Heading Date 3a Stimulates Tiller Bud Outgrowth in Oryza sativa L. through Strigolactone Signaling Pathway

<i>Heading date 3a (Hd3a</i>, a <i>FLOWERING LOCUS T</i> (<i>FT</i>) ortholog from rice) is well known for its important role in rice (<i>Oryza sativa</i> L.), controlling floral transition under short-day (SD) conditions. Although the effect of <i>Hd3a</i> on promoting branching has been found, the underlying mechanism remains largely unknown. In this report, we overexpressed an <i>Hd3a</i> and <i>BirAG</i> (encoding a biotin ligase) fusion gene in rice, and found that early flowering and tiller bud outgrowth was promoted in <i>BHd3aOE</i> transgenic plants. On the contrary, knockout of <i>Hd3a</i> delayed flowering and tiller bud outgrowth. By using the BioID method, we identified multiple Hd3a proximal proteins. Among them, D14, D53, TPR1, TPR2, and TPRs are central components of the strigolactone signaling pathway, which has an inhibitory effect on rice tillering. The interaction between Hd3a, on the one hand, and D14 and D53 was further confirmed by the bimolecular fluorescence complementation (BiFC), yeast two-hybrid (Y2H), and co-immunoprecipitation (Co-IP) methods. We also found that Hd3a prevented the degradation of D53 induced by rac-GR24 (a strigolactone analog) in rice protoplasts. RT-qPCR assay showed that the expression levels of genes involved in strigolactone biosynthesis and signal transduction were altered significantly between WT and <i>Hd3a</i> overexpression (<i>Hd3aOE</i>) or mutant (<i>hd3a</i>) plants. <i>OsFC1</i>, a downstream target of the strigolactone signaling transduction pathway in controlling rice tillering, was downregulated significantly in <i>Hd3aOE</i> plants, whereas it was upregulated in <i>hd3a</i> lines. Collectively, these results indicate that <i>Hd3a</i> promotes tiller bud outgrowth in rice by attenuating the negative effect of strigolactone signaling on tillering and highlight a novel molecular network regulating rice tiller outgrowth by <i>Hd3a</i>.