Concept 24.4: Domestication Has Altered Plant Form

We have seen in this chapter that a very simple plant body plan—with roots, stems, leaves, meristems, and relatively few tissue and cell types—underlies the diversity of the flowering plants that cover our planet. Although differences in form among species are expected, members of the same plant species can be remarkably diverse in form as well. From a genetic perspective, this observation suggests that minor differences in genomes or gene regulation can underlie dramatic differences in plant form. (Nevertheless, some plant species do differ greatly in genome size, content, and organization.)

The form of the plant body is subject to natural selection. Phenotypes that can outcompete their peers for scarce resources have the highest fitness. Some plants have developed a viny, climbing phenotype that gives them access to light in crowded conditions. Others have a branched phenotype that puts neighboring plants in the shade. Still others have a deep and extensive root system that gives them access to water. Clearly, the vegetative body plan is as important in terms of evolution as are phenotypes relating directly to reproduction, such as flower morphology.

When humans domesticated crops and chose from nature those phenotypes most suitable for cultivation, they set in motion a long experiment in artificial selection of the plant body. For example, in the case of corn (maize), farmers selected plants with tall, compact growth and few branches, minimizing competition among individual plants. Corn was domesticated from the wild grass teosinte, which still grows in the hills of Mexico (FIGURE 24.16). One of the most conspicuous differences between teosinte and domesticated corn is that teosinte, like other wild grasses, is highly branched, with many shoots, whereas domesticated corn has a single shoot. This morphological difference is due in large part to the activity of a single gene called TEOSINTE BRANCHED 1 (TB1). The protein product of TB1 regulates the growth of axillary buds. The allele of TB1 in domesticated corn represses branching, whereas the allele in teosinte permits branching. Similarly, in the domestication of rice, farmers selected for a vertical orientation of the upper leaves so that sunlight could reach all the leaves and photosynthesis was maximized.

It may be hard to believe that a single species, Brassica oleracea (wild mustard), is the ancestor of so many familiar and morphologically diverse crops, including kale, broccoli, brussels sprouts, and cabbage (see Figure 15.5). An understanding of how the basic body plan of plants arises makes it possible to appreciate how each of these crops was domesticated. Starting with morphologically diverse populations of wild mustard, humans selected and planted the seeds from variants with the trait they found desirable. Many generations of such artificial selection produced the crops that fill the produce section of the supermarket and the stands of the farmers’ market.

ANSWER Kenaf is a biennial, herbaceous plant that grows rapidly from very active shoot apical meristems (Concept 24.1), reaching a height of 4–5 meters in 5 months. There are more than 500 known genetic strains of kenaf. Over the centuries since its domestication, kenaf has been selected to grow taller and to branch less (Concept 24.4). In addition, the plant’s adventitious roots have become longer, at about 2 meters (Concept 24.2), and more numerous, better adapting the plant to soils that are waterlogged.

Kenaf stems are harvested for fiber (FIGURE 24.17). The phloem fibers are 3 millimeters long, somewhat longer than the fibers in wood; longer fibers are desirable for stronger products. In addition, the cell walls in kenaf stems are rich in cellulose but relatively poor in lignin. This makes kenaf easier to pulp than wood, which requires strong chemical treatments to hydrolyze the lignin in its cell walls. Furthermore, 1 hectare of kenaf produces 11–17 tons of fiber suitable for pulp and paper production; this is at least three times more than a hectare of southern pine trees produces. And bear in mind that it takes 5 months to grow the kenaf and 20 years to grow the pines!

The main uses of kenaf today are for the manufacture of rope, fiber-based bags, particleboard, and most recently, paper. The last use looks increasingly viable economically.