Theme 3-Topic 13

Topic 13: Evolution of Plants

Overview of Terrestrial Plant Evolution

  • Earth’s Terrestrial Surface: Lifeless for over 3 billion years.

    • Colonization by Simple Algae: Occurred around 1.2 billion years ago (bya).

    • Emergence of Small Plants: Approximately 470 million years ago (mya).

    • Plants evolved adaptations for terrestrial life, increasing height by around 385 mya.

  • Plant Diversity: Approximately 325,000 living species.

  • Ecological Role of Plants:

    • Provide atmospheric oxygen.

    • Primary food source for land animals.

  • Habitats of Plants:

    • Thrive in various terrestrial environments such as grasslands, forests, and deserts.

    • Some species, with terrestrial ancestors, adapted to aquatic habitats.

    • Notably, about 60 species of seagrasses inhabit freshwater and marine waters.

Characteristics of Plants and Protist Clades

  • Shared Characteristics with Protists: Many characteristics of plants are also present in various protist clades, mainly algae.

    • Multicellular Photosynthetic Eukaryotes:

    • Plants are multicellular and photosynthetic (photoautotrophic).

    • Many protists, especially algae, share these traits.

    • Cell Walls:

    • Composed of cellulose, a feature shared among red, green, and brown algae, as well as some dinoflagellates.

    • Chloroplasts:

    • Plants contain chloroplasts that harbor chlorophyll a and b; this trait is also found in certain protists (e.g., green algae, euglenids).

    • Chloroplasts are considered an ancestral trait for plants, highlighting their evolutionary link to protists from the supergroup Archaeplastida.

    • Plants descend from protists that underwent primary endosymbiosis of cyanobacteria.

Evolutionary Relationship to Freshwater Green Algae

  • Ancestry of Plants: Freshwater green algae (charophytes) are the closest living relatives of land plants.

    • Specific Taxa: Zygnema and related taxa are identified as the closest charophyte relatives to plants.

    • Common Ancestry: While plants are not derived from extant charophytes, they share a common ancestor.

  • Characteristics of Extant Charophytes:

    • Multicellular and morphologically complex; examples include species of Chara and Zygnema.

    • Many living in shallow freshwater environments prone to drying.

    • Natural selection favors charophytes that can survive submersion and desiccation cycles.

Shared Traits Between Charophytes and Land Plants

  • Clade Streptophyta: Includes both charophytes and land plants.

  • Distinctive Traits:

    • Circular rings of proteins in the plasma membrane that synthesize cellulose, closely resembling that in plants.

    • Sperm structure of plant species with flagellated sperm shows similarity to charophyte sperm.

Plant Kingdom Classification

  • Clade Consideration: Should charophytes be classified as ‘plants’?

    • Clade Streptophyta (meaning ‘twisted’, referring to sperm morphology).

  • Viridiplantae Clade: Includes chlorophytes (green algae), charophytes, and plants based on pigment similarities.

  • Strict Plant Definition: Kingdom Plantae (Embryophyta) incorporates only those species with embryos, termed embryophytes.

Advantages of Terrestrial Life for Charophyte Ancestors

  • Evolutionary Advantages from Land Transition:

    1. Decreased Competition: Land-based habitats had less competition initially.

    2. Increased Photosynthesis: Access to abundant sunlight, unfiltered by water.

    3. Abundant CO2: Rich atmospheric carbon dioxide for photosynthesis.

    4. Nutrient-rich Soil: Soils on land rich in minerals and nutrients.

    5. Limited Herbivores and Pathogens: Few initial threats allowed for early plant proliferation.

  • Challenges Faced by Early Plants:

    1. Desiccation: Water scarcity made survival challenging.

    2. Lack of Structural Support: Early plants had inadequate support to grow upright.

    3. Reproduction and Dispersal Issues: Needed methods to reproduce and disperse in a terrestrial environment.

Key Adaptations for Terrestrial Survival

  1. Sporopollenin:

    • Secreted by charophyte zygotes; a durable polymer in cell walls.

      • Resistant to degradation by enzymes and inorganic chemicals.

      • Protects zygotes from desiccation, UV light, and physical stress.

      • Present in spores and pollen of plants, enhancing resistance.

