Why Plants Tell Time

Question timeLast time we had a taste of HOW plants tell time.

But what, if any, are the adaptive advantages to plants for doing so?

It has long been presumed that the ability to anticipate day/night cycles gives organisms a fitness advantage.

For instance, this would allow plants to anticipate daylight and adjust their photosynthetic metabolism accordingly, perhaps getting a “head-start” on plants that didn’t.

But the ability of plants to tell time also allows them to adjust their development to seasonal variations in the environment.

Probably the best examples of this have to do with flower formation.

Time to Flower?

What factors influence when a plant flowers?

Unlike animals, plants don’t start out with their “naughty bits” – they have no sexual organs, a.k.a., flowers. They are said to grow “vegetatively”, that is, to produce just stems and leaves (and roots, of course).

Cactus clockIt’s a very big deal (way bigger than puberty, for example) to the plant when it makes the transition from a vegetative plant to a flowering plant. This involves the “flipping” of some major genetic “switches”, that is, major changes in gene regulation.

Thus, perhaps a better question is: what factors influence the flowering transition in plants?

The Length of the Night

It turns out that one of the most important factors involved in the transition to flowering in many plants is photoperiodism.

By definition, the photoperiod is the duration of an organism’s daily exposure to light.

For plants – particularly 175262601 78dd6804c3 min temperate zones – it’s conceivably a way for them to tell the time of year.

The discovery of photoperiodism and flowering in plants is attributed to Garner and Allard. In 1920 they published their findings on how day length affected flowering in certain varieties tobacco.

From their research, they determined that the plants could be divided into three general groups by how they flowered in response to relative day lengths:

Short-Day Plants (SDP) flowered after exposure to relatively short days; Long-Day Plants (LDP) that flowered after relatively long days; and Day-Neutral Plants (DNP) that didn’t seem to flower in response to photoperiod.

From experiments that interrupted the night with a brief period of light, we now know that it’s the night length that is critical in the photoperiodic control of flowering.

This means that SDP are really Long-Night Plants! (confusing?…more about this at another time, or see here).
Plant clock
Photoperiod + Circadian Rhythm

To further complicate our attempts at understanding of the photoperiodic control of flowering, it’s clear that the photoperiodic time-keeping mechanism is coupled with the plant’s internal circadian clock .

Though this complex mechanism is currently not fully understood, a simplified explanation may be as follows.

In plants that flower in response to photoperiod, a flowering signal (called florigen) may fluctuate in the leaves with a circadian rhythm. (LDP and SDP may differ in how the level of florigen is coupled to the circadian rhythm.)

When the external photoperiod (sensed by the leaves) is coincident with a certain phase of the internal clock (i.e., level of florigen in the leaves), then leaves send enough florigen to the apical meristem (via the phloem) to trigger the floral transition. (For a more thorough explanation see Ref. 2 below.)

Much recent progress has been made in identifying florigen and how it triggers the massive changes in gene regulation that lead to the floral transition. (Much more about this in upcoming weeks.)

Time Affects More Than Flowering

Recent scientific evidence indicates that plants’ responses to the environment are also affected by the biological clock.

“The circadian clock is likely to increase the fitness of plants through many mechanisms, including (1) temporal compartmentation of metabolic processes; (2) anticipation of daily environmental changes; (3) optimization of the turnover rate of proteins; (4) anticipation of seasonal environmental changes; and (5) gating of environmental signals.” (from Ref. 3 below)

Recent News: Plants use circadian rhythms to prepare for battle with insects. A Rice University study shows that plants make predawn preparations to fend off hungry caterpillars.

Bottom line: The ability of plants to tell time allows them to adjust their metabolism, development and environmental responses on a daily, as well as seasonal, basis.

References

1. Gardner, M. J., K. E. Hubbard, C. T. Hotta, A. N. Dodd, and A. A. R. Webb (2006) “How plants tell the time.” Biochemical Journal, Vol. 397(Pt. 1), pp.15–24. (Full Text)

2. Hayama, R. and G. Coupland (2004) “The molecular basis of diversity in the photoperiodic flowering responses of Arabidopsis and rice.” Plant Physiology, Vol. 135, pp. 677-684. (Full Text)

3. Hotta, C. T., et al. (2007) “Modulation of environmental responses of plants by circadian clocks.” Plant, Cell & Environment, Vol. 30, pp. 333–349. (Full Text)

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