Stigma exsertion is an important agricultural trait that facilitates the use of heterosis in crop breeding. Although several quantitative trait loci associated with stigma exsertion have been fine mapped or cloned, the underlying genetic basis, particular in legumes, remains unclear. In this study, we identified and characterized an exserted stigma mutant stigma exsertion 1 (sge1) in the model legume Medicago truncatula. The exserted stigma phenotype of sge1 is mainly caused by physical interaction between floral organs, in which normal petal and stamen elongation are inhibited due to flower cuticle defects. SGE1 encodes an ATP binding cassette G transporter (ABCG) that plays a critical role in regulating floral cutin and wax secretion in M. truncatula. SGE1 physically interacts with another half-size transporter, MtABCG13, to form a functional heterodimer. Mutation of MtABCG13 results in flower cuticle defects similar to those in sge1, as well as stigma exsertion, indicating that SGE1 and MtABCG13 are indispensable for flower cuticle secretion and collaboratively control stigma exsertion in M. truncatula. Our findings uncover novel functions for ABCG transporters in determining stigma exsertion by affecting the physical interactions of floral organs, providing insight into the molecular mechanism underlying stigma exsertion in leguminous plants with complex zygomorphic flowers.

By Lin Hao (linhao@caas.cn)