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Characterization of Epicuticular Wax and Cuticular Features of the Sorghum Internode
Abstract
Bioenergy sorghum is a drought-tolerant high-biomass C4 grass targeted for production on annual cropland that is marginal for food crops, due primarily to abiotic constraints. To better understand the overall contribution of stem wax to bioenergy sorghum’s resilience, the current study characterized sorghum stem cuticular wax loads, composition, morphometrics, wax pathway gene expression and regulation using vegetative phase Wray (Sweet Sorghum), R07020 (Late Flowering), and TX08001 (Bioenergy) genotypes. Wax loads on sorghum stems (~103-215 µg/cm^2) were much higher than arabidopsis stem and leaf wax loads (~10 times higher). Wax on developing sorghum stem internodes was enriched in C28/30 primary alcohols (~65%) while stem wax on fully developed stems was enriched in C28/30 aldehydes (~80%). Scanning electron microscopy showed minimal wax on internodes prior to the onset of elongation and that wax tubules first appear associated with cork-silica cell complexes when internode cell elongation is complete. Wax was enriched in alkanes, then primary alcohols and finally aldehydes during stem development consistent with developmental changes in the expression of SbCER1-2 & SbCER3-2 (alkanes), SbCER4 (primary alcohols) and SbCER3-2 (aldehydes). Epicuticular wax tubule formation was closely associated with the papillae, a formation of the cork cell. A unique suberin-like compound selectively accumulated around the outer periphery of cork cells and in association with the papillae at the onset of wax tubule formation. A sorghum mutant bm40-1/2 that does not accumulate epicuticular wax or wax tubules did not accumulate the suberin-like substance on cork cells/papillae but instead the suberin-like material accumulated on epidermal cells but not cork cells. This suggests proper localization of the suberin-like compound is necessary for epicuticular wax secretion in addition to wax tubule formation. AFM-IR and fluorescence microscopy suggest that this suberin-like compound is aromatic and crosslinked to the cuticle, indicating that this is a modification of the cuticle not the cell wall. Gene regulatory network analysis targeting epicuticular wax biosynthesis and maturation aided in the identification of sorghum homologs of transcription factors that regulate wax biosynthesis (i.e., SbSHN1, SbWRI1/3, SbMYB94/96/30/60, MYS1) and other transcription factors (i.e., SbMYB73, SbMYB93) that could regulate suberin biosynthesis in epidermal cells during cuticle maturation. All of these results suggest sorghum epicuticular wax secretion is dependent upon a specialized suberin biosynthetic pathway to modify the cuticle structure.
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Citation
Chemelewski, Robert Nicholas (2023). Characterization of Epicuticular Wax and Cuticular Features of the Sorghum Internode. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /203057.