  2. Adaptations for Water Conservation:

    • Waxy Cuticle: Protective layer on the epidermis for water conservation and microbial defense.

    • Stomata: Tiny epidermal pores facilitating gas exchange and water evaporation.

      • Stomata can close to reduce water loss in dry conditions.

  3. Lignified Vascular Tissue:

    • Xylem: Transports water and minerals from roots via tubes made of lignified dead cells.

      • Lignin strengthens and waterproofs xylem walls.

    • Phloem: Distributes organic products of photosynthesis, essential for plant survival.

    • Vascular Tissue Functions:

      • Provides vertical growth support through lignified xylem.

      • Enables water transport, necessary for survival in arid conditions.

  4. Functional Compartmentalization:

    • Plants exhibit structural specialization:

      • Roots: Explore underground for water and nutrients.

      • Shoots: Seek light and gas above ground.

      • Elongation and Branching: Optimize exposure to environmental resources, promoting growth in resource-rich areas.

Distinctive Characteristics of the Plant Kingdom

  1. Alternation of Generations:

    • The life cycle alternates between two genetically distinct multicellular stages,

      • Sporophyte Stage (diploid, 2n): Specialized for spore dispersal through meiosis.

      • Gametophyte Stage (haploid, 1n): Specialized for fertilization, producing haploid gametes via mitosis.

    • Unlike animals that possess only unicellular haploid stages (gametes).

    • The alternation of generations evolved independently from some algae but not from charophytes.

  2. Multicellular, Dependent Embryos:

    • Retained within the gametophyte for nutritional support, termed embryophytes due to this dependency.

    • Nutrient transfer occurs through specialized placental transfer cells.

  3. Walled Spores in Sporangia:

    • Spores produced within structures called sporangia from diploid sporocytes through meiosis, showcasing resistance due to sporopollenin.

    • Adaptation for dry conditions crucial for terrestrial survival.

  4. Apical Meristems:

    • Regions for continuous growth found at the tips of roots and shoots, enabling plant height and branching.

    • New cells produced by meristems differentiate into various tissues vital for plant function.

  5. Cuticle and Stomata:

    • The cuticle minimizes water loss and protects against microbes.

    • Stomata, formed by guard cells, regulate gas exchange and water loss.

Timeline of Plant Evolution

  • ~1.2 bya: Colonization of terrestrial environments by photosynthetic organisms (cyanobacteria and algae).

  • At least 470 mya: Evidence of land plants through fossilized spores and tissues.

  • ~450 mya: Significant presence inferred from abundant fossil records of sporophyte tissues.

  • ~425 mya: Appearance of larger structures like sporangia in fossil records.

  • Genetic studies indicate land plants originated between 490–425 mya.

Diversity of Plants

  • Classification Based on Vascular Tissue:

    • Vascular plants possess vascular tissues whereas nonvascular plants (bryophytes: liverworts, mosses, hornworts) do not.

    • Relationships among bryophytes and between vascular plants are still debated.

  • Seedless Vascular Plants:

    • Two main clades:

    • Lycophytes (e.g., club mosses)

    • Monilophytes (ferns and relatives).

  • Seed Plants: Characterized by seeds consisting of an embryo enclosed in a protective coat, representing the third clade of vascular plants.

    • Divided into Gymnosperms (e.g., conifers) and Angiosperms (flowering plants, most diverse and dominant).

Current Plant Diversity

  • Number of Known Species:

    • Nonvascular Plants (Bryophytes):

    • Phylum Hepatophyta: 9,000 (Liverworts)

    • Phylum Bryophyta: 13,000 (Mosses)

    • Phylum Anthocerophyta: 225 (Hornworts)

    • Vascular Plants:

    • Seedless Vascular Plants:

      • Phylum Lycophyta: 1,200 (Lycophytes)

      • Phylum Monilophyta: 12,000 (Monilophytes)

    • Seed Plants:

      • Phylum Ginkgophyta: 1 (Ginkgo)

      • Phylum Cycadophyta: 350 (Cycads)

      • Phylum Gnetophyta: 75 (Gnetophytes)

      • Phylum Coniferophyta: 600 (Conifers)

      • Phylum Anthophyta: 290,000 (Flowering plants)

  • Vascular plants constitute approximately 93% of all plant species, with nonvascular plants making up about 7%.