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A co-opted steroid synthesis gene, maintained in sorghum but not maize, is associated with a divergence in leaf wax chemistry
Monday, 2021/03/29 | 08:43:17
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Lucas Busta, Elizabeth Schmitz, Dylan K. Kosma, James C. Schnable, and Edgar B. Cahoon PNAS March 23, 2021 118 (12) e2022982118 SignificanceVirtually all above-ground plant surfaces, such as leaf and stem exteriors, are covered in a cuticle: a wax-infused polyester. This waxy biocomposite is the largest interface between Earth’s biosphere and atmosphere. Its chemical composition is not only highly tuned to mediate nonstomatal water loss, but it also self-assembles to produce superhydrophobic surfaces, protects against UV radiation, and contains bioactive compounds that help resist microbial attack. Developing fundamental knowledge of waxy biocomposites, particularly those on crop species, is a prerequisite for an understanding of their structure–function relationships. Here, we uncover a likely genetic basis for the presence and absence, respectively, of triterpenoids in the leaf waxes of sorghum and maize—compounds previously associated with creating heat-tolerant cuticular water barriers. AbstractVirtually all land plants are coated in a cuticle, a waxy polyester that prevents nonstomatal water loss and is important for heat and drought tolerance. Here, we describe a likely genetic basis for a divergence in cuticular wax chemistry between Sorghum bicolor, a drought tolerant crop widely cultivated in hot climates, and its close relative Zea mays (maize). Combining chemical analyses, heterologous expression, and comparative genomics, we reveal that: 1) sorghum and maize leaf waxes are similar at the juvenile stage but, after the juvenile-to-adult transition, sorghum leaf waxes are rich in triterpenoids that are absent from maize; 2) biosynthesis of the majority of sorghum leaf triterpenoids is mediated by a gene that maize and sorghum both inherited from a common ancestor but that is only functionally maintained in sorghum; and 3) sorghum leaf triterpenoids accumulate in a spatial pattern that was previously shown to strengthen the cuticle and decrease water loss at high temperatures. These findings uncover the possibility for resurrection of a cuticular triterpenoid-synthesizing gene in maize that could create a more heat-tolerant water barrier on the plant’s leaf surfaces. They also provide a fundamental understanding of sorghum leaf waxes that will inform efforts to divert surface carbon to intracellular storage for bioenergy and bioproduct innovations.
See: https://www.pnas.org/content/118/12/e2022982118
Figure 2 Coverage and composition of wax on juvenile and adult leaves of S. bicolor. (A) Bar chart showing total wax coverage (in μg/cm2; y axis) on leaves produced by juvenile (Left bar) and adult (Right bar) S. bicolor plants. Each bar is given a border to reflect plant maturity, where those colored gray correlate to data for juvenile plants and those bordered black are adult plants. (B) Bar chart showing the relative abundance (percent total wax from that leaf; y axis) of each compound class (x axis) present on leaves produced by juvenile (Top, gray bordered bars) and adult (Bottom, black bordered bars) S. bicolor plants. Bars are color filled according to compound class, and as defined in Fig. 1A: yellow: alkanes, blue: aldehydes, orange: triterpenoids, red: primary alcohols, purple: methyl alkylresorcinols, pink: fatty acids, and gray: unidentified. (C) Bar chart showing the relative abundance (percent total wax from that leaf; y axis) of each compound identified on juvenile (Top, gray bordered bars) and adult (Bottom, black bordered bars) leaves of S. bicolor. Numbers below each bar indicate the total carbon number of that compound, and the text below each group of bars indicates the compound class represented by those bars. The chemical structures of these compounds are presented in Fig. 1B. Throughout, bar heights and error bars represent the mean and SE of five independent samples, respectively. |
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