Reduced Expression of C-class Genes Is Associated with the Multiple-petal Phenotype in Nelumbo nucifera

Abstract

The sacred lotus (Nelumbo nucifera) is an aquatic basal eudicot that is widely cultivated for food, medicine, and ornamental use. Ornamental lotus cultivars form flowers of various colors and shapes. The ABCE model for flower development has been proposed for model core eudicots such as Arabidopsis and snapdragon, but little is known about the molecular mechanisms of floral organ specification and development in N. nucifera. Here, we identified ABCE-class genes in the N. nucifera genome and compared their gene structures and spatial expression patterns in floral organs between single- and double-petalled flower-type lotus cultivars. Two lotus homologs of the C-class gene AGAMOUS (NnAG1, NnAG2) in a double-petal flower cultivar had intact transcripts, suggesting that loss of function of AG is not the cause of the double-petal phenotype in N. nucifera. NnAG1 and NnAG2 expression were specifically observed in stamens and carpels of a single-petalled flower cultivar, but were slightly reduced in the double-petalled flower cultivar. In situ hybridization analysis further revealed that these two C-class homologs are specifically expressed in inner whorls such as stamens and carpels in a single-petal flower cultivar, but the signals are diminished in double- and proliferating-petalled cultivars. These findings suggest that down-regulation of C-class genes is responsible for the multiple-petal phenotype in N. nucifera.