Overexpression of SlMBP22 in Tomato Affects Flower Morphology, Fruit Set and Development

MADS-domain transcription factors have been clarified as key regulators involved in proper flower and fruit development in angiosperms. Bs genes, as members of the MADS-box subfamily, have been suggested to play an important role during the evolution of the reproductive organs in seed plants. Our knowledge about their effects on reproductive development in fruit crops like tomato (Solanum lycopersicum), however, is still unclear. Here, we found that the overexpression of SlMBP22 (SlMBP22-OE) resulted in considerable alterations regarding floral morphology, and affected the expression levels of several floral homeotic genes. Further analysis by yeast-two-hybrid assays demonstrated that SlMBP22 could form dimers with class A protein MACROCALYX (MC) and with SEPALLATA (SEP) floral homeotic proteins TM5 and TM29, respectively. In addition, pollen viability and cross-fertilization assays suggested that the defect in female reproductive development was responsible for infertility phenotype observed in the strong overexpression transgenic plants. The mild overexpression transgenic fruits were reduced in size, as a result of reduced cell expansion, rather than impaired cell division. Additionally, overexpression of SlMBP22 in tomato not only affected proanthocyanidin (PA) accumulation but also altered seed dormancy. Taken together, these findings may provide new insights into the knowledge of Bs MADS-box genes in flower and fruit development in tomato.