How the Plants Colonized Land: From Algal Protists to Flowering Plants
How Plants Colonized Land
Origins and Evolution of Plants
Archaeplastidia:
Red algae and green algae are the closest relatives of plants.
Taxonomic classification: Excavata, SAR, Archaeplastida, Red algae, Chlorophytes, Charophytes, Green algae, Plants, Unikonta.
Evolution from Green Algae to Land Plants
Plants evolved from green algae:
Green algae known as charophytes are identified as the closest relatives of plants.
Characteristics of Charophytes
Definition of Charophytes:
Example: Chara sp.
Common names: muskgrass or skunkweed due to its odor.
Habitat: Shallow freshwater, branched multicellular algae that can extend above the water surface.
Key Traits Shared by Charophytes and Plants
Rings of cellulose-synthesizing proteins:
Shared with charophytes and land plants, but absent in other protists.
Structure: Cellulose microfibrils synthesis via distinctive cellular rings within the plasma membrane.
Non-charophytes display linear arrangement of proteins.
Peroxisome enzymes:
Specific enzymes that minimize loss of organic products during photorespiration.
Structure of flagellated sperm:
Similar structure between land plants and charophytes, indicating an evolutionary connection.
Formation of a phragmoplast:
During cell division, microtubules form a phragmoplast between nuclei, which becomes the new cell wall.
Presence of sporopollenin-like polymer:
Charophytes produce sporopollenin, a durable polymer preventing zygote desiccation.
Major component of spore and pollen walls in many plants.
Sequence similarities:
Similar nuclear and chloroplast DNA sequences.
Highlights that plants may resemble the algal ancestors of plants; however, it doesn't confirm direct descent.
Adoption of Terrestrial Life
Advantages of Living Above Waterline:
More sunlight available for photosynthesis.
Increased carbon dioxide availability.
Nutrient accessibility from the soil.
Potential reduction in herbivores.
Disadvantages of Terrestrial Life:
Increased scarcity of water.
Risks of desiccation (drying out).
Structural support challenges due to gravity.
Derived Traits of Land Plants
Four Key Traits Present in Most Land Plants:
Alternation of generations and multicellular, dependent embryos:
Gametophyte stage (haploid) produces haploid gametes via mitosis.
Fertilization leads to forming a diploid sporophyte (2n) that produces haploid spores via meiosis.
Representation in Figure:
Gametophyte (n) → Mitosis → Gametes → Fertilization → Zygote (2n) → Sporophyte → Meiosis → Spores.
Walled spores produced in sporangia:
Sporophyte generates spores within sporangia.
Diploid sporocytes undergo meiosis to create haploid spores, which have sporopollenin in their walls for resistance to harsh environments.
Multicellular gametangia:
Female gametangia (archegonia) produce eggs and facilitate fertilization.
Male gametangia (antheridia) produce and release sperm.
Apical meristems:
Regions of localized growth sustaining continual growth.
Involved in differentiation into specialized organs above and below ground.
Additional Derived Traits of Land Plants
Cuticle:
A protective waxy layer over the epidermis, preventing desiccation and resistance to microbial attack.
Stomata:
Openings for gas exchange (CO2 intake).
About 95% of water loss occurs through stomata during gas exchange.
Interactions with Fungi (Mycorrhizae):
Fossils indicate early plants (420 mya) were associated with endomycorrhizae, enhancing nutrient and water absorption from soil.
Production of Secondary Metabolites:
Derived from primary metabolic pathways (alkaloids, terpenes, tannins, flavonoids).
Used to deter herbivory.
The Origin and Diversification of Plants
Fossil Evidence:
Indications that plants colonized land at least 475 million years ago (mya).
Early fossil records show grouped spores from ancient plants rather than single grains.
Extant Plant Lineages and Evolution Timeline:
Timeline:
Origin of land plants ~475 mya.
Origin of vascular plants ~425 mya.
Origin of extant seed plants ~305 mya.
Grouping based on Vascular Tissue:
Vascular plants with vascular tissues vs. nonvascular plants (bryophytes).
Bryophytes as a non-monophyletic group with unresolved relationships.
Seed Plants Classification:
Seed plants can be divided into two major clades:
Gymnosperms (naked seed, including conifers).
Angiosperms (flowering plants).
Nonvascular Plants (Bryophytes):
Dominated by three phyla:
Liverworts (phylum Hepatophyta).
Mosses (phylum Bryophyta).
Hornworts (phylum Anthocerophyta).
Bryophyte gametophytes dominate life cycles, remaining larger and longer-lived compared to sporophytes, which depend on them.
Morphology of Bryophyte Gametophytes:
Forms ground-hugging carpets; too delicate for large heights due to lack of vascular tissues.
Rhizoids anchor gametophytes to substrates, helping maintain stability.
Morphology of Bryophyte Sporophytes:
Shoots from archegonia, consisting of foot, seta (stalk), and sporangium (capsule) to discharge spores.
Ecological and Economic Importance of Mosses
Common in cold, wet environments:
Moss bogs cover roughly 3% of Earth's land surface, with 25% of such bogs located in Canada.
Growth rate: 1/16 inch per year.
Peat Moss Incorporation:
Sphagnum decomposed material yields peat moss, crucial for global carbon reservoirs (containing 30% of soil carbon).
Concerns over overharvesting leading to CO2 release due to peatland drying.
Evolution and Diversity of Vascular Plants
Rise of Seedless Vascular Plants:
First tall plants including ferns developed during the Devonian and Carboniferous periods (425-300 mya).
Vascular tissues enabled significant growth, allowing height for competitive advantage.
Early Vascular Plant Fossils:
First records ~425 mya showing early vascular plants around 15cm tall without roots.
The appearance of branching structures opened avenues for complex body plans possibly driven by space and light competition.
Vascular Plant Anatomy:
Sporangium: Tissue for spore formation found on modified leaves called sporophylls.
Clusters of sporangia called sorus under sporophylls.
Heterospory vs. Homospory:
Most seedless vascular plants are homosporous, producing one spore type leading to bisexual gametophytes.
Seed plants are heterosporous: they produce megaspores (female gametophytes) and microspores (male gametophytes).
Vascular Tissue Transport:
Vascular tissue is crucial for nutrient and water transport:
Xylem: Conducts water/minerals, includes dead cells called tracheids, strengthened by lignin for support.
Phloem: Living cells that distribute organic products.
Impact of Lignified Vascular Tissue:
Enabled taller plants to effectively compete for sunlight.
Increased dispersal distance of spores for rapid colonization of environments.
Natural selection favored taller plants, contributing to the rise of the first forests (~385 mya).
Evolution of Roots and Leaves:
Roots: Essential for anchoring and stability of vascular plants and for water/nutrients absorption from soil. Evolved from subterranean stems.
Leaves: Increase surface area for photosynthesis, can be categorized as:
Megaphylls: Highly branched vascular system.
Microphylls: Single vein structure, appearing in the fossil record ~410 mya.
The Role of Decay in Coal Formation
Decay of early vascular plants during Carboniferous led to coal formation.
This era's increased plant growth likely removed CO2 from the atmosphere, influencing global cooling post-Carboniferous.
Coal formation from decaying plants of this period contributed to energy resources.
Conclusion
The evolution of land plants from charophytes and their adaptation to terrestrial environments has significant evolutionary implications, showcasing traits that have allowed them to thrive in a variety of environments, significantly influencing global ecosystems and climate.