Genome-Wide Classification and Evolutionary Analysis of the KNOX Gene Family in Plants

The Knotted1-like homeobox (KNOX) gene family plays a pivotal role in regulating meristem activity, organ differentiation, and cell meristematic identity. However, there has been a lack of large-scale, systematic, and comprehensive comparative analyses to explore their expression patterns and evolutionary mechanisms. In this study, a total of 1425 KNOX genes were identified across 118 plant species. The result showed that higher plants exhibited a significantly higher abundance of KNOX genes compared to lower plants. Phylogenetic analysis revealed that all KNOX genes can be divided into two classes (class I and II) and evolved independently after species differentiation. An analysis of gene duplication or loss showed that gene loss was more common than gene duplication in lower plants within the KNOX gene family. These findings suggest that gene loss in the KNOX gene family occurs after events such as whole-genome duplication (WGD) or whole-genome triplication (WGT). In addition, conserved motif analysis was also conducted to uncover the evolutionary trajectories of KNOX genes. We found that three motifs (M1, M2, and M4) were present in nearly all KNOX genes, while four novel motifs (M7-M10) were lost in lower plants but present in higher plants. Moreover, the loss of certain motifs in the KNOX genes was also observed in higher plants, indicating sequence divergence in KNOX genes throughout evolution. To understand the expression patterns of KNOX genes, a gene expression pattern analysis was performed in A. thaliana and O. sativa. The results showed that class I KNOX genes exhibit conserved high expression in stems, suggesting their potential similar biological roles across different plant species and the conservation of their functions during evolution. Additionally, we analyzed the KNOX genes in the Citrus genus and closely related species, and we found that the number of KNOX genes evolved at a slower rate in these species, indicating a relatively conservative nature. In conclusion, this study provides valuable resources for the investigation of KNOX gene family evolution and function in plants.