BIF 26: The Colonization of Land by Plants and Fungi

BIF 26: The Colonization of Land by Plants and Fungi

© 2011 Pearson Education, Inc.

Overview: The Greening of Earth

• For more than the first 3 billion years of Earth’s history, the terrestrial surface was lifeless– Cyanobacteria likely existed on land 1.2 billion years ago

• Around 500 million years ago, small plants, fungi, and animals emerged on land

• Since colonizing land, plants have diversified into roughly 290,000 living species– Land plants are defined as having terrestrial ancestors, even

though some are now aquatic– Land plants do not include photosynthetic protists (algae)

• Plants supply oxygen and are the ultimate source of most food eaten by land animals


• Since colonizing land, plants have diversified into roughly 290,000 living species

• Land plants are defined as having terrestrial ancestors, even though some are now aquatic

• Land plants do not include photosynthetic protists (algae)

• Plants supply oxygen and are the ultimate source of most food eaten by land animals


1 m

Figure 29.1

FOSSILS SHOW THAT PLANTS COLONIZED LAND MORE THAN 470 MILLION YEARS AGO

Green algae called charophytes are the closest relatives of land plants.


Morphological and Molecular Evidence of Algal Ancestry

• Many characteristics of land plants also appear in a variety of protist clades, mainly algae

• However, land plants share four key traits with only charophytes

– Rings of cellulose-synthesizing complexes– Peroxisome enzymes– Structure of flagellated sperm– Formation of a phragmoplast

30 nm


• 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


1 m

Figure 26.3

Chara species, a pond organism

Coleochaete orbicularis, adisk-shaped charophytethat also lives in ponds (LM)

40 m

5 mm


Adaptations Enabling the Move to Land

• In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out– Sporopollenin is also found in plant spore walls

• 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 define plants as embryophytes, plants with embryos


1 m

Figure 29.4

Red algae

Chlorophytes

Charophytes

Embryophytes

ANCESTRALALGA

Viridiplantae

Streptophyta

Plantae


Derived Traits of Plants

• Four key traits appear in nearly all land plants but are absent in the charophytes

– Alternation of generations and multicellular, dependent embryos

– Walled spores produced in sporangia– Multicellular gametangia– Apical meristems


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


1 m

Figure 29.5a

Gamete fromanother plant

Key

Haploid (n)

Diploid (2n)Gametophyte(n)

Mitosis Mitosis

Spore Gamete

MEIOSIS FERTILIZATION

Zygote

MitosisSporophyte(2n)

Alternation of generations

2n

n

n n

n


1 m

Figure 29.5b

Embryo

Maternal tissue

Embryo (LM) andplacental transfer cell (TEM)of Marchantia (a liverwort)

Wall ingrowths

Placental transfercell (outlined inblue)10 m

2 m


Walled Spores Produced in Sporangia

• The sporophyte produces spores in organs called sporangia

• Diploid cells called sporocytes undergo meiosis to generate haploid spores

• Spore walls contain sporopollenin, which makes them resistant to harsh environments


1 m

Figure 29.5c

SporesSporangium

Longitudinal section ofSphagnum sporangium (LM)

Sporophyte

Gametophyte

Sporophytes and sporangia of Sphagnum (a moss)


Multicellular Gametangia

• Gametes are produced within organs called gametangia

• Female gametangia, called archegonia, produce eggs and are the site of fertilization

• Male gametangia, called antheridia, produce and release sperm Female

gametophyte

Malegametophyte

Archegonia,each with anegg (yellow)

Antheridia(brown),containing sperm

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

1 m

Apical meristemof shoot

Developingleaves

Shoot 100 m100 mRoot

Apicalmeristemof root

Apical meristems of plantroots and shoots


Additional derived traits include

– Cuticle, a waxy covering of the epidermis– Mycorrhizae, symbiotic associations between fungi

and land plants that may have helped plants without true roots to obtain nutrients

– Secondary compounds that deter herbivores and parasites


The Origin and Diversification of Plants

• Fossil evidence indicates that plants were on land at least 475 million years ago

• Fossilized spores and tissues have been extracted from 475-million-year-old rocks

(a) Fossilizedspores

Fossilizedsporophytetissue

(b)


What differentiates early plants from their ancestors?

• Spores are different– Chemical composition of

spores– Wall structures of spores

• Vascular tissue• Stomata• Branched sporophytes• Many symbiotic with

algae

FUNGI PLAYED AN ESSENTIAL ROLE IN THE COLONIZATION OF LAND

Early plants formed symbiotic relationships with fungi. These associations are called mycorrhizae.


Fungi are diverse and widespread• They are essential for the well-being of most terrestrial

ecosystems because they break down organic material and recycle vital nutrients

• About 100,000 species of fungi have been described• It is estimated there are actually 1.5 million species of

fungi


Fungi are heterotrophs that feed by absorption

• Despite their diversity, fungi share key traits, most importantly the way in which they derive nutrition

• Fungi use enzymes to break down a large variety of complex molecules into smaller organic compounds

• The versatility of these enzymes contributes to fungi’s ecological success

• Fungi exhibit diverse lifestyles– Decomposers– Parasites– Mutualists


Body Structure

• The most common body structures are multicellular filaments and single cells (yeasts)

• Some species grow as either filaments or yeasts; others grow as both


© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.

