Plants commonly use photoperiod (day length) to control the timing of flowering during the year, and variation in photoperiod response has been selected in many crops to provide adaptation to different environments and farming practices. Positional cloning identified Ppd-H1, the major determinant of barley photoperiod response, as a pseudo-response regulator, a class of genes involved in circadian clock function. Reduced photoperiod responsiveness of the ppd-H1 mutant, which is highly advantageous in spring-sown varieties, is explained by altered circadian expression of the photoperiod pathway gene CONSTANS and reduced expression of its downstream target, FT, a key regulator of flowering.

The circadian clock is an autonomous oscillator that produces endogenous biological rhythms with a period of about 24 h. This clock allows organisms to coordinate their metabolism and development with predicted daily and seasonal changes of the environment. In plants, circadian rhythms contribute to both evolutionary fitness and agricultural productivity. Nevertheless, we show that commercial barley varieties bred for short growing seasons by use of early maturity 8 (eam8) mutations, also termed mat-a, are severely compromised in clock gene expression and clock outputs. We identified EAM8 as a barley ortholog of the Arabidopsis thaliana circadian clock regulator EARLY FLOWERING3 (ELF3) and demonstrate that eam8 accelerates the transition from vegetative to reproductive growth and inflorescence development. We propose that eam8 was selected as barley cultivation moved to high-latitude short-season environments in Europe because it allowed rapid flowering in genetic backgrounds that contained a previously selected late-flowering mutation of the photoperiod response gene Ppd-H1. We show that eam8 mutants have increased expression of the floral activator HvFT1, which is independent of allelic variation at Ppd-H1. The selection of independent eam8 mutations shows that this strategy facilitates short growth-season adaptation and expansion of the geographic range of barley, despite the pronounced clock defect.T he timing of flowering during the year is an important adaptive trait that strongly influences reproductive fitness. Many plants use the environmental cue of day length (photoperiod) to regulate flowering and this response can vary within species along a latitudinal cline (1-3). Photoperiod response has been extensively studied in Arabidopsis, where daily light oscillations entrain the circadian clock (4), which is the internal timepiece by which photoperiod is measured. In Arabidopsis, the circadian clock is composed of several feedback loops that interlock to provide robustness (5, 6). The related Myb transcription factors CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) have expression peaks in the morning and act antagonistically to the pseudoresponse regulator (PRR) TIMING OF CAB EXPRESSION1 (TOC1), which peaks in the evening. CCA1 and LHY promote the expression of PRR7 and PRR9, which themselves repress CCA1 and LHY, forming a morning feedback loop (5-7). This loop also involves EARLY FLOWERING3, EARLY FLOWERING4, and LUX ARHYTHMO (ELF3, ELF4, LUX), which repress PRR7 and PRR9 (8-10). An additional evening feedback loop involves GIGANTEA (GI), which promotes TOC1, which in turn represses GI (5). ELF3 also affects the evening loop by regulating GI protein turnover (11). Clock circuitry thus functions continuously during the day and this provides capacity for timed physiological outputs in response to the time of day.The clock is important for providing diurnal and seasonal control of flowering. In Arabidopsis, transcriptional outputs from the clock regulate CONSTANS (CO) expression so t...

The FLOWERING LOCUS T (FT ) gene plays a central role in integrating flowering signals in Arabidopsis because its expression is regulated antagonistically by the photoperiod and vernalization pathways. FT belongs to a family of six genes characterized by a phosphatidylethanolamine-binding protein (PEBP) domain. In rice (Oryza sativa), 19 PEBP genes were previously described, 13 of which are FT-like genes. Five FT-like genes were found in barley (Hordeum vulgare). HvFT1, HvFT2, HvFT3, and HvFT4 were highly homologous to OsFTL2 (the Hd3a QTL), OsFTL1, OsFTL10, and OsFTL12, respectively, and this relationship was supported by comparative mapping. No rice equivalent was found for HvFT5. HvFT1 was highly expressed under long-day (inductive) conditions at the time of the morphological switch of the shoot apex from vegetative to reproductive growth. HvFT2 and HvFT4 were expressed later in development. HvFT1 was therefore identified as the main barley FT-like gene involved in the switch to flowering. Mapping of HvFT genes suggests that they provide important sources of flowering-time variation in barley. HvFTI was a candidate for VRN-H3, a dominant mutation giving precocious flowering, while HvFT3 was a candidate for Ppd-H2, a major QTL affecting flowering time in short days.