BIOL120 Lectures 10–16: Plants, Algae & Land-Plant Evolution

Know what a land plant is, and recognise the major groups of extant (living) land plants.

Land plants are organisms characterized by several key evolutionary features, or synapomorphies, that enabled their transition to and survival in terrestrial environments:

• Multicellular forms with both haploid (n) and diploid (2n) generations.

• Sporopollenin-walled spores, which are resistant to desiccation.

• Multicellular gametangia (structures producing gametes) and multicellular embryos, protected within the female parent.

• Apical meristems, regions of continuous cell division at the tips of shoots and roots, allowing for indeterminate growth.

Major extant (living) land plant groups include:

• Non-Vascular Plants (Bryophytes): Liverworts, mosses, hornworts.

• Seedless Vascular Plants: Lycophytes, monilophytes (ferns, horsetails, whisk ferns).

• Seed Plants (Gymnosperms): Cycads, ginkgo, gnetophytes, conifers.

• Seed Plants (Angiosperms): Flowering plants.

Understand the concepts of the general land plant life cycle and alternation of generations, and that these apply to all land plants.

All land plants exhibit a life cycle characterized by alternation of generations, which involves a multicellular diploid (2n) sporophyte generation and a multicellular haploid (n) gametophyte generation, alternating through their life cycle:

• Sporophyte (2n): The diploid stage that produces haploid spores through meiosis (2n o n).

• Gametophyte (n): The haploid stage that produces haploid gametes (sperm and egg) through mitosis (n o n).

Fertilisation (fusion of gametes, n+n o 2n) results in a zygote that develops into a new sporophyte, completing the cycle.

Know some of the major variations in the life cycles of the major land plant groups.

The general pattern of alternation of generations varies in dominance and reproductive strategies among different land plant groups:

• Non-Vascular Plants (Bryophytes):

  • Gametophyte dominant, meaning the multicellular haploid stage is the larger, more visible plant.

  • Sporophyte is dependent on the gametophyte for nutrition and remains attached.

  • Require water for the dispersal of flagellated sperm to reach the egg.

• Seedless Vascular Plants:

  • Sporophyte dominant, being the larger, independent plant.

  • Gametophyte is small and independent, but often short-lived.

  • Possess lignified tracheids (xylem) for water transport and structural support, and phloem for nutrient transport.

  • Reproduction can be homosporous (producing one type of spore) or heterosporous (producing microspores and megaspores).

  • Water is still required for external fertilisation due to flagellated sperm.

• Seed Plants (Gymnosperms):

  • Sporophyte dominant; gametophytes are microscopic and retained within the sporophyte's reproductive structures.

  • Exhibit extreme heterospory, with separate male and female gametophytes develops from different spores types:

  • Ovule: Consists of a megasporangium and integuments, developing into a seed after fertilisation.

  • Pollen: Represents the male gametophyte, delivering non-motile or reduced sperm directly to the ovule via a pollen tube, eliminating the need for external water for fertilisation.

  • Seed provides embryo protection and nourishment.

• Seed Plants (Angiosperms):

  • Sporophyte dominant; gametophytes are even more reduced than in gymnosperms.

  • Distinct reproductive innovations:

  • Flowers: Specialized structures for sexual reproduction, attracting pollinators.

  • Fruits: Mature ovaries that enclose seeds, aiding in their protection and dispersal.

  • Double fertilisation: A unique process where one sperm fuses with the egg to form a zygote (2n), and another sperm fuses with two polar nuclei to form the endosperm (3n), which provides nourishment to the developing embryo.

Understand the broad evolutionary relationships of major land plant groups.

Land plants evolved from green algal ancestors, specifically charophytes, around ext{470 MYA}. Key evolutionary transitions marked the diversification of land plant groups:

• Transition to land: Initial evolutionary step, ~ ext{470 MYA}.

• Evolution of vascular tissue: Appearance of xylem (with lignified tracheids) and phloem, ~ ext{425 MYA}, enabling efficient transport of water and nutrients and allowing for greater plant height.

• Evolution of seeds: Emergence of seed plants, ~ ext{360 MYA}, providing enhanced embryo protection, nourishment, and dispersal capabilities, reducing reliance on water for reproduction.

These innovations show a progression from water-dependent, gametophyte-dominant forms to highly terrestrial, sporophyte-dominant forms with complex reproductive strategies.

Understand the major steps in the evolution of land plants and how they relate to the evolutionary of features relating to a terrestrial environment.

Major steps in land plant evolution reflect adaptations to the challenges of a terrestrial environment (desiccation, structural support, reproduction without water):

1. Cuticle and stomata: Early adaptations to prevent water loss and regulate gas exchange.

2. Alternation of generations: Allowed for independent sporophyte and gametophyte stages, with sporophyte becoming dominant for efficient spore dispersal in air.

3. Spores with sporopollenin: Provided desiccation resistance for dispersal in air.

4. Apical meristems: Enabled indeterminate growth and efficient light capture and resource acquisition from soil.

5. Multicellular gametangia and protected embryo: Provided protection for developing gametes and embryos from desiccation and environmental stress.

