Genetic Interactions Underlying the Biosynthesis and Inhibition of β-Diketones in Wheat and Their Impact on Glaucousness and Cuticle Permeability

<div><p>Cuticular wax composition greatly impacts plant responses to dehydration. Two parallel pathways exist in Triticeae for manipulating wax composition: the acyl elongation, reduction, and decarbonylation pathway that is active at the vegetative stage and yields primary alcohols and alkanes, and the β-diketone pathway that predominates at the reproductive stage and synthesizes β-diketones. Variation in glaucousness during the reproductive stage of wheat is mainly controlled by the wax production genes, <em>W1</em> and <em>W2</em>, and wax inhibitor genes, <em>Iw1</em> and <em>Iw2</em>. Little is known about the metabolic and physiological effects of the genetic interactions among these genes and their roles in shifting wax composition during plant development. We characterized the effect of <em>W1, W2, Iw1,</em> and <em>Iw2</em> and analyzed their interaction using a set of six near-isogenic lines (NILs) by metabolic, molecular and physiological approaches. Loss of functional alleles of both <em>W</em> genes or the presence of either <em>Iw</em> gene depletes β-diketones and results in the nonglaucous phenotype. Elimination of β-diketones is compensated for by an increase in aldehydes and primary alcohols in the <em>Iw</em> NILs. Accordingly, transcription of <em>CER4-6</em>, which encodes an alcohol-forming fatty acyl-CoA reductase, was elevated 120-fold in <em>iw1Iw2</em>. <em>CER4-6</em> was transcribed at much higher levels in seedlings than in adult plants, and showed little difference between the glaucous and nonglaucous NILs, suggesting that <em>Iw2</em> counteracts the developmental repression of <em>CER4-6</em> at the reproductive stage. While <em>W1</em> and <em>W2</em> redundantly function in β-diketone biosynthesis, a combination of both functional alleles led to the β-diketone hydroxylation. Consistent with this, transcription of <em>MAH1-9</em>, which encodes a mid-chain alkane hydroxylase, increased seven-fold only in <em>W1W2</em>. In parallel with the hydroxyl-β-diketone production patterns, glaucousness was intensified and cuticle permeability was reduced significantly in <em>W1W2</em> compared to the other NILs. This suggests that both <em>W1</em> and <em>W2</em> are required for enhancing drought tolerance.</p>