Genome-wide analysis of MADS-box genes reveals their roles in floral organ development in Lagerstroemia

Crape myrtle is an important summer flowering shrub with abundant flowers and long flowering period, and widely used in gardens around the world. Though the MADS-box genes have profound impacts on floral organogenesis in other plants, their function and expression patterns in Lagerstroemia remain unclear. In the study, we conducted the comprehensive analysis of the MADS-box family from three Lagerstroemia species, including 94 genes in L. suprareticulata, 83 genes in L. speciosa and 79 genes in L. indica. The MADS-box genes were classified into Type I (104 genes) and Type II (152 genes). The difference in gene numbers observed among the three species was closely related to tandem and segmental duplication. Homologous gene pairs were prevalent among Lagerstroemia species, offering a profound understanding of the evolutionary dynamics of MADS-box gene family. The promoter regions of type II genes are mostly composed of light-responsive and hormone-inducible elements. Expression profiles of ABCDE-class genes showed differences in the four-whorl flower organ. Yeast two-hybrid assay and luciferase complementation assay indicated that LiMADS53 and LiMADS60 synergistically regulated sepal initiation while LiMADS49 and LiMADS42 exhibited expression in petals and stamens. LiMADS42 and LiMADS53 contributed to stamen development. These results provide a basis for mining the genes that determine flower organs of crape myrtle, and lay a solid foundation for improving flower type and breeding of new cultivars.