Viridiplantae

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The Viridiplantae (alias:Chlorobionta)  are a clade of eukaryotic organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems. Like all eukaryotes, the Viridiplantae have cells with membrane-bound organelles, including a nucleus (containing chromosomes composed of linear chains of DNA bound to proteins, that are sorted during cell division by mitosis), microtubules, mitochondria, an endoplasmic reticulum, vesicles, and golgi bodies. Although the interrelationships of the non-land plant Viridiplantae will not be covered in detail here, it is important to realize that some of the evolutionary innovations, or apomorphies, that we normally associate with land plants actually arose before plants colonized the land.

In some classification systems, the group has been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional plant kingdom to include the green algae. Adl et al., who produced a classification for all eukaryotes in 2005, introduced the name Chloroplastida for this group, reflecting the group having primary chloroplasts with green chlorophyll. They rejected the name Viridiplantae on the grounds that some of the species are not plants, as understood traditionally. The Viridiplantae are made up of two clades: Chlorophyta and Streptophyta as well as the basal Mesostigmatophyceae and Chlorokybophyceae. Together with Rhodophyta and glaucophytes, Viridiplantae are thought to belong to a larger clade called Archaeplastida or Primoplantae.

Several apomorphies unite the Viridiplantae. One possible novelty for this group is a cellulosic cell wall. Cellulose, like starch, is a polysaccharide, but one in which the glucose sugar units are bonded in the beta-1,4 position (=β-1,4-glucopyranoside). This slight change in chemical bond position results in a very different molecule. Cellulose is secreted outside the plasma membrane as microscopic fiber-like units called microfibrils that are further intertwined into larger fibril units, forming a supportive meshwork. The function of cellulose is to impart rigidity to the cells, acting as a sort of cellular exoskeleton. The evolution of a cellulosic cell wall was a preamble to the further evolution of more complex types of growth, particularly of self-supporting shoot systems. It is not clear if a cellulosic cell wall constitutes an apomorphy for the Viridiplantae alone, as it may have evolved much earlier, constituting an apomorphy for the Viridiplantae plus one or more other groups; in any case, its adaptive significance seems clear.

Perhaps the primary apomorphy for the Viridiplantae is a specialized type of chloroplast. As discussed in Chapter 1, chloroplasts are one of the major defining characteristics of traditionally defined “plants”; their adaptive significance as organelles functioning in photosynthesis, the conversion of light energy to chemical energy, is unquestioned. Chloroplasts in the Viridiplantae, the green plants, differ from those of most other organisms, such as the red and brown “algae,” in (1) containing chlorophyll b in addition to chlorophyll a, the former of which acts as an accessory pigment in light capture; (2) having thylakoids, the chlorophyll-containing membranes, that are stacked into grana, which are pancakelike aggregations; and (3) manufacturing as a storage product true starch, a polymer of glucose sugar units (= polysaccharide) in which the glucose molecules are chemically bonded in the alpha-1,4 position (α-1,4-glucopyranoside). Thus, all green plants, from filamentous green “algae” in a pond or tide pool to giant sequoia or Eucalyptus trees have this same type of chloroplast. Recent data imply that chloroplasts found in the green plants today were modified from those that evolved via endosymbiosis, the intracellular cohabitation of an independently living, unicellular prokaryote inside a eukaryotic cell.

The Viridiplantae as a whole are classified as two sister groups: chlorophytes, or Chlorophyceae, and streptophytes, or Streptophyceae. The traditional “green algae” are a paraphyletic group (which is why the name is placed in quotation marks) and are defined as the primarily aquatic Viridiplantae, consisting of all chlorophytes and the nonland plant streptophytes. “Green algae” occur in a tremendous variety of morphological forms. These include single cells with or without flagella, thalloid forms, motile and nonmotile colonies , and non-motile filaments. Many have flagellated motile cells in at least one phase of their life history. “Green algae” inhabit fresh and marine waters and some live in or on soil (or even on snow!) or in other terrestrial but moist habitats.

The primitive type of green plant sexual reproduction seems to have been the production of flagellate, haploid (n) gametes that are “isomorphic,” that is, that look identical. Fertilization occurs by union of two of these gametes, resulting in a diploid (2n) zygote. The zygote, which is free-living, then divides by meiosis to form four haploid spores, each of which may germinate and develop into a new haploid individual, which produces more gametes, completing what is termed a haplontic (or “haplobiontic”) life cycle.

Within the streptophyte lineage that gave rise to the land plants, a few innovations evolved that may have been “preadaptations” to survival on land. First of these was the evolution of oogamy, a type of sexual reproduction in which one gamete, the egg, becomes larger and nonflagellate; the other gamete is, by default, called a sperm cell. Oogamy is found in all land plants but independently evolved in many other groups, including many other “algae” and in the animals.

Several other apomorphies of and within the Viridiplantae include ultrastructural specializations of flagella and some features of biochemistry. Although these have been valuable in elucidating phylogenetic relationships, their adaptive significance is unclear, and they will not be considered further here.

An apomorphy for the charophytes, a clade within the streptophytes that includes Coleochaete, Charales, and the land plants, are plasmodesmata. Plasmodesmata are essentially pores in the primary (1 °) cell wall through which membranes traverse between cells, allowing for transfer of compounds between cells. Plasmodesmata may function in more efficient or rapid transport of solutes, including regulatory and growth-mediating compounds, such as hormones.