A model for soybean inflorescence architecture based on morphological and gene expression analysis

Soybean inflorescence architecture is controversial and regulation of its development unresolved. Our study provides an integral view of its architecture and critical information on the gene network controlling its development. Inflorescence architecture is a highly important trait depending on the arrangement and number of flowers in the inflorescence stem. It strongly contributes to plant morphological diversity, and since it determines the number of flowers and fruits, it has strong potential to influence crop yield. Soybean (Glycine max) is a highly relevant grain crop. However, despite many studies involving soybean inflorescence, no clear descriptions of its architecture are available, and the information on this question is controversial. In addition, though a model for the gene network controlling inflorescence meristem identity is established for other legumes, such as pea (Pisum sativum) or Medicago truncatula, regulation of soybean inflorescence development is not resolved, with the nature of the gene specifying I2 meristem identity not clear yet. Here, we use macroscopic and microscopic observation to analyze soybean inflorescence architecture and RNA in situ hybridization to study the expression of the meristem genes DT1, DT2 and GmAP1a, to analyze the control of soybean inflorescence development. Our data demonstrate that, as pea and Medicago, soybean has a compound inflorescence, with flowers formed in secondary inflorescences (I2), and suggest that it is a compound raceme. Our expression study supports that DT1 and AP1 specify the identity of I1 and floral meristems, respectively. Importantly, the specific expression of Dt2 in I2 meristems strongly indicates I2 meristem identity specification by Dt2 and conservation of the inflorescence gene regulatory network with other legumes. Our study fills an important gap, providing an integral view of soybean inflorescence architecture and novel critical information on the gene network that controls its development.