Circadian oscillator

In living organisms, circadian oscillators are cyclic biochemical processes that produce daily rhythms. In Arabidopsis, ELF3, ELF4, and LUX comprise the evening complex, or EC, which regulates circadian rhythm in the growth and development of plants. The evening complex works in conjunction with the morning complex to form Arabidopsis’ repressilator circadian oscillator. Throughout the day, CCA1 and LHY, morning complex transcription factors, regulate transcription of evening complex proteins by binding to the evening element regulatory areas, promoters of the evening complex genes. The evening complex, as a result, sees its activity peak at dusk, which is when ELF3, ELF4, and LUX see their expression peak.^[1]

At night, the EC is responsible for repressing TIMING OF CAB EXPRESSION 1 (TOC1), GI, and the mid-day genes PSEUDO-RESPONSE REGULATOR 7 and 9 (PRR7 and 9), among other genes.^[8] While TOC1 and the PRR proteins are repressed during the day by CCA1 and LHY, their expression peaks at dusk, repressing the morning complex proteins. The EC gradually inhibits TOC1 and the PRR proteins throughout the night, indirectly promoting CCA1 and LHY until they peak at dawn, promoting morning complex activity and inhibiting evening complex activity. The EC also represses LUX and ELF4, auto-regulating its own transcription.^[9]

Inhibition of GI at night causes reduced expression of the flowering-promoting genes CONSTANS (CO) and FLOWERING LOCUS T (FT). The evening complex is also responsible for repressing the transcription growth factors PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5. Components of the EC, including ELF3 and LUX, interact with PIF4 and PIF5 to inhibit growth during nighttime.^[1][10]

Because ELF3 is essential for circadian rhythmicity, loss of ELF3 function in Arabidopsis renders the clock arrhythmic, as seen by observing outputs such as leaf movement and clock gene expression in constant conditions.^[5]

Regulation of hypocotyl elongation

ELF3 is a negative regulator of seedling stem length, known as hypocotyl elongation. The EC suppresses hypocotyl elongation by inhibiting the expression of growth-promoting factors PIF4 and PIF5 in the evening.^[9] At elevated temperatures, COP1-dependent degradation of ELF3 is enhanced, which allows the expression of genes like PIF4 and PIF5, promoting hypocotyl elongation in seedlings.^[2] Independent of the evening complex, ELF3 directly interacts with the PIF4 protein to prevent it from activating its downstream gene targets, which would lead to elongation growth if activated.^[2] ELF3 also inhibits hypocotyl elongation during shade avoidance response. Similar to its interaction with PIF4, under shaded conditions, ELF3 binds to and sequesters the transcriptional activator PIF7, preventing the activation of downstream, growth-promoting genes.^[2]

Mutations in ELF3 in Arabidopsis promote the growth of long hypocotyls. Defects persist in both red-light and blue-light conditions, although they are less severe when the plants are grown in constant white light.^[11][12] Studies have shown that ELF3 mutations have additive effects on hypocotyl elongation (in interaction with phyB mutations).^[11][12]

Temperature signal integration

The EC is responsible for integrating temperature inputs into the circadian clock by regulating the expression of both core clock genes, such as PRR7/9, LUX, and GI, and clock outputs, like the gene PIF4, which controls hypocotyl elongation.^[9] As part of the EC, association of ELF3 with the promoters of PRR9, LUX, and PIF4 is less stable at high temperatures, indicating that temperature inputs might directly control the recruitment of the evening complex to promoters.^[1] In Arabidopsis seedlings shifted to warmer temperatures (from 22°C to 28°C or from 16°C to 22°C), ELF3 was required to regulate the expression of GI, LUX, PIF4, PRR7, and PRR9.^[1]

Light signal transduction

The ELF3 protein is also a component of the PHYB signaling complex to control hypocotyl elongation. PHYB is a protein that mediates the plant's responses to continuous red light. However, ELF3 and PHYB act on independent signal transduction pathways to mediate plant flowering.^[13] It is suggested that ELF3 is able to regulate the photoperiodic induction of flowering by interacting with cryptochrome or other blue-light receptors such as ZEITLUPE (ZTL) and FLAVIN-BINDING, KELCH REPEAT, F-BOX1 (FKF1) in Arabidopsis thaliana.^[13]

Regulation of flowering time

In the long day facultative plant Arabidopsis, ELF3 is a key inhibitor of photoperiodic flowering when plants are grown in non-inductive environmental conditions.^[1] The EC represses the expression of GI, a positive regulator of flowering, in the early night by binding to the GI promoter and preventing its activation.^[2] Independent of the EC complex, ELF3 also regulates the accumulation of GI protein.^[14][2]

Mutations in ELF3 result in plants with a loss of photoperiod sensitivity and circadian regulation.^[5][1] Plants with elf3 mutations flower early in long day and short day conditions at the same developmental stage regardless of photoperiod.^[5] elf3 mutations also result in plants that are not as responsive to some light wavelengths, especially blue and green light. However, elf3 single mutants retain functional phytochrome-mediated pathways that regulate plant flowering.^[5][15]

Regulation of plant senescence

Mutations in ELF3 have also been associated with the speed of plant aging, or plant senescence, through the EC inhibition of PIF4 and PIF5.^[16] This prevents the process of leaf yellowing, an indicator of plant aging. In plants with ELF3 mutations, leaf yellowing occurred at a faster rate than wild type plants.^[16] However, ELF3's regulation pathway for senescence has not been fully established.^[16]

Angiosperm orthologs

Soybean (Glycine max) has two ELF3 orthologs in its genome. One of these two paralogs, GmELF3, has been implicated in regulating photoperiodic flowering. Recently, GmELF3 was identified as being responsible for the long-juvenile trait in some varieties of soybean, which permits its cultivation in the tropics and other low-latitude areas.^[17] Unlike in long-day facultative plants like Arabidopsis or barley, in short-day facultative plants (plants that reproduce in response to short days and long night) ELF3 appears to promote the transition from vegetative growth to reproductive growth. Mutations in GmELF3 delay flowering, allowing soybean plants to grow under the short days of low-latitude regions and still produce a reasonably sized harvest.^[17]

In rice (Oryza sativa), the ELF3 ortholog OsELF3 is required for robust circadian oscillation and is involved in the photoperiodic regulation of flowering.^[18]

Barley (Hordeum vulgare) has an ELF3 ortholog that is also necessary for circadian oscillation and regulates photoperiodic flowering, repressing the transition from vegetative to reproductive development under short days.^[9][19] In addition to an arrhythmic and early flowering phenotype, loss of elf3 function in barley causes plants grown under long-day conditions to reach reproductive maturity even faster in response to elevated temperatures (15 °C to 25 °C). Under short-days the same increase in temperature inhibited early/juvenile development. This suggested that the HvELF3 ortholog is also involved in temperature perception and temperature-mediated development.^[20] Specific varieties of barley with mutations in HvELF3 have been developed to tolerate the extremely short growing seasons and long days in high-latitude regions, such as in Scandinavia.^[20]

Other homologs

Multiple ELF3 homologs have been found in the genome of the model moss Physcomitrella patens, but it is currently unknown whether or not any of these PpELF3 paralogs have conserved function in circadian regulation or photoperiodic regulation of reproduction.^[21]

No homolog of ELF3 has been found in either Chlamydomonas reinhardtii or Ostreococcus tauri, two representative species of green algae, the organisms that are thought to be the evolutionary ancestors of land plants.^[21]