Abstract
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, W1 and W2, and wax inhibitor genes, Iw1 and Iw2. 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 W1, W2, Iw1, and Iw2 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 W genes or the presence of either Iw 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 Iw NILs. Accordingly, transcription of CER4-6, which encodes an alcohol-forming fatty acyl-CoA reductase, was elevated 120-fold in iw1Iw2. CER4-6 was transcribed at much higher levels in seedlings than in adult plants, and showed little difference between the glaucous and nonglaucous NILs, suggesting that Iw2 counteracts the developmental repression of CER4-6 at the reproductive stage. While W1 and W2 redundantly function in β-diketone biosynthesis, a combination of both functional alleles led to the β-diketone hydroxylation. Consistent with this, transcription of MAH1-9, which encodes a mid-chain alkane hydroxylase, increased seven-fold only in W1W2. In parallel with the hydroxyl-β-diketone production patterns, glaucousness was intensified and cuticle permeability was reduced significantly in W1W2 compared to the other NILs. This suggests that both W1 and W2 are required for enhancing drought tolerance.
Highlights
The aerial organs of terrestrial plants are coated with an extracellular layer of hydrophobic lipids, termed cuticle
All of the nearisogenic lines (NILs) were positive for all simple sequence repeat (SSR) markers, indicating that the mutations were not caused by chromosome deletions
We found that the W1 and W2 genes in wheat are each sufficient for the deposition of b-diketone and complement each other, but that both are required for the synthesis of hydroxyl-b-diketones
Summary
The aerial organs of terrestrial plants are coated with an extracellular layer of hydrophobic lipids, termed cuticle. The cuticle plays important roles in plant growth and development and, as the interface between sessile plants and the environments they live in, in the interaction with abiotic and biotic elements [1,2]. Based on solubility in organic solvents, the cuticle is composed of insoluble cutin and soluble cuticular wax. A cell wall-bound ester polymer of modified fatty acids and glycerol, serves as the backbone of the cuticle [3,4,5]. Wax composition varies with developmental stage, between organs, and with genetic and environmental conditions [1,4,6], causing the plant to be bluishwhite (glaucous) or nonglaucous. Glaucousness is the visible form of densely distributed epicuticular wax crystallites
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