June 9, 2006 | David F. Coppedge

Plant Hula-Hoop Railroads Build Cell Walls

Solving a long-standing mystery about how plants build cell walls, Stanford scientists imaged molecular machines traveling along hoop-shaped rings around the inside of the cell.  Publishing in Science, Paradez, Somerville and Ehrhardt proved that cellulose synthase (CESA), a machine that manufactures cellulose composed of six subunits arranged in rosettes, rides like a rail car on microtubules that encircle the inside of the plasma membrane.  From there, the machine extrudes the complex molecule to the exterior, building the rigid cell wall.
    Clive Lloyd, commenting on this finding in the same issue of Science,2 seemed happily astonished, not only at the scientific achievement, but at the plants themselves:

In a remarkable series of biological transformations, green plants convert carbon dioxide into cellulose fibers stronger than steel.  These thin threads of polymeric glucose are wrapped around growing cells, lending structural support to the plant as it extends further into the environment.  The fibers are not simply secreted into the plant cell wall in a haphazard fashion but are deposited in ordered layers that still allow the cell to expand.  For more than 40 years, it has been known that the alignment of these cellulose fibers (microfibrils) in the cell wall often coincides with cytoskeletal microtubules tethered to the cytoplasmic side of the plasma membrane… Despite this coincidence, there has never been direct proof that microtubules provide a guidance mechanism for the alignment of cellulose microfibrils.  Now, on page 1491 of this issue, Paredez et al. (1) provide that proof.

Lloyd described the cell-encircling hoops as a “microtubule railroad” providing tracks for the cellulose-synthesizing machines.  Apparently these tubules can reorient themselves, perhaps in hula-hoop fashion, allowing the machines to stitch cross-hatch patterns of cellulose for added strength (see 01/16/2003) for analogous process).

1Paradez, Somerville and Ehrhardt, “Visualization of Cellulose Synthase Demonstrates Functional Association with Microtubules,” Science, 9 June 2006: Vol. 312. no. 5779, pp. 1491 – 1495, DOI: 10.1126/science.1126551.
2Clive Lloyd, “Microtubules Make Tracks for Cellulose,” Science, 9 June 2006: Vol. 312. no. 5779, pp. 1482 – 1483, DOI: 10.1126/science.1128903.

Should we not gasp and applaud over how a blade of grass stands up?  Plants don’t just happen.  A plant could not grow upward against gravity without a complex, programmed arrangement of tools and parts that build the structure piece by piece in an ordered fashion.  Now, we see that this construction process involves railroad tracks and rail cars loaded with organic-chemistry wizards.
    The ways the components of the cell wall are manufactured and assembled are wonders in themselves (see 10/26/2001 for a glimpse).  Did you realize plants contain a substance stronger than steel?  And that they make it starting with just carbon dioxide (a gas) through a “remarkable series of biological transformations”?  What an amazing creation!  (We must add the obligatory observation that neither of these papers said anything about how these biological marvels might have evolved.)

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