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Favorite plant of molecular biologists Favorite plant of molecular biologists Thale cress, Arabidopsis thaliana, a member of the Cruciferae (cabbage family). It is also used by molecular biologists studying developmental processes in plants. A number of factors make A. thaliana ideal for these applications: a short generation time (5 weeks); a high seed number per plant (10,000); a small genome (about one tenth the number of base pairs as wheat); a tendency to self – fertilize (which leads to genetic uniformity & stability); and a susceptibility to infection by Agrobacterium tumifaciens (which means that the plants can be genetically transformed by plasmids). Plant diversity is the major theme of this topic.

Learning Objectives

After completing the topic, the student will be able to:

  • Define plant kingdom and appreciate the role of plants on Earth.
  • Develop a conceptual framework how plants evolved from green algae.
  • Classify plant kingdom.
  • Understand and explore how human welfare depends greatly on seed plants.
  • Compare and contrast the differences between seed and seed – less plants.
  • Define and distinguish the terms vascular and non-vascular plants.
  • Identify the key components of the gymnosperm and angiosperm life cycles.
  • Explore some of the applications of algae to the human welfare.
Plant Kingdom Plant Kingdom A plant is a living organism. It is made up of different parts, each of which has a particular purpose, or specialized funtion.
The Greening of Earth

Looking at a lush landscape, such as the forest scene in figure, it is difficult to imagine the terrestrial environment without any plants or other organisms. Yet for more than 3 billion years of Earth’s history, the land surface was largely lifeless.

Geochemical evidence suggests that thin coatings of cyanobacteria existed on land about 1.2 billion years ago. But it was only within the last 500 million years that small plants as well as fungi and animals joined them ashore. Finally, by about 385 million years ago, some plants appeared that could grow much taller, leading to the formation of the first forests.

Since colonizing land, plants have diversified widely; today, there are more than 290,000 known plant species. Plants inhabit all but the harshest environments, such as some mountaintops, a few desert areas, and the polar regions. A few plant species, such as seagrasses, returned to aquatic habitats during their evolution, but most present-day plants live in terrestrial environments.

In this chapter, we’ll refer to all plants as land plants, even those that are now aquatic, to distinguish them from algae, which are photosynthetic protists. The presence of land plants has enabled other life-forms– including animals to survive on land. Plants supply oxygen and ultimately most of the food eaten by terrestrial animals.

Also, plant roots create habitats for other organisms by stabilizing the soil in sand dunes and many other environments. This chapter traces the first 100 million years of plant evolution, including the emergence of seedless plants such as mosses and ferns and also examines the later evolution of seed plants.

Charophytes Charophytes Examples of charophytes, the closest algal relatives of land plants.

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Evolution of plants

Land plants evolved from green algae. Researchers have identified green algae called charophytes as the closest relatives of land plants. What is the evidence for this relationship, and what does it suggest about the algal ancestors of land plants?

Morphological and Molecular Evidence Many key traits of land plants also appear in some protists, primarily algae. For example, plants are multicellular, eukaryotic, photosynthetic autotrophs, as are brown, red, and certain green algae. Plants have cell walls made of cellulose, and so do green algae, dinoflagellates, and brown algae. And chloroplasts with chlorophylls a and b are present in green algae, euglenids, and a few dinoflagellates, as well as in plants.

However, the charophytes are the only algae that share the following four distinctive traits with land plants, strongly suggesting that they are the closest relatives of plants:

  • Rings of cellulose-synthesizing proteins. The cells of both land plants and charophytes have distinctive circular rings of proteins in the plasma membrane . These protein rings synthesize the cellulose microfibrils of the cell wall. In contrast, noncharophyte algae have linear sets of proteins that synthesize cellulose.
  • Peroxisome enzymes. The peroxisomes of both land plants and charophytes contain enzymes that help minimize the loss of organic products resulting from photorespiration.
  • Structure of flagellated sperm. In species of land plants that have flagellated sperm, the structure of the sperm closely resembles that of charophyte sperm.
  • Formation of a phragmoplast. Particular details of cell division occur only in land plants and certain charophytes, including the genera Chara and Coleochaete. For example, in land plants and certain charophytes, a group of microtubules known as the phragmoplast forms between the daughter nuclei of a dividing cell.

    A cell plate then develops in the middle of the phragmoplast, across the midline of the dividing cell. The cell plate, in turn, gives rise to a new cross wall that separates the daughter cells.