Green algae and land plants share key traits that hint at their common ancestry. Chloroplasts, cellulose cell walls, and starch storage are hallmarks of these photosynthetic organisms. Their life cycles alternate between haploid and diploid stages, with flagellated sperm for aquatic reproduction.

Charophytes, a group of algae, are the closest living relatives of land plants. They share features like oogamous reproduction, phragmoplasts, and plasmodesmata. DNA analysis has confirmed this relationship, revealing the evolutionary journey from water to land and the adaptations that made it possible.

Green Algae and Land Plants

Traits of algae and land plants

• Chloroplasts containing chlorophyll a and b pigments enable photosynthesis (green color)
  • Originated from endosymbiotic event where ancestral eukaryotic cell engulfed cyanobacteria
• Cell walls made of cellulose provide structural support and protection
  • Some land plants also incorporate lignin for additional strength
• Starch stored in plastids serves as energy reserve polysaccharide (glucose polymer)
• Alternation of generations life cycle with distinct haploid gametophyte and diploid sporophyte stages
  • Gametophyte produces gametes (sperm and eggs) for sexual reproduction
  • Sporophyte produces spores that develop into new gametophytes
• Flagellated sperm cells swim through water to reach eggs for fertilization (aquatic reproduction)

Charophytes as plant relatives

• Multicellular thalli with branching filaments resemble early land plant morphology
• Oogamous sexual reproduction involves large, non-motile eggs and smaller, flagellated sperm (similar to bryophytes)
• Phragmoplast forms during cell division guides cell plate formation (also in land plants)
  • Microtubule structure that separates daughter cells
• Plasmodesmata connections allow communication and transport between cells (symplastic pathway)
• Contain chlorophyll a, b and carotenoid pigments in plastids (like land plants)
• Cell walls contain cellulose, pectins and hemicelluloses (similar composition to land plants)
• Share many genetic markers and have closely related gene sequences
• Phylogenetic analyses consistently place charophytes as sister group to land plants

DNA analysis in plant evolution

• Morphological traits can be misleading due to convergent evolution (similar adaptations in unrelated organisms)
• Comparing DNA sequences of nuclear, chloroplast and mitochondrial genomes provides evolutionary insights
• Phylogenetic trees constructed from molecular data reveal evolutionary relationships and distances between taxa
  • Monophyletic groups consist of an ancestor and all its descendants (natural classification)
• DNA evidence resolves uncertainties in plant classification based on morphology alone
  • Confirms charophytes as closest living relatives of land plants (consistent with other evidence)
• Molecular clock analyses estimate timing of divergence events and origins of key adaptations
  • Transition to terrestrial habitats
  • Development of vascular tissues, seeds, flowers, etc.
• Advances in sequencing technologies and bioinformatics continue to reshape our understanding of plant evolution and diversity

Adaptations for terrestrial life

• Cuticle: Waxy layer on plant surfaces that reduces water loss
• Stomata: Pores in plant surfaces that regulate gas exchange and water loss
• Reproductive structures:
  • Archegonia: Female reproductive organs that produce and protect eggs
  • Antheridia: Male reproductive organs that produce sperm
  • Sporangia: Structures that produce and contain spores
  • Gametangia: General term for reproductive organs that produce gametes