The Evolution of Plants on Land

Chapter 29, 30 & 31 Plant Diversity I: How Plants Colonized Land Biology in Focus: Ch. 26—The colonization of land by plants and fungi

Key Concepts • Fossils show that plants colonized the land more than 470 million years ago • Fungi played an essential role in the colonization of land • Early land plants radiated into a diverse set of lineages • Seeds and pollen grains are key adaptations for life on land • Land plants and fungi fundamentally changed chemical cycling and biotic interactions

Overview: The Greening of Earth • Looking at a lush landscape, it is difficult to imagine the land without any plants or other organisms • For more than the first 2 billion years of Earth’s history, the terrestrial surface was lifeless • Since colonizing land, plants have diversified into roughly 290,000 living species • Plants supply oxygen and are the ultimate source of most food eaten by land animals

Concept 1: Evidence of Algal Ancestry • Green algae called charophytes are the closest relatives of land plants

Morphological and Molecular Evidence • Many characteristics of land plants also appear in a variety of algal clades, mainly algae • However, land plants share four key traits only with charophytes: • Rose-shaped complexes for cellulose synthesis • Peroxisome enzymes • Structure of flagellated sperm • Formation of a phragmoplast

Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plants • Note that land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes

Fig. 29-3 • Chara species, a pond organism • 5 mm • Coleochaete orbicularis, a • disk-shaped charophyte that • also lives in ponds (LM) • 40 µm

Adaptations Enabling the Move to Land • In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out • The movement onto land by charophyte ancestors provided unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogens • Land presented challenges: a scarcity of water and lack of structural support

The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants • Systematists are currently debating the boundaries of the plant kingdom • Some biologists think the plant kingdom should be expanded to include some or all green algae • Until this debate is resolved, we will retain the embryophyte definition of kingdom Plantae

Fig. 29-4 • Red algae • ANCESTRAL • ALGA • Chlorophytes • Viridiplantae • Charophytes • Streptophyta • Plantae • Embryophytes

Derived Traits of Plants • Four key traits appear in nearly all land plants but are absent in the charophytes: • Alternation of generations (with multicellular, dependent embryos) • http://education-portal.com/academy/lesson/alternation-of-generations-the-gametophyte-and-sporophyte.html • Walled spores produced in sporangia • Multicellular gametangia • Apical meristems

Additional derived traits such as a cuticle and secondary compounds evolved in many plant species • Symbiotic associations between fungi and the first land plants may have helped plants without true roots to obtain nutrients

Alternation of Generations and Multicellular, Dependent Embryos • Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations • The gametophyte is haploid and produces haploid gametes by mitosis • Fusion of the gametes gives rise to the diploid sporophyte,which produces haploid spores by meiosis

The diploid embryo is retained within the tissue of the female gametophyte • Nutrients are transferred from parent to embryo through placental transfer cells • Land plants are called embryophytes because of the dependency of the embryo on the parent

Fig. 29-5a • Gamete from • another plant • Gametophyte • (n) • Mitosis • Mitosis • n • n • n • n • Spore • Gamete • MEIOSIS • FERTILIZATION • Zygote • 2n • Mitosis • Sporophyte • (2n) • Alternation of generations

Fig. 29-5b • Embryo • 2 µm • Maternal tissue • Wall ingrowths • 10 µm • Placental transfer cell • (outlined in blue) • Embryo (LM) and placental transfer cell (TEM) • of Marchantia (a liverwort)

Walled Spores Produced in Sporangia • The sporophyte produces spores in organs called sporangia • Spore walls contain sporopollenin, which makes them resistant to harsh environments

Fig. 29-5c • Spores • Sporangium • Longitudinal section of • Sphagnum sporangium (LM) • Sporophyte • Gametophyte • Sporophytes and sporangia of Sphagnum (a moss)

Fig. 29-5d • Archegonium • with egg • Female gametophyte • Antheridium • with sperm • Male • gametophyte • Archegonia and antheridia of Marchantia (a liverwort)

Apical Meristems • Plants sustain continual growth in their apical meristems • Cells from the apical meristems differentiate into various tissues

Fig. 29-5e • Apical meristems • Developing • leaves • Apical • meristem • of shoot • Apical meristem • of root • Shoot • Root • 100 µm • 100 µm

The Origin and Diversification of Plants (a) Fossilized spores • Fossil evidence indicates that plants were on land at least 470 million years ago • Fossilized spores and tissues have been extracted from 450-million-year-old rocks (b) Fossilized sporophyte tissue

