Hd1,a CONSTANS ortholog in rice, functions as an Ehd1 repressor through interaction with monocot‐specific CCT‐domain protein Ghd7

Abstract

SUMMARYFlowering time is an important agronomic trait that affects crop yields. In cereals, several CCT-domain proteins unique to monocots, such as Grain number, plant height, and heading date 7 (Ghd7) gene, have been identified as key floral repressors, although the corresponding molecular mechanisms have been unknown until now. In rice, a short-day plant, Heading date 1 (Hd1) gene, a rice ortholog of Arabidopsis floral activator CONSTANS (CO), represses flowering under non-inductive long-day (LD) conditions and induces it under inductive short-day (SD) conditions. Here, we report biological interactions between Ghd7 and Hd1, which together repress Early heading date 1 (Ehd1), a key floral inducer under non-inductive LD conditions. In addition to this genetic interaction between them, Co-IP experiments further demonstrated that a Ghd7-Hd1 protein formed a complex in vivo and ChIP and luciferase reporter analyses suggested that this complex specifically binds to a cis-regulatory region in Ehd1 and represses its expression. These findings imply that Hd1, an evolutionally conserved transcriptional activator, can function as a strong transcriptional repressor within a monocot-specific flowering-time pathway through with Ghd7.

Molecular Regulation of Flowering Time in Grasses

“…Indeed, Hd1 repressor activity requires a proper Ghd7 function under LD conditions to repress Ehd1 in the morning. In contrast, Ghd7 can repress the expression of Ehd1, Hd3a and RFT1 by itself under all photoperiod conditions [15]. …”

“…Genetic approaches in Arabidopsis have identified three genes that control flowering: GIGANTEA (GI)-CONSTANS (CO)-FLOWERING LOCUS (FT) [15,28]. Loss of function mutations in each gene for flowering control delay flowering under LD conditions but no effect is produced under SD [31].…”

“…In the last decades, studies on the model plant Arabidopsis have revealed that molecular mechanisms discovered in that species are evolutionally conserved in other species [15]. Genetic approaches in Arabidopsis have identified three genes that control flowering: GIGANTEA (GI)-CONSTANS (CO)-FLOWERING LOCUS (FT) [15,28].…”

“…OsGIGANTEA (OsGI) acts an activator of Heading date1 (Hd1), an ortholog of CO, and controls flowering time by modulating rhythmic flowering under SD [36]. Hd1 encodes a zinc finger type transcriptional activator with the conserved CCT (CO, CO-like, TIMING OF CAB EXPRESSION1 (TOC1)) domain [15,37]. In contrast to Arabidopsis CO, Hd1 promotes Heading date 3a (Hd3a) expression in SD but expression is modified in LD conditions [38,39] where Hd1 function is converted into a repressor.…”

“…Consideration of flowering time is an important strategy in the cultivation of grain crops in northern latitudes. Early flowering is useful in regions where growing seasons are short to enhance grain yield stability by avoiding drought or adverse temperatures [14][15][16]. Flowering time is also a major determinant of biomass yield in perennial C 4 bioenergy crops, because delayed flowering time allows an extended period of vegetative growth and produces more biomass.…”