The MADS6, JAGGED, and YABBY proteins synergistically determine floral organ development in rice

MADS6, JAGGED (JAG), and DROOPING LEAF (DL) are key regulators of floral organ patterns in rice (Oryza sativa); however, how they work together in specifying floral organs remains to be determined. Here, we extensively analyzed the coordination mechanism. Genetic interactions showed that all double/triple mutant combinations of mads6-5 with jag and/or dl-sup7 generated an inflorescence from the spikelet center and lemma-like organs (LLOs) at the periphery, indicating that these genes synergistically promote floral organ specification, inhibit inflorescence initiation, and terminate the floral meristem (FM). Particularly, a fully developed mads6-5 jag spikelet appeared as a large bouquet composed of numerous multi-floral complexes (MFC), while the triple mutant was generally similar to mads6-5 jag, except for a longer pedicel and fewer MFCs. Expression analysis revealed that JAG directly inhibits the transcription of MADS6 in stamens but not in pistils, as JAG and DL co-express in pistils and form a JAG-DL complex, indicating that JAG and DL may coordinate the transcription of MADS6 in sexual organs. Protein interactions revealed that MADS6 and JAG bind to five spikelet-related YABBY proteins (including DL), forming ten heterodimers, suggesting that they may promote floral differentiation through various pathways. However, MADS6 and JAG neither bound together nor formed a heterotrimer with any of the five YABBY proteins. These findings revealed specific synergistic patterns between MADS6, JAGGED, and YABBY proteins, which may contribute to the unique characteristics of rice spikelets and provide insights into the diversity regulation mechanisms of floral specification in plants.