact both in vitro and in planta with all secondary CESAs. Although CESAs are broadly capable of interacting in pairwise combinations, they are not all able to form functional complexes in planta. Analysis of transgenic lines showed that CESA7 can partially rescue defects in the primary cell wall biosynthesis in a weak cesa3 mutant. Green fluorescent protein-CESA protein fusions revealed that when CESA3 was replaced by CESA7 in the primary rosette, the velocity of the mixed complexes was slightly faster than the native primary complexes. CESA1 in turn can partly rescue defects in secondary cell wall biosynthesis in a cesa8ko mutant, resulting in an increase of cellulose content relative to cesa8ko. These results demonstrate that sufficient parallels exist between the primary and secondary complexes for cross-functionality and open the possibility that mixed complexes of primary and secondary CESAs may occur at particular times.

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“…Interestingly, there were also PCW CesAs that showed similar expression patterns as pectin and xyloglucan biosynthesis genes, with high transcript abundances during SCW biosynthesis, bringing into question their PCW specificity. A functional role for PCW CesAs during SCW biosynthesis is supported by the presence of protein complexes containing both PCW and SCW CesAs in xylem cells synthesizing SCW layers (Song et al, 2010;Carroll et al, 2012). Thus, what are referred to as PCW CesA genes might have a more general role than previously thought, including the possibility that they enter into complexes with SCW CesAs.…”

Cellulose Synthesis and Its Regulation

“…Furthermore, ectopic overexpression of fra5, a dominant mutant of CESA7 affects cell wall formation in both types of walls (Zhong et al, 2003). In a genetic swap experiment, CESA7, under the expression of the CESA3 promoter, can partially complement the growth defect of cesa3 je5 , indicating that CESA7 may have structural properties allowing its incorporation into primary CSCs (Carroll et al, 2012). On the other hand, CESA1, under a promoter of a secondary CESA gene, partially complemented cesa8 irx1 null mutant phenotypes, including defects in cellulose content, plant stature and collapsed xylem vessels (Carroll et al, 2012;.…”

“…In a genetic swap experiment, CESA7, under the expression of the CESA3 promoter, can partially complement the growth defect of cesa3 je5 , indicating that CESA7 may have structural properties allowing its incorporation into primary CSCs (Carroll et al, 2012). On the other hand, CESA1, under a promoter of a secondary CESA gene, partially complemented cesa8 irx1 null mutant phenotypes, including defects in cellulose content, plant stature and collapsed xylem vessels (Carroll et al, 2012;. Interestingly, a phylogenetic analysis revealed that the separation of primary and secondary CESAs may have occurred before the appearance of vascular plants (Carroll et al, 2011).…”

Functional analysis of complexes with mixed primary and secondary cellulose synthases

“…Carroll et al reported that all primary CESAs (CESA1, CESA3 and CESA6) interacted with secondary CESAs (CESA4, CESA7 and CESA8) in a split-ubiquitin Y2H assay. 12 We cannot examined in all transgenic lines. P7C1 partially complemented several irx1 cesa8 growth defects such as dwarf stature, reduced fertility, reduced leaf size and short siliques (Fig.…”