Leaf angle is defined as the inclination between the midrib of the leaf blade and the vertical stem of a plant. This trait has been identified as a key component in the development of high-yielding varieties of cereal species, particularly maize, rice, wheat, and sorghum. The effect of leaf angle on light interception efficiency, photosynthetic rate, and yield has been investigated since the 1960s, yet, significant knowledge gaps remain in understanding the genetic control of this complex trait. Recent advances in physiology and modeling have proposed a plant ideotype with varying leaf angles throughout the canopy. In this context, we present historical and recent evidence of: (i) the effect of leaf angle on photosynthetic efficiency and yield; (ii) the hormonal regulation of this trait; (iii) the current knowledge on its quantitative genetic control; and (iv) the opportunity to utilize high-throughput phenotyping methods to characterize leaf angle at multiple canopy levels. We focus on research conducted on grass species of economic importance, with similar plant architecture and growth patterns. Finally, we present the challenges and strategies plant breeders will need to embrace in order to manipulate leaf angle differentially throughout the canopy and develop superior crops for food, feed, and fuel production.

Leaf angle across the sorghum canopy is genetically controlled by both common and layer-specific loci, which holds potential for the development of an optimized canopy associated with higher yields.

Background Leaf angle is an important plant architecture trait, affecting plant density, light interception efficiency, photosynthetic rate, and yield. The “smart canopy” model proposes more vertical leaves in the top plant layers and more horizontal leaves in the lower canopy, maximizing conversion efficiency and photosynthesis. Sorghum leaf arrangement is opposite to that proposed in the “smart canopy” model, indicating the need for improvement. Although leaf angle quantitative trait loci (QTL) have been previously reported, only the Dwarf3 (Dw3) auxin transporter gene, colocalizing with a major-effect QTL on chromosome 7, has been validated. Additionally, the genetic architecture of leaf angle across canopy layers remains to be elucidated. Results This study characterized the canopy-layer specific transcriptome of five sorghum genotypes using RNA sequencing. A set of 284 differentially expressed genes for at least one layer comparison (FDR < 0.05) co-localized with 69 leaf angle QTL and were consistently identified across genotypes. These genes are involved in transmembrane transport, hormone regulation, oxidation-reduction process, response to stimuli, lipid metabolism, and photosynthesis. The most relevant eleven candidate genes for layer-specific angle modification include those homologous to genes controlling leaf angle in rice and maize or genes associated with cell size/expansion, shape, and cell number. Conclusions Considering the predicted functions of candidate genes, their potential undesirable pleiotropic effects should be further investigated across tissues and developmental stages. Future validation of proposed candidates and exploitation through genetic engineering or gene editing strategies targeted to collar cells will bring researchers closer to the realization of a “smart canopy” sorghum.