Expression level of B- and C-class MADS-box genes is associated with the petaloidy of stamens in cultivated amaryllis (Hippeastrum hybridum)

One of the most important attributes of Hippeastrum hybridum (amaryllis) flower is its double-flower trait, which is partly derived from the petaloidy of its stamens. Three types of floral organs in whorl 3 of double flowers from H. hybridum cv. Jewel, namely normal stamens, partly petaloid stamens, and completely petaloid stamens, were sampled for RNA-seq analysis to identify candidate genes related to the petaloidy of stamens. 12,584 differentially expressed genes between normal stamens and petaloid stamens were obtained, in which several MADS-box genes and a series of genes related to morphogenesis, pigment, and cell wall development were identified. Quantitative real-time PCR analysis of nine MADS-box genes clustered to the ABCE-class was performed in three cultivars 'Jewel', 'Merry Christmas', and 'Red Nymph'. Consistent with the transcriptome data, higher expression of the B-class genes HhAP3.2 and HhPI1, but lower expression of the C-class gene HhAG1 was observed in petaloid stamens than those in normal stamens, and this trend was enhanced with an increase in the degree of petaloidy. These results suggest that the expression level of B- and C-class MADS-box genes is associated with the petaloidy of stamens in H. hybridum, which provide candidate genes for double-flower breeding in H. hybridum.