September 22, 2008 | David F. Coppedge

Leaves Don’t Fall; They’re Pushed

Rocks may fall (thus the need for warning signs on highways), but leaves are pushed off of trees by a genetic program.  The process, called abscission, has been mysterious for a long time.  A team from the University of Missouri has mapped out, for the first time, the abscission pathway in one plant.  Being this is the first day of fall, it would be worthwhile to think about the processes behind autumn’s colorful cascade of leaves.
    The opening paragraph in PNAS1 shows why leaf fall doesn’t just happen.  Look at just a few of the processes involved:

Abscission is a physiological process that involves the programmed separation of entire organs, such as leaves, petals, flowers, and fruit.  Abscission allows plants to discard nonfunctional or infected organs, and promotes dispersion of progeny.  At the cellular level, abscission is the hydrolysis of the middle lamella of an anatomically specialized cell layer, the abscission zone (AZ), by cell wall-modifying and hydrolyzing enzymes.  Thus, abscission requires both the formation of the AZ early in the development of a plant organ and the subsequent activation of the cell separation response.

Gene knockout experiments showed that proteins missing from a signalling cascade formed plants deficient in abscission ability.  “A growing paradigm in signal transduction pathways,” they explained, “features receptor modules that perceive signals and modules such as MAPK cascades that relay and amplify this information to downstream effectors.”  Because little is known about this signalling process, they studied it in the common lab plant Arabidopsis (a European/Asian herb also called thale cress).
    A press release about the study posted on PhysOrg was titled, “When leaves fall, more is occurring than a change of weather.”  That can be illustrated by the researchers’ ending paragraph.  It shows they uncovered the workings of only a small part of a very complex process:

Multiple gene products, including potential signaling ligands, membrane receptors, protein kinase cascades, regulators of hormone responses, and transcription factors have been implicated in the regulation of abscission in plants.  We have demonstrated, by several different lines of evidence, that there is a signaling cascade (Fig. 5B), from putative ligand (IDA) to receptors (HAE HSL2) to cytoplasmic effectors (MKK4, MKK5, MPK3, and MPK6), which function together to control cell separation during abscissionAdditional gene products are also likely to play important roles in abscission and the relationships between them and the signaling pathway outlined here remain to be determined.  However, based on the genetic interactions between IDA, HAE, HSL2, MKK4, and MKK5, it seems that this core signaling cascade is an important regulator of floral abscission.

All this for something we take for granted this time of year: colorful leaves drifting by the window.

1.  Cho, Larue, Chevalier, Wang, Jinn, Zhang, and Walker, “Regulation of floral organ abscission in Arabidopsis thaliana,” Proceedings of the National Academy of Sciences USA, published online before print September 22, 2008, doi: 10.1073/pnas.0805539105.

The autumn leaves drift by my window,
The autumn leaves of red and gold;
I dream of genes and MAPK modules,
Of signal pathways yet scarcely told.

When evolutionists continue to proclaim glib generalities about how plants evolved this and animals evolved that, it’s essential to look in detail at some of the structures and processes they’re talking about.  Even something as common as leaf fall is not simple.  The plant has to sense the time of year.  It has to signal the nucleus to translate genes and produce the right proteins in the right quantities.  These form a cascade of signals, with feedback loops, that instigate changes in cell adhesion.  The right cells have to start separating in the right order.  Simultaneously, the photosynthetic organs have to shut down.  The changes in pigments have to be expressed to provide plant protection (10/27/2007).  The stems have to weaken so the leaves will drop only when the plant has enough resources for the coming winter.  These are just a few considerations behind the programmed, coordinated, environmentally-responsive genetic program devoted just to this one operation.
    The PhysOrg article tried to explain why leaves fall.  “Aged leaves, for example, may be shed to facilitate the recycling of nutrients, ripening fruits dropped to promote seed dispersal and infected or diseased floral organs discarded to prevent the spread of disease.”  Whoa… that’s teleology-talk.  Stop right there on that first suggestion.  How could a tree plan its own recycling program?  After the leaves have dropped, the nutrients are gone.  They’re lying on the ground.  it doesn’t make any sense to say that the plant facilitated a recycling program, nor that it was trying to promote its own seed dispersal, or trying to prevent the spread of disease.  The plant is a brainless machine programmed with these functions.  If you don’t believe computers can emerge and program themselves, then plants cannot do such things, either.  Such subtle personification fallacies are ubiquitous in evolutionary literature.  Plants do these things because they were programmed to do them.
    Many questions remain.  How does the whole plant know to change color all at once?  Since abscission also relates to fruit and seed dispersal, how does the abscission program know when the seed ripening program has completed?  How do the stems on maple seeds loosen at precisely the time when the seeds, that work like marvelous propellers in the wind, are ready to fly?  Let’s teach our kids to see beyond the surface properties of nature into its marvelous secrets.  This is good inoculation against dogmas that would have them believe complex programmed operations just happen.
    Suggested visual resources: Journey of Life and Wonders of God’s Creation from Moody Video, and Incredible Creatures that Defy Evolution from Exploration Films.  Or, take a walk in the woods for a 360-degree, surround-sound demonstration.

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Categories: Amazing Facts, Genetics

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