July 11, 2011 | David F. Coppedge

Plant Patterns Prolong Perplexity

Plants perform a wonder that has attracted the admiration of scholars from ancient Egypt, Greece and Rome to modern times: the ability to reproduce mathematically perfect patterns.  This ability, called phyllotaxis, can be described mathematically with the Fibonacci Series and the Golden Angle.  The beautiful spirals in sunflowers, artichokes, cacti, dandelion heads and other plants continue to fascinate children and adults today, but those are not the only examples.  Leaves on a stem can emerge in phyllotactic patterns like a spiral staircase, and depending on the environment, plants can switch patterns at different stages in development.  Scientists have learned a lot about the players in the phyllotaxis game, but still do not understand the script.  The details of how genes and proteins produce the patterns remain elusive.

In Current Biology,1 French biologists Jean-Christophe Palauqui and Patrick Laufs recounted some of the theories that have tried to explain phyllotaxis.  Scientists know that the plant hormone auxin becomes concentrated in the shoot meristem where new organ primordia emerge, and that the PIN1 auxin transporter is able to polarize the localization of auxin.  New work reported in the same issue of Current Biology implicates the PLETHORA (PLT) gene family, known to be active with root formation, with the processes going on in the meristem.   Tinkering with the players can enhance or inhibit pattern formation.

Just how these players interact, though, is not well understood.  It's not a simple case of gene turning on protein turning on hormone; each of the players signal each other back and forth in a complex choreography.  In addition, the PLT genes seem able to stimulate mechanical forces in the primordia by the way they regulate PIN1 polarity and hence auxin distribution.  There are also time delays between gene expression and downstream effects, such as 4 hours from the time PLT genes activate to the time PIN1 transcript levels are seen to increase.  But then, auxin level can also feed back to regulate PIN1 expression.  It appears, therefore, that the intra-player signaling is indirect and complex.

The authors stated that the bewildering interactions of these players keeps biologists busy: “Elucidating the mechanism underlying PLT-mediated control of phyllotaxis will be challenging and likely depend on quantitative descriptions and modeling of PLT expression, PIN1 levels and polarization, auxin distribution, growth and mechanics,” they said.  Even if these problems are solved with mechanistic theories some day, questions may still remain about how a seed with no phyllotaxis results in a mature plant with it.  And beyond that, philosophers of aesthetics may continue to ponder how plants – and many other phenomena in nature unrelated to them, like spiral galaxies, hurricanes, conch shells, and the cochlea in the human ear – reproduce “divine proportions” that humans find beautiful (see article by Fred Willson at ICR and the 11/20/2003 entry).

1. Jean-Christophe Palauqui, Patrick Laufs, “Phyllotaxis: In Search of the Golden Angle,” Current Biology, Volume 21, Issue 13, R502-R504, 12 July 2011, DOI 10.1016/j.cub.2011.05.054.

Explaining the mechanics of something does not explain its origin.  You can understand how a robot on an assembly line works, and describe its structures and functions with flowcharts, equations and blueprints.  That knowledge will not explain why it produces a Mercedes. 

Wikipedia trivializes the explanation as a consequence of natural selection, claiming that the solution was found within a decade of Darwin by Wilhelm Hofmeister.  “Questions remain about the details,” the entry oversimplifies.  If that were true, the scientists publishing in this week’s Current Biology would not remain baffled by it.  The devil is often in the details.  Hofmeister knew nothing of PIN1 and PLT, let alone the genetic code.  His simplistic model of competing mechanical forces is so 1896; it cannot satisfy observers today, with our newer knowledge of genetic codes, proteins, and cell signals.

If evolutionists do some day get all the mechanics worked out, the questions stated above will still remain.  Considering that not all plants employ phyllotaxis, and that the patterns seem unnecessary for survival, unguided evolution reduces to an empty hand-waving story that “amazing stuff happens sometimes.”  The same criticism can be leveled at any explanation employing impersonal, material causes. 

Take a deeper look at that sunflower.  Here is a natural wonder that calls out for better science, deeper philosophy, and perhaps most of all, sound theology.

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