Plants can reproduce both sexually and asexually. When a plant reproduces asexually, it clones itself—it creates another plant with an identical genotype. In asexual reproduction, new individuals are produced from a single parent without the fusion of sex cells. While plants may still reproduce sexually, asexual reproduction is advantageous whenever conditions are stable and the plant is thriving in its current environment.

When environmental conditions vary or change suddenly, genetic diversity becomes important. Those organisms adapted to the environmental changes will have a better chance of surviving. Genetic diversity is generated through sexual reproduction—a process that involves the fusion of two reproductive cells, or gametes, such as an egg and a sperm. In sexual reproduction, genetic diversity is achieved through recombination and independent assortment of gamete chromosomes.

Both sexual and asexual reproduction are important in agriculture. For example, most fruit trees are grown from grafts. A graft is a shoot or bud, called the scion, that has been joined to another plant, called the stock. Grafting allows for optimal combinations of root systems, good tasting fruit, and resistance to pests. Grafting is a form of asexual reproduction.

Sexual reproduction and selection are at the heart of plant breeding. Many characteristics, such as resistance to disease and pests, have been introduced into crop plants over hundreds of years. One example is resistance to leaf rust, a damaging disease caused by a fungal pathogen. Wheat-breeding programs have developed varieties of wheat that are resistant to the rust fungus. Resistant varieties typically possess a leaf rust resistance gene, called an Lr gene.

Biotechnology provides another route to introducing desired genes and traits into plants. It can accomplish things that traditional breeding techniques cannot. For example, resistance to leaf rust that is introduced through traditional cross breeding, is often associated with a single gene product. Resistance to this gene product may be overcome by a resistant strain of rust fungus that evolves through natural selection. After a few years, the once-resistant varieties of wheat lose their effectiveness against the disease. This process is similar to the way bacteria develop resistance to antibiotics.

To get around these problems, scientists are in the process of bioengineering a more durable variety of wheat that has multiple-gene resistance. This is like using a multiple drug cocktail in the treatment of HIV, the virus that causes AIDS. The wheat’s fungus-resistant gene products work together like HIV drugs. Even if the fungus becomes resistant to one of the gene products, the other gene products still prevent the fungus from growing on the wheat.

In this activity we’ll learn how plants reproduce and develop. We’ll start by examining the different types of asexual reproduction. We’ll then learn about sexual reproduction and embryonic development in plants.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education