Plant Systematics: an Overview

I Systematics 1 Plant Systematics: An Overview PLANTS . 3 Evolution . 10 What Is a Plant? . 3 Taxonomy . 10 Plants and the Evolution of Life . 3 Phylogeny . 13 Land Plants . 5 Why Study Systematics? . 13 Why Study Plants? . 6 REVIEW QUESTIONS . 15 SYSTEMATICS . 7 EXERCISES . 16 What Is Systematics? . 7 REFERENCES FOR FURTHER STUDY . 16 This book is about a fascinating fi eld of biology called plant defi ned by the common (but independently evolved) characteristic systematics. The purpose of this chapter is to introduce the of photosynthesis. However, delimiting organismal groups based basics: what a plant is, what systematics is, and the reasons on evolutionary history has gained almost universal acceptance. for studying plant systematics. This latter type of classifi cation directly refl ects the patterns of that evolutionary history and can be used to explicitly test evolutionary hypotheses (discussed later; see Chapter 2). PLANTS An understanding of what plants are requires an explanation of the evolution of life in general. WHAT IS A PLANT? This question can be answered in either of two conceptual PLANTS AND THE EVOLUTION OF LIFE ways. One way, the traditional way, is to defi ne groups of Life is currently classifi ed as three major groups (sometimes organisms such as plants by the characteristics they possess. called domains) of organisms: Archaea (also called Archae- Thus, historically, “plants” included those organisms that pos- bacteria), Bacteria (also called Eubacteria), and Eukarya or sess photosynthesis, cell walls, spores, and a more or less sed- eukaryotes (also spelled eucaryotes). The evolutionary relation- entary behavior. This traditional grouping of plants contained ships of these groups are summarized in the simplifi ed evolu- a variety of microscopic organisms, all of the “algae,” and tionary tree or cladogram of Figure 1.1. The Archaea and the more familiar plants that live on land. A second way to Bacteria consist of small, mostly unicellular organisms that answer the question “What is a plant?” is to evaluate the possess circular DNA, replicate by fi ssion, and lack membrane- evolutionary history of life and to use that history to delimit bound organelles. The two groups differ from one another the groups of life. We now know from repeated research studies in the chemical structure of certain cellular components. that some of the photosynthetic organisms evolved indepen- Eukaryotes are unicellular or multicellular organisms that dently of one another and are not closely related. possess linear DNA (organized as histone-bound chromo- Thus, the meaning or defi nition of the word plant can be somes), replicate by mitotic and often meiotic division, and ambiguous and can vary from person to person. Some still like possess membrane-bound organelles such as nuclei, cytoskeletal to treat plants as a “polyphyletic” assemblage (see later discussion), structures, and (in almost all) mitochondria (Figure 1.1). 3 © 2010 Elsevier Inc. All rights reserved. doi: 10.1016/B978-0-12-374380-0.00001-5 4 CHAPTER 1 plant systematics: An Overview Eukarya (Eukaryotes) Stramenopiles (Heterokonta) Alveolates (Alveolata) (Green Plants) (Red Algae) (Browns) Opisthokonts yta Oomycota molds) (water Phaeophyta Bacteria Archaea Ciliates Ampicomplexans Glaucophyta Rhodophyta Chlorobionta/ Viridiplantae Fungi Animalia Diplomonads Parabasalids Kinetoplastids Amoebozoa Rhizaria Katablepharidae Cryptoph Haptophyta Euglenozoa Dinoflagellates red chloroplast *chloroplast *chloroplast green chloroplast *chloroplast *chloroplast *chloroplast *chloroplast chloroplast origin Primary Endosymbiosis Mitochondria (by endosymbiosis) Cytoskeletal/contractile elements (actin, myosin, tubulin) Other membrane-bound organelles (endoplasm. retic., golgi, lysosomes) Mitosis (+ meiosis in sexually reproducing organisms) Nucleus (membrane bound), enclosing chromosomes DNA linear, bound to histones = endosymbiotic origin of mitochondrion and chloroplast from ancestral Bacterium Figure 1.1 Simplifi ed cladogram (evolutionary tree) of life (modifi ed from Kim and Graham 2008, Moreira et al. 2007, and Yoon et al. 2008), illustrating eukaryotic apomorphies (the relative order of which is unknown) and the hypothesis of a single origin of mitochondria and chloroplasts via endosymbiosis (arrows). Note modifi cation of chloroplast structure in the red and green plants, and subsequent secondary endosymbiosis in numerous other lineages (indicated by *). Eukaryotic groups with photosynthetic members are in bold. Some of the unicellular bacteria (including, e.g., the resemble photosynthetic bacteria in having pigment-containing Cyanobacteria, or blue-greens) carry on photosynthesis, a thylakoid membranes. biochemical system in which light energy is used to synthesize How did