Concept 2: Essential role of fungi in colonization • How did early land plants obtain nutrients without roots and leaves? • Fossil evidence reveals that early land plants formed symbiotic associations with fungi to aid in nutrient absorption from the soil. (mycorrhizae)

Chapter 31 Fungi

Overview: Mighty Mushrooms • Fungi are diverse and widespread • Essential to most terrestrial ecosystems because they break down organic material and recycle vital nutrients • Fungi exhibit diverse lifestyles: • Decomposers • Parasites • Mutualists

Fungal Nutrition • Fungi are heterotrophs that feed by absorption. • Fungi use enzymes to break down complex molecules into smaller organic compounds that can be absorbed. • Fungi can digest compounds from a wide range of sources, living or dead.

Adaptations for Feeding by Absorption • Most fungi have cell walls made of chitin • The most common body structures are multicellular filaments and single cells (yeasts) • Morphology enhances absorption. Some species grow as either filaments or yeasts; others grow as both • Fungi consist of mycelia, networks of branched hyphae adapted for absorption

Specialized Hyphae in Mycorrhizal Fungi • Some unique fungi have specialized hyphae called haustoriathat allow them to penetrate the tissues of their host • Mutually beneficial relationships between such fungi and plant roots are called mycorrhizae(“fungus roots”) • Mycorrhizal fungi deliver phosphate ions and other minerals to plants in exchange for organic nutrients such as carbohydrates

Fig. 31-4b Plant cell wall Fungal hypha Plant cell Plant cell plasma membrane Haustorium (b) Haustoria

Two main types of mycorrhizal fungi: • Ectomycorrhizal fungi (ektos, out)form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex • Arbuscularmycorrhizal fungi (arbor, tree) extend hyphae through the cell walls of root cells and into tubes formed by invagination (pushing inward) of the root cell membrane

Fig. 31-2 Reproductive structure Hyphae Spore-producing structures 20 µm Mycelium

Sexual and Asexual Reproduction • Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually • Plasmogamyis the union of two parent mycelia • Hours, days, or even centuries may pass before the occurrence of karyogamy, nuclear fusion • During karyogamy, the haploid nuclei fuse, producing diploid cells • The diploid phase is short-lived and undergoes meiosis, producing haploid spores

Fig. 31-5-1 Key Haploid (n) Heterokaryotic (unfused nuclei from different parents) Diploid (2n) Spore-producing structures Spores ASEXUAL REPRODUCTION Mycelium GERMINATION

Fig. 31-5-2 Key Heterokaryotic stage Haploid (n) Heterokaryotic (unfused nuclei from different parents) PLASMOGAMY (fusion of cytoplasm) Diploid (2n) KARYOGAMY (fusion of nuclei) Spore-producing structures Zygote SEXUAL REPRODUCTION Spores ASEXUAL REPRODUCTION Mycelium GERMINATION

Fig. 31-5-3 Key Heterokaryotic stage Haploid (n) Heterokaryotic (unfused nuclei from different parents) PLASMOGAMY (fusion of cytoplasm) Diploid (2n) KARYOGAMY (fusion of nuclei) Spore-producing structures Zygote SEXUAL REPRODUCTION Spores ASEXUAL REPRODUCTION Mycelium MEIOSIS GERMINATION GERMINATION Spores

Asexual Reproduction • In addition to sexual reproduction, many fungi can reproduce asexually • Molds produce haploid spores by mitosis and form visible mycelia • Other fungi that can reproduce asexually are yeasts, which inhabit moist environments • Instead of producing spores, yeasts reproduce asexually by simple cell division and the pinching of “bud cells” from a parent cell

Fig. 31-7 10 µm Parent cell Bud

The Origin of Fungi • Fungi and animals are more closely related to each other than they are to plants or other eukaryotes • DNA evidence suggests that fungi are most closely related to unicellular nucleariids while animals are most closely related to unicellular choanoflagellates

Fig. 31-8 Animals (and their close protistan relatives) UNICELLULAR, FLAGELLATED ANCESTOR Nucleariids Opisthokonts Chytrids Fungi Other fungi

The Move to Land • Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants • Among fossil evidence, molecular studies on sym genes have shown evidence of gene conservation for hundreds of millions of years. • Molecular analyses have also helped clarify evolutionary relationships among fungal groups, although areas of uncertainty remain

Fig. 31-11 Hyphae 25 µm Chytrids (1,000 species) Zygomycetes (1,000 species) Fungal hypha Glomeromycetes (160 species) Ascomycetes (65,000 species) Basidiomycetes (30,000 species)

The Evolution of Plants on Land