Hershey, D.R. 2004. Plants Have Polarity Too. American Biology Teacher (submitted).

Plants Have Polarity Too

The August 2004 Biology Today (Flannery 2004) contained a key error (C⁺ on page 452 should be Ca²⁺) and several important omissions including no mention of plants. There is much recent polarity research on plants, especially on the model plant Arabidopsis thaliana (Betts & Moore 2003, Geisler et al. 2003, Grebe 2004, Kidner & Martienssen 2004, Lindsay 2004). Much early polarity research involved plants too. Hermann von Vöchting first applied the term polarity to plants. In 1878, he reported that, in high humidity chambers, willow stem cuttings formed roots at the morphologically basal end and shoots at the morphologically apical end regardless of the cutting's orientation to gravity (Mohr & Schopfer 1995). Vöchting-type experiments are probably simpler for students to conduct than current model systems.

C. elegans was discussed but the genus was not mentioned. The following multiple choice question illustrates why the genus name must be given the first time a species is mentioned in an article:

The C. elegans used in polarity research is

a. Corydoras elegans
b. Calochortus elegans
c. Cymbidium elegans
d. Cestrum elegans
e. Chamaedorea elegans
f. Caenorhabditis elegans

The answer is given at the end of this letter.

Biology Today was unable to come up with a simple definition for polarity. Taiz & Zeiger (1991) succinctly defined polarity as "the opposite ends of the axis are different." Another concise definition is "the difference of one end from the other" (Purves et al. 1998). David Webb defined it simply as “regional differences.”

The acronym PAR was also not defined. PAR is often used as an acronym for photosynthetically active radiation. In this case PAR apparently stood for protease-activated receptor, which should have been the heading for the PAR section.

Lewis Thomas is correct that we don't know much compared to what is left to discover. It is also important to keep in mind that some of what we think we know today will someday turn out to be inaccurate. For example, biology texts usually state that photorespiration in plants is simply a wasteful process with no purpose. Recent research reveals that photorespiration plays a necessary role in nitrogen assimilation (Rachmilevitch et al. 2004). In addition, some of what we do know is often stated incorrectly in the biology teaching literature (Hershey 2004). Satirist Ambrose Bierce (1958) expressed our uncertainty memorably,

"Education, n. That which discloses to the wise and disguises from the foolish their lack of understanding."

Here's the answer to the multiple choice question posed earlier. Corydoras elegans is a catfish from Brazil. Calochortus elegans (mariposa lily) is a wildflower collected by the Lewis and Clark Expedition in Idaho. Cymbidium elegans is an orchid native to India. Chamaedorea elegans (parlor palm) is widely grown as a houseplant and is native to Mexico and Guatemala. Cestrum elegans is a climbing Mexican shrub cultivated for its red flowers. Caenorhabditis elegans, a 1 mm-long soil nematode of temperate regions, is one model organism for polarity research. The genus is often pronounced as see-no-rab-DIE-tis.

David R. Hershey, Ph.D.
dh321z@yahoo.com

References

Betts, H. & Moore, I. (2003). Plant cell polarity: the ins-and-outs of sterol transport. Current Biology, 13, R781-783.

Bierce, A. (1958). The Devil's Dictionary. New York: Dover.

Flannery, M. (2004). Polarity at many levels. American Biology Teacher, 66, 449-453.

Geisler, M.J., Deppong, D.O., Nadeau, J.A. & Sack, F.D. (2003). Stomatal neighbor cell polarity and division in Arabidopsis. Planta, 216, 571-579.

Grebe, M. (2004). Ups and downs of tissue and planar polarity in plants. Bioessays, 26, 719-729.

Hershey, D.R. (2004). Avoid misconceptions when teaching about plants. ActionBioscience.org

Kidner, C.A. & Martienssen, R.A. (2004). Spatially restricted microRNA directs leaf polarity through ARGONAUTE1. Nature, 428, 81-84.

Lindsay, K. (ed.) (2004). Polarity in Plants. Volume 12 of Annual Plant Reviews.

Mohr, H. & Schopfer, P. (1995). Plant Physiology. Berlin: Springer-Verlag.

Purves, W.K., Orians, G.H., Heller, H.C. & Sadava, D. (1998). Life: The Science of Biology. Sunderland, MA: Sinauer Associates.

Rachmilevitch, S., Cousins, A.B. and Bloom, A.J. 2004. Nitrate assimilation in plant shoots depends on photorespiration. Proc. Nat. Acad. Sci. 101(31): 11506-11510.

Taiz, L. & Zeiger, E. (1991). Plant Physiology. New York: Benjamin/Cummings.