chloroplasts evolve? It is now largely accepted that high-energy compounds from simpler starting compounds, chloroplasts of eukaryotes originated by the engulfment of an carbon dioxide and water. These photosynthetic bacteria have ancestral photosynthetic bacterium (probably a cyanobacterium) a system of internal membranes called thylakoids, within by an ancestral eukaryotic cell, such that the photosynthetic which are embedded photosynthetic pigments, compounds bacterium continued to live and ultimately multiply inside the that convert light energy to chemical energy. Of the several eukaryotic cell (Figures 1.1, 1.2). (Mitochondria also evolved groups of eukaryotes that are photosynthetic, all have special- by this process, from an ancestral, nonphotosynthetic bacterium; ized photosynthetic organelles called chloroplasts, which see Figure 1.1.) The evidence for this is the fact that chloroplasts, unit I systematics 5 self-replicating chloroplasts ancestral . photosynthetic . .. bacterium . .. .. .. .. .. .. .. .. .. .. .. .. .. eukaryotic cell photosynthetic eukaryotic cell Figure 1.2 Diagrammatic illustration of the origin of chloroplasts by endosymbiosis of ancestral photosynthetic bacterium within ancestral eukaryotic cell. like bacteria today, (a) have their own single-stranded, circular distinctive characteristics of the green plant chloroplast with DNA; (b) have a smaller sized, 70S ribosome; and (c) replicate respect to photosynthetic pigments, thylakoid structure, and stor- by fi ssion. These engulfed photosynthetic bacteria provided age compounds (see Chapter 3 for details). Green plants include high-energy products to the eukaryotic cell; the “host” eukary- both the predominately aquatic “green algae” and a group known otic cell provided a benefi cial environment for the photosynthetic as embryophytes (formally, the Embryophyta), usually referred bacteria. The condition of two species living together in close to as the land plants (Figure 1.3). The land plants are united contact is termed symbiosis, and the process in which symbiosis by several evolutionary novelties that were adaptations to the results by the engulfment of one cell by another is termed en- transition from an aquatic environment to living on land. These dosymbiosis. Over time, these endosymbiotic, photosynthetic include (1) an outer cuticle, which aids in protecting tissues from bacteria became transformed structurally and functionally, re- desiccation; (2) specialized gametangia (egg and sperm produc- taining their own DNA and the ability to replicate, but losing ing organs) that have an outer, protective layer of sterile cells; and the ability to live independently of the host cell. In fact, over (3) an intercalated diploid phase (sporophyte) in the life cycle, time there has been a transfer of some genes from the DNA of the early, immature component of which is termed the embryo the chloroplast to the nuclear DNA of the eukaryotic host cell, (hence, “embryophytes”; see Chapter 3 for details). making the two biochemically interdependent. Just as the green plants include the land plants, the land Although knowledge of eukaryotic relationships is still in plants are inclusive of the vascular plants (Figure 1.3), the latter fl ux, the most recent data from molecular systematic studies being united by the evolution of an independent sporophyte and indicates that this so-called “primary” endosymbiosis of the xylem and phloem vascular conductive tissue (see Chapter 4). chloroplast probably occurred one time, a shared evolutionary The vascular plants are inclusive of the seed plants (Figure novelty of the red algae (Rhodophyta) and green plants (Viri- 1.3), which are united by the evolution of wood and seeds diplantae or Chlorobionta; Figure 1.1). This early chloroplast (see Chapter 5). Finally, seed plants include the angiosperms became modifi ed with regard to photosynthetic pigments, thy- (Figure 1.3), united by the evolution of the fl ower, including lakoid structure, and storage products into forms characteristic carpels and stamens, and by a number of other specialized of the red algae and green plants (see Figure 1.1). In addition, features (see Chapters 6–8). several lineages of photosynthetic organisms—including the For the remainder of this book, the term plant is treated as euglenoids, dinofl agellates, and brown algae (Phaeophyta), equivalent to the embryophytes, the land plants. The rationale and a few other lineages—may have acquired chloroplasts for this is partly that land plants make up a so-called mono- via “secondary” endosymbiosis, which occurred by the en- phyletic group, whereas the photosynthetic eukaryotes as a gulfment of an ancestral chloroplast-containing eukaryote by whole are not monophyletic and, as a group,

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