Genetic control over silica deposition in wheat awns

Awns are long, stiff filamentous extensions of glumes in many grasses. In wheat, awns contribute up to 40% of the grain's photosynthetic assimilates, and assist in seed dispersal. Awns accumulate silica in epidermal hairs and papillae, and silica has been positively associated with yield and environmental stress tolerance. Here, the awns of a set of domesticated wheat genotypes and their direct progenitor, Triticum turgidum ssp. dicoccoides were characterized. In addition, the silica concentration in awns was genetically dissected in a tetraploid wheat population of recombinant inbred lines (RILs) derived from a cross between durum wheat (cv. Langdon) and wild emmer (accession G18-16). Scanning electron micrographs revealed a continuous silica layer under the cuticle. Extended silicification was identified in the epidermis cell wall and in sclerenchyma cells near the vascular bundles, but not in the stomata, suggesting that an active process directs the soluble silica away from the water evaporation stream. The number of silicified cells was linearly correlated to silica concentration in dry weight (DW), suggesting cellular control over silicification. Domesticated wheat awns contained up to 19% silica per DW, as compared with 7% in the wild accessions, suggesting selection pressure associated with the domestication process. Six quantitative trait loci (QTLs) for silica were identified in the awns, with a LOD score of 3.7-6.3, three of which overlapped genomic regions that contribute to high grain protein. Localization of silica in the awns and identification of QTLs help illuminate mechanisms associated with silica metabolism in wheat.