Lecture 41 May 9 2003 R. Jones Chapters 33 and 36

1. Legumes form associations with bacteria of the genus Rhizobium (but other plant species such as Ceanothus, alder and other non-agricultural species harbor N-fixing prokaryotes). The bacteria live in nodules on the roots of plants and fix large amounts of N using the energy provided by the plant (Figure 32.13). This is another form of mutualism. The plant gets fixed N and the bacterium gets shelter and food. In older forms of agriculture, crop rotation was commonly practiced where legumes where grown in alternate cycles with other non-N fixing crops as a means of enhancing soil nitrate levels. This practice not so common now. Wheat farmers, for example, grow only wheat, but modern agricultural practices such as no-till farming are much superior to older farming methods!

2. Plants absorb nitrate, phosphate as well as other essential elements such as sulfur, potassium, iron, calcium etc. from the soil and they incorporate these elements into their tissues. Th elements required by plants at the highest concentrations are C, H, O N, P and S. With the exception of C, these elements are absorbed through the roots. Plants have developed interesting mechanisms to forage for these elements-elaborate root systems, mycorrhiza, root nodules and plants are pretty good at foraging for mineral elements.

3. Because of this plants are also used for phytoremediation. Plants can take up beneficial elements but also non-beneficial ones as well. For example, selenium (Se) is a very common naturally occurring element in soil in California and Se accumulates in many plants. Se can cause death or blind staggers in grazing animals. Not too good for humans either. Some plants can accumulate and tolerate lots of aluminum. Al is abundant in soil and soluble in acid soils (30% of US agricultural soils are acidic and therefore have high Al). Researchers trying to make plants more Al tolerant. Maybe good, maybe bad because if plants accumulate high Al perhaps this may not be beneficial to human health. Some species of plants can take up heavy metals such as cadmium, mercury, lead etc. This useful for phytoremediation but not so good for dietary applications.

4. Hormones were first discovered in plants. Plants have an array of chemical compounds that enable them to respond to cues from the environment. Because plants are sessile (i.e. stuck in the ground) they must have particularly good defenses. Running and hiding is not an option. For example, abscisic acid (ABA) is a hormone that regulates stomatal opening and closing. Plants produce ABA in response to water stress and close their stomates to reduce water loss.

5. Charles Darwin and his son Francis were the first to realize that plants responded to their environment and that this response was regulated by some internal chemical factor. They showed that the bending of young grass shoots (coleoptiles) to light was caused by some diffusible chemical substance. They were the first to recognize that a substance produced in one part of the organism could influence growth in another part. This is the classical definition of what we now know as a hormone.

6. The nature of this hormone was discovered in the 1930s and found to be quite simple compound related in structure to the amino acid tryptophan and called auxin. These auxins regulate responses of plants to light (phototropism) gravity (gravitropism) and apical dominance (branching pattern) by moving around the plant and stimulating growth.

7. Ethylene is a simple gas that plants produce that regulates processes such as fruit ripening, senescence of leaves and flowers etc. It is a very important gas commercially because it hastens ripening and spoils fruits and vegetables when they are transported.

8. Cytokinins are cell division hormones in plants. These were discovered by people in the 1950s who were trying (unsuccessfully) to grow plant cells in culture. Before the discovery of the cytokinins plant cells would not grow but it was discovered that coconut milk could stimulate cell division if added to cells. Found that the active ingredient in coconut milk was similar to the base adenine found in DNA and RNA and was a cytokinin.

9. Gibberellins are stem growth regulating compounds that cause stems to be extremely elongated. Discovered in Japan by researchers investigating a disease of rice called foolish seedling disease. This disease was caused by a fungus and caused the stems of rice to be very long and finally the stems fell over-thus foolish stems. It was discovered that the fungus (called Gibberella) causing this disease makes a hormone that when applied to rice (or any other stems) causes dramatic stem growth. This compound is also important in stimulating fruit growth, seed germination etc. Commercially very important. Every table grape in California that's not organically grown is sprayed with gibberellin.