American Scientist

To the Editors:

The phytolith story presented by Thomas Hart (“Phytoliths: The Storytelling Stones Inside Plants,” March–April) was quite exciting. The new discoveries reveal so much detail about the diets and surroundings of extinct animals, as well as the flora of ancient environments. I anticipate that my paleo- and ethnobotanical students will enjoy reading this article. I did find, however, that some botanical details, which were admittedly peripheral to Hart’s main story, were erroneous or misleading.

For example, lignin is not a tissue, as claimed. Tissues are aggregates of cells that work in concert to undertake their characteristic useful functions. Lignin, on the other hand, is a type of macromolecule that particular cell types infuse into their cell walls in order to provide specific functionality for the several kinds of tissues that contain them. The author described lignin as providing “a defensive function.” While this is doubtless true when lignin is produced by phloem fiber or certain periderm cells (for instance, cork cells), the primary role of lignin is to increase the physical strength of cell types found mainly in xylem tissue. Lignified walls resist collapse due to transpiration tension occurring in tracheary elements and increase the plant’s weight-bearing ability. Lignin is the molecular feature that makes wood woody; its main function is increasing structural strength.

The claim that “the same genes that regulate the production of lignin ... also regulate phytolith development” is misleading. None of the entries in the article’s bibliography support this statement. Although my survey of the literature was not exhaustive, I was only able to find one study that correlates one gene with the presence of both lignified cells and phytolith-bearing tissue. This research studied one genus in the periderm (rind) of its fruits, meaning one specific kind of tissue in one kind of organ. Phytoliths are usually not found accompanying lignified cells, and they are often found associated with cells that do not contain lignin. This gene locus (Hr) appears to control the distribution of specialized cell types, rather than the molecular synthesis (production) of the materials, one an organic phenylpropanoid macromolecule and the other a mineral inclusion. These are compositionally very different from each other, so their actual “production” is unlikely to involve the same genes.

Daniel K. Gladish
Miami University
Hamilton, OH

Dr. Hart responds:

Thank you to Dr. Gladish for his kind words and for the clarification regarding lignin and its role within plants. There were two publications I had in mind when discussing lignin and phytolith production. The first publication was written by Jane E. Dorweiler and John Doebley and was titled “Developmental analysis of Teosinte glume architecture1: A key locus in the evolution of maize (Poaceae)” ( American Journal of Botany, 1997). In this paper, the authors discuss the pleiotropic properties [meaning multiple traits resulting from the expression of a single gene locus] of the locus Teosinte glume architecture1 (tga1) in domesticated maize (Zea mays L. ssp. mays) and its wild progenitor, teosinte (Zea mays ssp. parviglumis Iltis and Doebley). They discuss how tga1 regulates cell production, lignification, and silicification of plant cells in the glumes of these taxa. The second publication was titled “Evidence for the control of phytolith formation in Cucurbita fruits by the hard rind (Hr) genetic locus: Archaeological and ecological implications” by Dolores R. Piperno, Irene Holst, Linda Wessel-Beaver, and Thomas C. Andres (Proceedings of the National Academy of Science of the U.S.A., 2002). In this paper, they argue that Hr regulates lignin and silica production in the rinds of Cucurbita taxa. They suggest that when there is an increase in the amount of lignin, such as can be found in the rind of wild Cucurbita, there is a corresponding increase in the amount of silica deposited in the rind. While this may not be the case in all plants, both of these papers suggest that there is at least some sort of relationship between lignin and silica production and that this relationship can be traced to a single pleiotropic locus.