6. Vascular tissue (xylem and phloem): Revolutionary for upright growth, allowing plants to access more light and disperse spores/seeds further, and for efficient transport of water and nutrients from the soil.

7. Roots: Anchoring and efficient absorption of water and minerals.

8. Leaves: Increased surface area for photosynthesis.

9. Seeds: Provided a protected, nourished embryo that could survive harsh conditions and disperse widely without free water.

10. Pollen: Delivered male gametes without requiring water, crucial for widespread terrestrial reproduction.

11. Flowers and Fruits (Angiosperms): Enhanced pollination efficiency (often via animals) and seed dispersal.

Appreciate the huge diversity of land plants, and their importance to other life on earth, in the >400 million years since plants evolved.

Land plants are fundamental to life on Earth, composing approximately 80 ext{%} of Earth’s biomass and evolving over ext{400 million years}. They store approximately ext{450 Gt C} (living) and ext{550 Gt C} (dead), playing critical ecological roles:

• Oxygen Release: Through photosynthesis, they are the primary producers of atmospheric oxygen ( ext{O}_2).

• Carbon Fixation: Convert atmospheric carbon dioxide ( ext{CO}_2) into organic compounds.

• Ecosystem Services:

  • Provisioning: Provide essential resources like food, fiber, timber, medicines, and biofuels.

  • Regulating: Contribute to climate regulation through ext{CO}_2 sequestration and local climate cooling, as well as erosion and water control.

  • Supporting: Crucial for soil formation, nutrient cycling, habitat structure, and biodiversity maintenance.

  • Cultural: Offer aesthetic beauty and support traditions.

Threats like plant blindness, deforestation, and biodiversity loss highlight the critical need for education and conservation focus on plants due to their immense importance.

Understand what are 'algae'.

"Algae" is a functional (polyphyletic) term primarily used for diverse aquatic photosynthetic protists. They are not a single taxonomically unified group but rather a collection of distantly related organisms that share the characteristic of conducting photosynthesis. Their wide-ranging evolutionary origins are reflected in their presence across multiple eukaryotic supergroups, including Excavata, SAR, and Archaeplastida. Despite their taxonomic diversity, algae are fundamental to aquatic ecosystems, acting as crucial primary producers, generating significant amounts of atmospheric oxygen, and forming the base of many food webs.

Know the different autotrophic supergroups.

Photosynthetic lineages (which include algae) are found in three major eukaryotic supergroups:

• Excavata: Includes euglenids, which are photosynthetic.

• SAR (Stramenopiles, Alveolates, Rhizarians): Contains important photosynthetic groups like diatoms, dinoflagellates, and brown/golden algae.

• Archaeplastida: Consists of red and green algae, and glaucophytes. This supergroup is significant as it includes the direct ancestors of land plants.

Key photosynthetic protist groups include:

• Dinoflagellates: Characterized by cellulose plates and two flagella, known for harmful algal blooms (red tides) and bioluminescence.

• Diatoms: Possess silica frustules, contribute to approximately 45 ext{%} of oceanic ext{O}_2 production, and play a role in the carbon pump.

• Brown algae (kelps): The largest protists, forming 3-D coastal forests, and a source of alginates.

• Red algae: Contain phycoerythrin pigment, form coralline reefs, and are used for agar/carrageenan and as a food crop (nori).

• Green algae: Major primary producers in freshwater; charophytes within this group are considered ancestral to land plants.

Understand how light capture by pigments and photosynthesis function.

Photosynthesis is the process by which light energy is converted into chemical energy, primarily in the form of glucose. The overall equation for oxygenic photosynthesis is:

• 6\,CO2 + 6\,H2O \xrightarrow{\text{light}} C6H{12}O6 + 6\,O2

Light capture is facilitated by various pigments that absorb light within the photosynthetically active radiation (PAR) range (400 ext{–}700 ext{ nm}):

• Chlorophyll a: Universal pigment found in all photosynthetic organisms.

• Chlorophyll b: Found in green algae and land plants, extending their light absorption spectrum.

• Chlorophyll c: Present in red, brown algae, and diatoms.

• Carotenoids: (e.g., fucoxanthin) Broaden the range of light absorption and provide photoprotection.

• Phycocyanin and Phycoerythrin: Accessory pigments (found in red algae and cyanobacteria) that allow light capture in deeper waters where other wavelengths are scarce, thus extending organisms' depth niches.

The diversity of pigments allows different photosynthetic organisms to optimize light capture in various environments, leading to a wide range of ecological niches.

The origin of plastids (chloroplasts) in eukaryotic cells is traced back to endosymbiotic events:

• Primary Endosymbiosis: A eukaryotic host cell engulfed a cyanobacterium, leading to the formation of plastids with two membranes, as seen in red/green algae and glaucophytes.

• Secondary Endosymbiosis: A eukaryotic host cell engulfed an alga (which already had a plastid from primary endosymbiosis), resulting in plastids with three or four membranes, characteristic of stramenopiles, alveolates, euglenids, and chlorarachniophytes.