Time Changes Everything

Time Changes Everything (Photo credit: Thomas Hawk)

With the weekend’s change back to standard time, many of you may find yourselves still ‘adjusting’ to the shift. For some, the annual clock fall back means brighter mornings or cozier darker evenings, but those of you that are caretakers of animals or young children know that it really just means you are getting up an hour earlier than the rest of the world and trying your best to stay awake to a reasonable hour at the end of the day. Oh sure, others will tell you all kinds of tricks. “Shift 10 minutes at a time the week before!”* “Stay busy all day and stay up late the night before, so you’ll sleep later.”** Despite these admonitions and our best efforts, we still spend the week adjusting.***

Don’t forget that plants and other photosynthetic organisms have their own ways of telling time on a variety of scales.They also have their own ways of adjusting these clocks.

They may not care what the time is on our clocks, but the number of light hours in the day entrains a robust circadian cycle for plants. Being able to program their physiology with the Earth’s daily rotation gives plants a significant advantage. In last week’s Nature****, Haydon and co-workers describe a component at the intersection of metabolism and gene regulation that keeps plants like Arabidopsis on time. Because of the obvious and intimate relationship that the photosynthetic machinery has with light, the products of that process- sugars- are a key output that influences the timing of the endogenous circadian clock. Haydon and co-authors were able to show that sugars also act to entrain the internal clock. In this way the system acts on light inputs as well as the metabolic output of a light-dependent process. In particular, they identify PSEUDO-RESPONSE REGULATOR 7 (PRR7) as a critical component necessary for sensing information about the photosynthetically-derived sugars and relaying it to the circadian clock machinery. Who needs a winding mechanism when plants are able to integrate so many metabolic signals to keep track of their days?

In addition to day-to-day timekeeping, some plants must also keep time with the seasons for their metabolic programming. For trees counting down the days until spring, it turns out that temperature is a bigger factor in telling time compared to day length. Remember, photosynthesis is shut down for the season at this point and the trees are relying on all their hard work during spring and summer. By the time early spring rolls around, trees must integrate information on

1 how long it has been cold (winter temperature)

2 the current warming temperature (spring temperature)

3 increasing day length (photoperiod)

Researchers Laube et al tried to disentangle the relative importance of these variables on the budburst of a number of different woody species, and the results have been published in the journal Global Change Biology. While there isn’t much changing about the seasonal day length over the course of the year on Earth for any given point on the globe, climate change is starting to have subtle effects on the winter temperature and spring temperature variables. Even these small changes can have an impact on which species are the most successful in a forest population. Their research suggests that the critical variable is the winter temperature. Some species need colder winters to accurately tell time when spring comes. Thus, warmer winters disrupt the timing of ‘leafing-out’ of these species. Other species from warmer climates do not have this strong chilling effect. This means that over time even these small temperature changes can result in ecological shifts in forest species. Species from more southern climates can start to out-compete native northern species for those first rays of spring.

I’m not sure if colder temperatures would be useful for my adjustment this week, but anything is worth a try at this point.


*Right, but this assumes that my family’s schedule is precise on that scale. Sorry, our stochastic fluctuations are considerably greater than 10 minutes, and last week (Halloween and concomitant sugar highs and lows) didn’t really lend itself to staying on a schedule.

** No, this usually just means crashing harder than normal suddenly and not as late as you would like. By that point your four-year-old is so tired he falls in the toilet as you make him ‘go’ one last time before bed. He and the dogs still wake up an hour early leaving you more exhausted than you would have been if you would have tried a more low-key approach to Saturday.

***Given my current tired state of mind and the topic of adjusting, I can’t help but reference the song from Death to Smoochy.  In my case, “you’re mom’s not mean, she’s just adjusting.”

****Sorry, paywall warning.



6 thoughts on “Adjusting

  1. John

    Hi Johnna, I have a question about the naming of the regulator in this circadian rhythm of plants. Do you know why it is named “pseudo-response regulator 7”, as opposed to simply “response regulator 7” or something of the like? I’ve looked at the paper that describes it, and it appears to function like a standard regulator – I don’t understand why Haydon chose to add the “pseudo-response” onto the name. Do you know why this is the case?
    Thank you!

    1. johnnaroose Post author

      The name ‘Pseudo-response regulator’ was not designated by Haydon et al. They just further defined what it does with respect to the circadian clock. PRR7 is how the gene is known in a number of different articles. Check out the TAIR page for the gene (Pro Tip: scroll all the way down to the bottom to see a curated list of all papers that reference that gene):
      Going back in the literature from some of those references, you will find that there is a whole family of PRRs in Arabidopsis and a number of them have roles in circadian rhythms (Check out this reference- Going back even further… you find this paper which describes the different classifications authentic response regulators (ARRs) vs. pseudo-response regulators (PRRs). Quoting from that abstract “Here we identified and characterized a set of novel Arabidopsis genes whose products considerably resemble the authentic response regulators (ARR-series) of Arabidopsis in the sense that they have a phospho-accepting receiver-like domain. However, they should be discriminated from the classical ones in the strict sense that they lack the invariant phospho-accepting aspartate site.” Apparently it only takes the absence of a single aspartate to get yourself considered a ‘second-class’ aka pseudo response regulator. Not sure if anyone has fully nailed down the functional ramifications of that difference at the molecular level.

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