<i>BpFLC</i> coordinates seasonal and age-related flowering in <i>Betula platyphylla</i> through environmental cues and epigenetic regulation.

Abstract

The <i>Flowering Locus C</i> (<i>FLC</i>) gene is a flowering repressor in annual plants, but its function in perennial woody species remains less understood. Here, we explored the role of <i>BpFLC</i> in <i>Betula platyphylla</i> using a genotype that flowers after 5 to 6 y of growth in the field. Overexpression of <i>BpFLC</i> extended vegetative growth in transgenic <i>B. platyphylla</i>, without flowering even after 7 y in the field. CRISPR/Cas9-mediated loss of function in <i>BpFLC</i> led to early flowering after a 3-year growth. Integrated ATAC-seq, RNA-seq, and Flowering-Interactive Database (Flor-ID) analyses of <i>BpFLC</i> overexpression transgenics against the wild type identified a flowering-promoting TF <i>BpGI</i> (<i>GIGANTEA</i>) as a unique direct target of BpFLC. The BpFLC-<i>BpGI</i> binding represses <i>BpGI</i>'s flowering-promoting functions. Full genome Y1H identified BpHEN4 (HUA ENHANCER4) and BpELIP1 (EARLY LIGHT-INDUCED PROTEIN1) as the only two <i>BpFLC</i> binding genes that shared synchronized upregulation patterns with <i>BpFLC</i> under various environmental cues, including temperatures and photoperiods. Overexpression of <i>BpHEN4</i> or <i>BpELIP1</i> in <i>B. platyphylla</i> activated <i>BpFLC</i> expression. BpHEN4 and BpELIP1 dimerize and cobind to the G-box in <i>BpFLC</i>'s first intron but not the promoter for <i>BpFLC</i> upregulation. Our study suggested a <i>FLC</i>-mediated flowering repressor model in <i>B. platyphylla</i> where <i>BpHEN4</i> and <i>BpELIP1</i> are signals transducing environmental cues and cobind to <i>BpFLC</i>'s first intron to activate <i>BpFLC,</i> leading to elevated <i>BpGI</i> chromatin accessibility for more effective BpFLC-<i>BpGI</i> interactions to drive <i>BpGI</i>-mediated downregulation of flowering-promoting TFs to delay flowering. The greater transgene containment with growth/biomass benefit would be a unique economic trait for any key forest species.