Abstract
Arabidopsis thaliana CELLULOSE SYNTHASE A4/7/8 (CESA4/7/8) are three non-redundant subunits of the secondary cell wall cellulose synthase complex. Transcript abundance of these genes can vary among genotypes and expression quantitative trait loci (eQTL) were identified in a recombinant population of the accessions Bay-0 and Shahdara. Genetic mapping and analysis of the transcript levels of CESAs between two distinct near isogenic lines (NILs) confirmed a change in CESA4 expression that segregates within that interval. We sequenced the promoters and identified 16 polymorphisms differentiating CESA4Sha and CESA4Bay. In order to determine which of these SNPs could be responsible for this eQTL, we screened for transcription factor protein affinity with promoter fragments of CESA4Bay, CESA4Sha, and the reference genome CESA4Col. The wall thickening activator proteins NAC SECONDARY WALL THICKENING PROMOTING FACTOR2 (NST2) and NST3 exhibited a decrease in binding with the CESA4Sha promoter with a tracheary element-regulating cis-element (TERE) polymorphism. While NILs harboring the TERE polymorphisms exhibited significantly different CESA4 expression, cellulose crystallinity and cell wall thickness were indistinguishable. These results suggest that the TERE polymorphism resulted in differential transcription factor binding and CESA4 expression; yet A. thaliana is able to tolerate this transcriptional variability without compromising the structural elements of the plant, providing insight into the elasticity of gene regulation as it pertains to cell wall biosynthesis and regulation. We also explored available DNA affinity purification sequencing data to resolve a core binding site, C(G/T)TNNNNNNNA(A/C)G, for secondary wall NACs referred to as the VNS element.
Highlights
While a primary cell wall surrounds all plant cells, a secondary cell wall is found in xylem cells responsible for water transportation, structural fibers, and cells that serve as an outside barrier to the external environment
There was a continuous range of values and normal distributions were observed for the ten Cellulose Synthase A (CESA) genes known to play a role in the biosynthesis of cellulose in secondary cell walls (Figure 1A and Supplementary Figure 1)
We investigated the variable expression of CESA4 in a recombinant inbred line (RIL) population of A. thaliana
Summary
While a primary cell wall surrounds all plant cells, a secondary cell wall is found in xylem cells responsible for water transportation, structural fibers, and cells that serve as an outside barrier to the external environment These thick and relatively inflexible walls are composed of a complex of cellulose, hemicelluloses, and the polyphenolic polymer lignin. Cellulose chains coalesce in parallel to form a single microfibril via hydrogen bonding and van der Waals forces. Depending on their density and nature of the commingling polymers and linkages, microfibrils can contribute to a matrix that ranges from fairly elastic to extremely rigid. Understanding the regulation of secondary cell wall composition, especially the effects of natural genetic variation, will facilitate enhanced gene modification and plant breeding for more efficient biomass production
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