PHYTOCHROME C regulation ofPHOTOPERIOD1is mediated byEARLY FLOWERING 3inBrachypodium distachyon

Abstract

Daylength sensing in many plants is critical for coinciding the timing of flowering with the appropriate season. Temperate-climate-adapted grasses such as Brachypodium distachyon flower during the spring when days are becoming longer. The photoreceptor PHYTOCHROME C is essential for long-day (LD) flowering in B. distachyon. PHYC is required for the LD activation of a suite of genes in the photoperiod pathway including PHOTOPERIOD1 (PPD1) that, in turn, result in the activation of FLOWERING LOCUS T (FT1)/FLORIGEN, which causes flowering. Thus, phyC mutants are extremely delayed in flowering. Here we show that PHYC-mediated activation of PPD1 occurs via EARLY FLOWERING 3 (ELF3), a component of the evening complex in the circadian clock. The extreme delay of flowering of the phyC mutant disappears when combined with an elf3 loss-of-function mutation. Moreover, the dampened PPD1 expression in phyC mutant plants is elevated in phyC/elf3 mutant plants consistent with the rapid flowering of the double mutant. We show that loss of PPD1 function also results in reduced FT1 expression levels and extremely delayed flowering consistent with reports from wheat and barley. Additionally, elf3 mutant plants have elevated expression levels of PPD1 and we show that overexpression of ELF3 results in delayed flowering, which is associated with a reduction of PPD1 and FT1, demonstrating ELF3 represses PPD1 transcription, consistent with previous studies showing that ELF3 binds to the PPD1 promoter. Indeed, PPD1 is the main target of ELF3-mediated flowering as elf3/ppd1 double mutant plants are delayed flowering. Our results indicate that ELF3 operates downstream from PHYC and acts as a repressor of PPD1 in the photoperiod flowering pathway of B. distachyon.

EARLY FLOWERING 3interactions withPHYTOCHROME BandPHOTOPERIOD1are critical for the photoperiodic regulation of wheat heading time

“…The same result was reported for Brachypodium plants grown under 20 h light / 4 h darkness, where elf3 ppd1 headed 16 d earlier than ppd1 [55]. However, when the same Brachypodium genotypes were grown under SD (8 h light) or LD (16 h light), the elf3 ppd1 plants headed significantly later (24.4 and 13.7 d) than the ppd1 plants [55]. These results suggest differences between Brachypodium and wheat in the PPD1-independent effect of ELF3.…”

“…A similar effect was reported in barley, where a strong QTL for heading time under LD was found at the CO1 locus, but only among accessions carrying the ppd-H1 photoperiod insensitive allele [82]. These results demonstrate that in wheat whereas in Brachypodium CO1 is downregulated in phyC and ppd1 [55]. These differences may explain the earlier heading of the co1 mutant relative to the wildtype in wheat [5], and the delayed heading of CO1 RNA interference knock-down plants in Brachypodium under LD [83].…”

“…The elf3 ppd1 combined mutant headed earlier than the ppd1 mutant both in LD (37 d) and SD (>48 d) indicating that ELF3 can delay wheat heading time independently of PPD1. The same result was reported for Brachypodium plants grown under 20 h light / 4 h darkness, where elf3 ppd1 headed 16 d earlier than ppd1 [55]. However, when the same Brachypodium genotypes were grown under SD (8 h light) or LD (16 h light), the elf3 ppd1 plants headed significantly later (24.4 and 13.7 d) than the ppd1 plants [55].…”

“…The combined mutant also restored the expression of PPD1, which was repressed in phyB. Similar genetic interactions between elf3 and phyC are reported in the companion study in Brachypodium [55], suggesting that these interactions are conserved in the temperate grasses. Taken together, these results indicate that the critical role of PHYB or PHYC in the light activation of PPD1 is mediated by ELF3 in both wheat and Brachypodium.…”

“…A comparison of the CO1 expression results in wheat and Brachypodium revealed differences between these two species. In wheat, CO1 is upregulated in phyB (S3E Fig) and ppd1 (Fig 7K), whereas in Brachypodium CO1 is downregulated in phyC and ppd1 [55]. These differences may explain the earlier heading of the co1 mutant relative to the wildtype in wheat [5], and the delayed heading of CO1 RNA interference knock-down plants in Brachypodium under LD [83].…”