Branching angles in modulating plant architecture: Molecular mechanisms, dynamic regulation, and evolution

Plants develop branches to expand areas for assimilation and reproduction. The branching angles coordinate with branching types, creating diverse plant shapes that adapt to varied environments. Two types of branching angles - the angle between shoots, and the angle in relation to gravity or the gravitropic set-point angle (GSA) along shoots - determine the spacing between shoots and the shape of the above-ground parts. However, it remains unclear how these branching angles are modulated throughout shoot development, and how they interact with other factors contributing to plant architecture. In this review, we systematically focus on the molecular mechanisms that regulate branching angles across various species, including gravitropism, anti-gravitropic offset, phototropism, and other regulatory factors, which collectively highlight the comprehensive mechanisms centered on auxin. We also discuss the dynamics of branching angles during development, and their relationships with branching number, stress resistance, and crop yield. Finally, we provide an evolutionary perspective on the conserved role of auxin in regulating branching angles.