Mechanical Strength: An Unrecognised Target in the Genetic Improvement of Crops.

Leaf angle (LA) is a crucial agronomic trait influencing planting density and crop yield. Previous research highlighted the importance of cellular variations in the ligular region for determining LA, but the underlying regulatory mechanisms remain unclear. Here, we demonstrate LA is not a static trait, but rather represents a dynamic equilibrium between mechanical forces maintaining leaf erectness and those promoting blade drooping. To quantify the drooping tendency, we introduce gravitational moments, which show positive correlations with LA, blade length (BL) and blade weight (BWt). Notably, the mechanical forces are tightly regulated by the sheath layers surrounding the stalk and by the thickness and lignin deposition on the ligular region of sclerenchyma (SC) cells. Furthermore, we applied single-nucleus transcriptome analyses (snRNA-seq) to construct a comprehensive transcriptional atlas spanning the ligular regions of compact-type (Z58), intermediate-type (B73) and expanded-type (W22) inbred lines. Through the comparative analysis of snRNA-seq and RNA-seq of three inbred lines, we identified the adaxial hypodermis (HP) cells as pivotal sites where lignin biogenesis and metabolism genes were specifically expressed in compact-type Z58, consistent with the lignin deposition pattern. Notably, we discovered that the NAM, ATAF and CUC (NAC) transcription factor-encoding genes NAC secondary wall thickening promoting factor 2 (NST2) and NST3, which mediate lignin biogenesis in the ligular region, especially on the adaxial side, play key roles in reinforcing mechanical support and reducing LA. Collectively, this study advances our understanding of ligular development and LA regulatory mechanisms and provides strategic insights for breeding crops with improved agricultural productivity.