Archaeplastida and Land Plant Adaptations

Archaeplastida

• Archaeplastida contains descendants from endosymbiotic cyanobacteria with chloroplasts
• Glaucophytes, also known as glaucocystophytes/glaucocystids, are a small group of unicellular algae found in freshwater and moist terrestrial environments
• These were present during the Proterozoic era but have decreased in abundance compared to other eras
• Glaucophytes are also known for their unique chloroplast structure preserving some ancient cyanobacteria traits.

Adaptations Enabling the Move to Land.

• Charophytes have a durable polymer called sporopollenin that prevents zygotes from drying out
• Moving onto land offered charophyte ancestors more sunlight, more CO2, nutrient-rich soil, and fewer herbivores/pathogens.
• Land presented challenges, including water scarcity and a lack of structural support.

Morphological and Molecular Evidence

• Many land plant characteristics also appear in various algal clades, mostly algae
• Land plants share several traits with charophytes, which include:
  • Chlorophylls a and b
  • Cellulose
  • Structure of flagellated sperm
  • Formation of sporopollenin

Land Plants Evolved from Green Algae.

• Green algae/Glaucophyta relatives called Charophytes seem to be the closest relatives of land plants.
• 290,000 living species of plants
• Comparisons of both nuclear and chloroplast genes demonstrate that charophytes are possibly the closest living relatives of land plants
• Note that land plants did not necessarily evolve from modern charophytes but share a common ancestor with modern charophytes.

Four Key Traits in Land Plants (Absent in Charophytes)

• Alternation of generations (with multicellular, dependent embryos)
• Walled spores produced in sporangia
• Multicellular gametangia
• Apical meristems
• Symbiotic relationships between fungi and the initial land plants aided in the obtaining of nutrients for plants not having true roots.

Alternation of Generations and Multicellular, Dependent Embryos.

• Plants alternate between two multicellular stages during a reproductive cycle called alternation of generations.
• The gametophyte is haploid and creates haploid gametes by mitosis.
• Fusion of the gametes results in the diploid sporophyte, which creates haploid spores by meiosis
• Land plants are called embryophytes due to the embryo's dependence on the parent.

Two Types of Gametophytes

• The diploid generation is called the sporophyte
• The haploid generation is called the gametophyte
• As plants evolved, the sporophyte tissue became dominant. This can be witnessed in examples such as mosses and vascular plants. The gametophyte is dominant in some primitive plants .

Derived Traits of Plants

• Accumulated derived traits aided in the survival of plants on land and potentially opened the way for plant colonization
• Protection from dehydration (cuticle)
• Protection from predators (secondary compounds)
• Eventually, water, mineral, and nutrient transport (xylem/tracheids and phloem/plumbing). Vascular tissue allowed enhanced internal transport.

Viridiplantae

• Viridiplantae includes the largest amount of genetic diversity among plant-like organisms
• Includes green algae, ancestors to land plants. Green algae includes Glaucocystophytes and diverse lineages.