Maize, as the largest crop in China, plays a vital role in ensuring national food security and driving economic development. The continuous increase in maize yield over the past few decades has been largely attributed to the breeding of compact-plant varieties and the adoption of higher planting densities. In comparison to traditional sprawling maize plants, compact plant architecture, characterized by narrower stem-leaf angles and upright leaf orientation, facilitates the achievement of higher yields through increased planting densities. Despite being widely recognized and utilized in modern maize breeding and production practices, the regulatory mechanisms underlying the compact plant architecture, particularly the regulation of stem-leaf angles, still require in-depth investigation. Therefore, a comprehensive understanding of the regulatory mechanisms governing stem-leaf angles holds significant importance for crop architecture improvement and yield enhancement.

 

On May 7, 2024, the research paper entitled "Histological and single-nucleus transcriptome analyses unveil the specialized functions of ligular sclerenchyma cells and key regulators of leaf angle in maize" was published online in Molecular Plant. This study, conducted jointly by Professor Gang Li's team from the College of Life Sciences at Shandong Agricultural University and Research Professor Bosheng Li's team from the Peking University Institute of Advanced Agricultural Sciences (PKU-IAAS), revealed the cytological characteristics of the critical region responsible for leaf angle establishment in maize, known as the ligule region. The researchers constructed a single-nucleus transcriptomic atlas of the ligule and identified a series of regulatory genes involved in maize ligule development. Functional validation of some key genes was also performed. This study not only deepens our understanding of leaf angle establishment and plant architecture regulation in maize but also provides potential targets for genetic improvement of dense-planting tolerant plant architectures, holding both theoretical and practical significance.

 

https://doi.org/10.1016/j.molp.2024.05.001

Figure 1 Influence of cell elongation and lignification in the proximal side of maize ligule on leaf angle establishment

 

Grass family plants exhibit complex leaf structures consisting of three components: the leaf blade, ligule, and leaf sheath. Among them, the ligule, located between the leaf blade and leaf sheath, resembles a "hinge" or "joint" structure and serves as a critical area influencing stem-leaf angles. In grass family plants, compared to rice and wheat, maize possesses broader, longer, and heavier leaves, suggesting the evolution of a unique regulatory mechanism for controlling stem-leaf angles, possibly to support the heavier leaf structure.

 

To delve into the regulatory mechanisms underlying maize stem-leaf angles, the research team conducted histological analysis and lignin staining on the ligule region of 140 maize inbred lines. For the first time, they discovered the presence of thick-walled cells and significant lignin deposition (dual-track pattern) in both the axial and distal sides of the ligule in compact inbred lines. In contrast, in the ligule of sprawling inbred lines, only the distal side exhibited thick-walled cells and lignin deposition (single-track pattern). This finding indicates that the dual-track distribution pattern of thick-walled cells in the ligule region is the main cause for reducing leaf angle in compact plant architectures.

 

The proximal thick-walled cells in the ligule region originate from the differentiation of subepidermal cells. In the process of cell specialization, differences in cell length and lignification directly determine the size of the stem-leaf angles, thereby affecting the overall compactness of the plant. The orientation deposition of lignin in proximal thick-walled cells may restrict the increase of stem-leaf angles by inhibiting cell elongation and enhancing mechanical strength.

 

Figure 2 Single-cell transcriptomics reveals cellular heterogeneity in the maize ligule region

 

Furthermore, in order to elucidate the process of specialization in the thick-walled cells of the proximal side of the ligule, the research team successfully constructed a single-cell transcriptomic atlas of the ligule region in the compact-plant inbred line Z58 using second-generation transcriptomic sequencing and single-nucleus RNA sequencing (SnRNA-seq) technology. They identified 16 distinct cell clusters and their specific highly expressed genes, predicting a series of leaf angle regulatory genes. Moreover, through molecular biology techniques such as CRISPR-Cas9, the research team conducted functional analysis on selected candidate genes, revealing the involvement of the highly expressed transcription factors bHLH30 and bHLH155 in the ligule proximal side. These transcription factors were found to regulate ligule development and plant architecture by activating the expression of genes related to cell elongation and lignification.

 

Figure 3 Molecular regulatory model of maize leaf angle establishment

 

This research not only reveals the significant role of thick-walled cells in the ligule of Poaceae plants in the establishment of plant architecture but also provides a valuable resource in the form of a high-resolution single-cell transcriptomic atlas of the ligule. This atlas is crucial for dissecting the complex regulatory networks underlying stem-leaf angles in crops, offering important scientific foundations and new perspectives for the precise design and genetic improvement of crop architecture for high plant density tolerance.

 

Dr. Qibin Wang (currently a postdoctoral researcher in the College of Agriculture at Shandong Agricultural University) and Ms. Qiuyue Guo (research assistant at the PKU-IAAS) contributed equally as first authors to this paper. Prof. Gang Li from the College of Life Sciences at Shandong Agricultural University and Dr. Bosheng Li from the IPKU-IAAS, served as corresponding authors of the paper. Prof. Pinghua Li from the College of Agriculture at Shandong Agricultural University also participated in this study. The research received funding from the National Natural Science Foundation of China, Key Research and Development Program of Shandong Province, Taishan Scholar Program, and China Postdoctoral Science Foundation.