CELLULOSE SYNTHASE9 Serves a Nonredundant Role in Secondary Cell Wall Synthesis in Arabidopsis Epidermal Testa Cells      

Abstract

Herein, we sought to explore the contribution of cellulose biosynthesis to the shape and morphogenesis of hexagonal seed coat cells in Arabidopsis (Arabidopsis thaliana). Consistent with seed preferential expression of CELLULOSE SYNTHASE9 (CESA9), null mutations in CESA9 caused no change in cellulose content in leaves or stems, but caused a 25% reduction in seeds. Compositional studies of cesa9 seeds uncovered substantial proportional increases in cell wall neutral sugars and in several monomers of cell wall-associated polyesters. Despite these metabolic compensations, cesa9 seeds were permeable to tetrazolium salt, implying that cellulose biosynthesis, via CESA9, is required for correct barrier function of the seed coat. A syndrome of depleted radial wall, altered seed coat cell size, shape, and internal angle uniformity was quantified using scanning electron micrographs in cesa9 epidermal cells. By contrast, morphological defects were absent in cesa9 embryos, visually inspected from torpedo to bent cotyledon, consistent with no reduction in postgermination radical or hypocotyl elongation. These data implied that CESA9 was seed coat specific or functionally redundant in other tissues. Assessment of sections from glutaraldehyde fixed wild-type and cesa9 mature seeds supported results of scanning electron micrographs and quantitatively showed depletion of secondary cell wall synthesis in the radial cell wall. Herein, we show a nonredundant role for CESA9 in secondary cell wall biosynthesis in radial cell walls of epidermal seed coats and document its importance for cell morphogenesis and barrier function of the seed coat.

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

“…Overexpression of a mutant allele of the Arabidopsis CESA7 gene, named fra5, resulted in changes in cellulose synthesis during primary wall formation (reduced thickness of the cell wall and cell elongation) as well as causing a dominant-negative effect on cellulose synthesis during secondary wall formation (Zhong et al, 2003), as was also suggested in the case of the widely recognized secondary wall-specific AtCESA7 (MUR10), being required for normal primary cell wall carbohydrate composition in mature leaves, normal plant growth, hypocotyl strength, and fertility (Bosca et al, 2006). Another study shows that despite CESA9 having already been classified as a primary cell wall CESA (Desprez et al, 2007;Persson et al, 2007), a nonredundant role was shown in secondary cell wall thickening in the seed coat (Stork et al, 2010). The rice (Oryza sativa) brittle culm11 mutant has shown both altered primary (increased callose, pectic arabinan, and xylan) and secondary (brittleness of the culm, abnormal secondary structure, decreased wall thickness, and reduced cellulose content) wall composition (Zhang et al, 2009), further supporting the possibility of cross-talk and overlapping functions between the primary and secondary CESAs.…”

CELLULOSE SYNTHASE-LIKE A2, a Glucomannan Synthase, Is Involved in Maintaining Adherent Mucilage Structure in Arabidopsis Seed

“…S2). As viewed by scanning electron microscopy (SEM), Arabidopsis seed coat epidermal cells appear as hexagonal epidermal cells outlined by reinforced radial cell walls with a volcanoshaped columella (Beeckman et al, 2000;Western et al, 2000;Stork et al, 2010). Inspection of mature dry seeds of the three mutant lines revealed no alteration in radial cell walls, columella, or mucilage pockets around the columella (Fig.…”

Functional analysis of complexes with mixed primary and secondary cellulose synthases

“…15,[17][18][19] The molecular details of the primary to secondary cell wall transition and mechanisms that underlie the cross talk between primary and secondary cell wall biosynthesis are largely unknown.…”

“…13 Another example came from the most recent finding that primary CESAs have function in the formation of secondary cell walls during seed coat development. 14,15 Specifically, CESA5 is responsible for mucilage attachment and CESA2, 5 and 9 contribute to secondary cell wall biosynthesis in the columella cells. 16 These findings suggest that the distinction between primary and secondary CESAs might not be as strict as initially defined.…”