Toward "Smart Canopy" Sorghum: Discovery of the Genetic Control of Leaf Angle Across Layers

Abstract

A "smart canopy" ideotype has been proposed with leaves being upright at the top and more horizontal toward the bottom of the plant to maximize light interception and conversion efficiencies, and thus increasing yield. The genetic control of leaf angle has, to date, been studied on one or two leaves, or data have been merged from multiple leaves to generate average values. This approach has limited our understanding of the diversity of leaf angles across layers and their genetic control. Genome-wide association studies and quantitative trait loci mapping studies in sorghum (Sorghum bicolor) were performed using layer-specific angle data collected manually and via high-throughput phenotyping strategies. The observed distribution of angles in indoor and field settings is opposite to the ideotype. Several genomic regions were associated with leaf angle within layers or across the canopy. The expression of the brassinosteroid-related transcription factor BZR1/BES1 and the auxin-transporter Dwarf3 were found to be highly correlated with the distribution of angles at different layers. The application of a brassinosteroid biosynthesis inhibitor could not revert the undesirable overall angle distribution. These discoveries demonstrate that the exploitation of layer-specific quantitative trait loci/genes will be instrumental to reversing the natural angle distribution in sorghum according to the "smart canopy" ideotype.

Figure 1.

Leaf angle distribution throughout the canopy and estimation of plot-based plant width (PPW) as a proxy for leaf angle. A, Natural leaf angle distribution throughout the canopy on diverse sorghum genotypes under field and controlled conditions. B, Original red-green-blue (RGB) images and derived 3D reconstruction of a sorghum plot with the independent estimation of PPW for each one-third of the canopy. Different colors indicate point clouds from each of the three sets of stereo cameras on the automated platform Phenobot 1.0 (Salas-Fernandez et al., 2017).

Figure 2.

Genome-wide associations for UPPW, MPPW, LPPW, and validated regions by coincident QTL discovered using three independent biparental populations. Validated regions are indicated by color-shaded areas. Colors indicate the QTL identifiers with a physical position coincident with an associated region by GWAS.

Figure 3.

Common QTL across leaves controlling leaf angle in three biparental populations (Pop.1, Pop.2, and Pop.3). A to C, Sections correspond to the QTL-controlling angle of PFL, L4, and L5 (A), PFL and L4 (B), and L4 and L5 (C).

Figure 4.

Leaf-specific QTL controlling leaf angle in three biparental populations (Pop.1, Pop.2, and Pop.3). A to C, Sections correspond to QTL specific for PFL (A), L4 (B), and L5 (C). “Pop.X-dw3” indicates QTL discovered using the Dw3 gene as a covariate in the analysis.

Figure 5.

Leaf angle and relative normalized expression of BZR1/BES1 and Dw3 by RT-qPCR in different genetic backgrounds under control conditions. Angles (gray bars) increase from L5 to the PFL leaf. The expression of BZR1/BES1 (blue) and Dw3 (red) also increase, as the plant develops, from the lower to the upper canopy. Error bars indicate se (n = 3).

Figure 6.

Phenotypic and gene expression response to the BR inhibitor pcz. A, Leaf angle under control and pcz treatment. Asterisks indicate significance by ANOVA (**P < 0.01). B, RT-qPCR expression profiles of BZR1/BES1 and Dw3 under treatment calculated as 2-ΔCT. Pearson correlation values were calculated between expression levels of each gene (indicated by colors) and the corresponding leaf angle after pcz treatment. C, RT-qPCR expression profiles of BZR1/BES1 and Dw3 under treatment relative to control calculated as 2^−ΔΔCT. Negative values indicate lower expression than under control conditions. C5, C8, and CPFL (leaves counted from bottom to top). Trt., pcz treatment at 50 μm. Error bars indicate se (n = 3).