Leaf angle regulation toward a maize smart canopy.

Dense planting of single-cross hybrids contributes to maize yield increase over the past decades. Leaf angle, an important agronomic trait relevant to planting density, plays a fundamental role in light penetration into the canopy and photosynthetic efficiency. Leaf angle is a key parameter of plant architecture in the concept of smart canopy. Maize smart-canopy-like plant architecture exhibits optimal leaf angle, resulting in erect upper leaves and intermediate or horizontal lower leaves. Leaf angle regulation is a promising way forward in the breeding of varieties with canopy ideotypes. In this review, we first describe the relationship between maize polarity axes and leaf angle formation. Then, we revisit advances in the mutant and quantitative genetics research of maize leaf angle, highlighting the biological implications of transcription factors for maize leaf angle regulation. We underscore that KNOX family is essential for the blade-sheath boundary establishment and brassinosteroid pathway components as well as regulator ZmRAVL1 serve as key hubs of the transcriptional hierarchy governing maize leaf angle formation. We also suggest potential avenues for manipulating maize leaf angles across canopy layers.