<i>INDETERMINATE1</i>-mediated expression of <i>FT</i> family genes is required for proper timing of flowering in <i>Brachypodium distachyon</i>.

Abstract

The transition to flowering is a major developmental switch in plants. In many temperate grasses, perception of indicators of seasonal change, such as changing day-length and temperature, leads to expression of <i>FLOWERING LOCUS T1</i> (<i>FT1</i>) and <i>FT-Like</i> (<i>FTL</i>) genes that are essential for promoting the transition to flowering. However, little is known about the upstream regulators of <i>FT1</i> and <i>FTL</i> genes in temperate grasses. Here, we characterize the monocot-specific gene <i>INDETERMINATE1</i> (<i>BdID1</i>) in <i>Brachypodium distachyon</i> and demonstrate that <i>BdID1</i> is a regulator of <i>FT</i> family genes. Mutations in <i>ID1</i> impact the ability of the short-day (SD) vernalization, cold vernalization, and long-day (LD) photoperiod pathways to induce certain <i>FTL</i> genes. <i>BdID1</i> is required for upregulation of <i>FTL9</i> (<i>FT-LIKE9</i>) expression by the SD vernalization pathway, and overexpression of <i>FTL9</i> in an <i>id1</i> background can partially restore the delayed flowering phenotype of <i>id1</i>. We show that BdID1 binds in vitro to the promoter region of <i>FTL</i> genes suggesting that ID1 directly activates <i>FTL</i> expression. Transcriptome analysis shows that <i>BdID1</i> is required for <i>FT1</i>, <i>FT2</i>, <i>FTL12</i>, and <i>FTL13</i> expression under inductive LD photoperiods, indicating that <i>BdID1</i> is a regulator of the <i>FT</i> gene family. Moreover, overexpression of <i>FT1</i> in the <i>id1</i> background results in rapid flowering similar to overexpressing <i>FT1</i> in the wild type, demonstrating that <i>BdID1</i> is upstream of <i>FT</i> family genes. Interestingly, <i>ID1</i> negatively regulates a previously uncharacterized <i>FTL</i> gene, <i>FTL4,</i> and we show that <i>FTL4</i> is a repressor of flowering. Thus, <i>BdID1</i> is critical for proper timing of flowering in temperate grasses.