These examples were based on the manuscript available here: http://www.plantphysiol.org/content/158/2/1079
PL Summary Example: Unstructured (wc: 116)
The circadian clock, often called the biological clock, is found in most organisms, including plants. One of its functions in flowering plants is to determine when flowers appear during the plant’s life cycle. Using a weed called Arabidopsis thaliana, a common model for learning about plants, we studied how two proteins, CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) and EARLY FLOWERING 3 (ELF3), determine when flowers appear. Our results showed that CCA1 and ELF3 work together or in parallel to affect the time of flowering, providing additional information about how the complex interactions between the circadian clock and the flowering-time pathways in plants occur and provides another example of how proteins act together in networks to affect developmental processes.
PL Summary Example: Structured (wc: 637)
Another Piece of the Puzzle: The Circadian Clock and Flowering Time
The circadian clock, often just called the biological clock, is something with which we’re all familiar. It’s that mechanism that generates circadian rhythms. Shift workers and those who do a lot of air travel are well aware of their clocks. But did you know that other organisms like plants have clocks, too? It’s not all prettily wrapped up in a brain like ours, but nonetheless, it’s there in the flower garden, quietly keeping time.
The circadian clock is called an endogenous, or internally generated, timekeeper that creates 24-hour rhythms in biological processes. Environmental signals, like the sun coming up, set the pace of the clock, after which point the clock maintains its own rhythms.
Conceptually, clocks are composed of three parts: inputs, the clock mechanism that keeps time, and outputs. In plants, inputs are environmental cues like light and temperature; outputs include flowering time and leaf movement. One thing the animal clock has in common with the plant clock is that we don’t completely understand how it works or how it controls other biological processes.
The Circadian Clock and Flowering Time
Because flowering is so crucial for plant reproduction, it is controlled by multiple pathways. One of these is called the photoperiodic pathway because it is controlled by the photoperiod, or day length, via the circadian clock.
In this study, we attempted to understand how the circadian clock-associated proteins EARLY FLOWERING TIME 3 (ELF3) and CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) affect each other and flowering time. In plants, the clock is run by a group of interacting proteins. CCA1 is thought of a core clock protein because it is central to clock function. ELF3’s roles in the circadian clock are less clear. It appears to have multiple roles, including in the modulation of light input and in the clock’s core mechanism.
ELF3 is Part of the Clock Mechanism
We showed that CCA1 could affect ELF3 protein levels by binding to its promoter, the region of a gene where transcription is initiated to produce a messenger RNA (mRNA) that can then be used as a template to produce a protein. When CCA1 binds to the promoter of the ELF3 gene, its transcription is blocked, and its protein levels decline. This result firmly places ELF3 in the core clock mechanism and completes a CCA1-ELF3 negative feedback loop via other proteins.
ELF3 and CCA1 Interact in Different Ways to Promote Flowering
To determine how CCA1 and ELF3 might interact to affect flowering time, the following plant types were examined: wild-type (no genetic alterations), elf3-1 (ELF3 is not produced), CCA1-ox (CCA1 is present in higher quantities than in wild-type plants), and elf3-1 CCA1-ox plants (no ELF3 protein and an overabundance of CCA1 in the same plant). Because Arabidopsis flowers much later in short days (8 h light, 16 h dark) than in long days (16 h light, 8 h dark), the mechanisms controlling flowering could differ, and therefore plants were examined under both conditions. The number of days until the plants flowered were determined, and the mRNA expression of five different genes whose proteins are part of the photoperiodic flowering pathway were observed using a technique called quantitative real-time polymerase chain reaction (qRT-PCR). Combining the flowering time and qRT-PCR data indicates that the relationship between CCA1 and ELF3 in the determination of when plants flower is complex. In short days, CCA1 could act in parallel or coordinate with ELF3 to determine when plants flower. In long days, ELF3 appears to act before CCA1 in the photoperiodic control of flowering.
A Network, Not a Pathway
For CCA1 and ELF3, like for many relationships, it’s complicated. Like other biological systems, the proteins of the circadian clock form more of a network than a series of linear pathways.