Animation: Fungal Reproduction and Nutrition Right-click slide / select “Play”


• The morphology of multicellular fungi enhances their ability to absorb nutrients

• Fungi consist of mycelia, networks of branched hyphae adapted for absorption

• A mycelium’s structure maximizes its surface area-to-volume ratio

• Fungal cell walls contain chitin


Reproductive structure

Hyphae

Spore-producingstructures

Mycelium

60 m

Figure 31.2


• Most fungi have hyphae divided into cells by septa, with pores allowing cell-to-cell movement of organelles

• Coenocytic fungi lack septa and have a continuous cytoplasmic mass with hundreds or thousands of nuclei

(a) Septate hypha (b) Coenocytic hypha

NucleiCell wall

Pore

Septum Nuclei

Cell wall


Specialized Hyphae in Mycorrhizal Fungi

• Some unique fungi have specialized hyphae called haustoria that allow them to penetrate the tissues of their host

(a) Hyphae adapted for trapping and killing prey

(b) Haustoria

Fungal hypha Plantcellwall

Plant cell

Plant cellplasmamembraneHaustorium

NematodeHyphae 25 m


Mycorrhizae are mutually beneficial relationships between fungi and plant roots

• Ectomycorrhizal fungi form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex

• Arbuscular mycorrhizal fungi extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane

• Mycorrhizal fungi deliver phosphate ions and minerals to plants

• Most vascular plants have mycorrhizae


Fungi produce spores through sexual or asexual life cycles

• Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually

• Fungi can produce spores from different types of life cycles


Figure 31.5-1

Key

Haploid (n)

Heterokaryotic

Diploid (2n)

Spores


ASEXUALREPRODUCTION

GERMINATION

Mycelium


Figure 31.5-2

PLASMOGAMY

Key

Haploid (n)

Heterokaryotic

Diploid (2n)

Spores


ASEXUALREPRODUCTION

SEXUALREPRODUCTION

GERMINATION

Zygote

Heterokaryoticstage

KARYOGAMY

Mycelium


Figure 31.5-3

PLASMOGAMY

Key

Haploid (n)

Heterokaryotic

Diploid (2n)

Spores


ASEXUALREPRODUCTION

SEXUALREPRODUCTION

GERMINATIONGERMINATION MEIOSIS

Spores

Zygote

Heterokaryoticstage

KARYOGAMY

Mycelium


The Origin of Fungi

• Fungi, animals, and their protistan relatives form the opisthokonts clade

UNICELLULAR,FLAGELLATEDANCESTOR

Animals (and their closeprotistan relatives)

Nucleariids

Chytrids

Other fungi

OpisthokontsFungi


• DNA evidence suggests that – Fungi are most closely related to unicellular

nucleariids – Animals are most closely related to unicellular

choanoflagellates• This suggests that multicellularity arose separately in

animals and fungi• The oldest undisputed fossils of fungi are only about

460 million years old


The Move to Land

• Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants


Fungi have radiated into a diverse set of

lineages• Molecular

analyses have helped clarify evolutionary relationships among fungal groups, although areas of uncertainty remain

Chytrids (1,000 species)

Zygomycetes (1,000 species)

Glomeromycetes (160 species)

Ascomycetes (65,000 species)

Basidiomycetes (30,000 species)

Hyphae 25 m

25 mFungal hypha

EARLY LAND PLANTS RADIATED INTO A DIVERSE SET OF LINEAGES

Figure 30.2

PLANT GROUP

Mosses and othernonvascular plants

Gametophyte

Sporophyte

Sporophyte(2n)

Gametophyte(n)

Dominant

Reduced, dependent ongametophyte for nutrition

Dominant

Reduced, independent(photosynthetic andfree-living)

Ferns and other seedlessvascular plants

Dominant

Reduced (usually microscopic), dependent on surroundingsporophyte tissue for nutrition

Seed plants (gymnosperms and angiosperms)

AngiospermGymnosperm

Microscopicfemalegametophytes(n) insidethese partsof flowers

Microscopic femalegametophytes (n) insideovulate cone

Microscopicmalegametophytes(n) insidethese partsof flowers

Microscopic malegametophytes (n)inside pollencone

Sporophyte (2n)Sporophyte (2n)

Sporophyte(2n)

Gametophyte(n)

Example


1 m

Figure 29.7

Origin of land plants (about 475 mya)

Origin of vascular plants (about 425 mya)

Origin of extant seed plants (about 305 mya)

2

1

3

2

1ANCESTRALGREENALGA

500 450 400 350 300 50 0

Millions of years ago (mya)

Liverworts

Mosses

Hornworts

Lycophytes (clubmosses, spikemosses, quillworts)

Pterophytes (ferns,horsetails, whisk ferns)

Gymnosperms

Angiosperms

Land plants

Vascular plants

Nonvascular

plants(bryophytes)

Seedlessvascularplants

Seed plants

3


Land plants can be informally grouped based on the presence or absence of vascular tissue

• Most plants have vascular tissue; these constitute the vascular plants

• Nonvascular plants are commonly called bryophytes– Bryophytes are not a monophyletic group; their

relationships to each other and to vascular plants are unresolved

Nonvascular plants (bryophytes)

Seedless vascular plants

Gymnosperms

Angiosperms


Plagiochila deltoidea- A liverwort

Polytrichum commune- a moss

Anthoceros- A hornwort


Land plants can be informally grouped based on the presence or absence of vascular tissue

• Seedless vascular plants can be divided into clades– Lycophytes (club mosses and their relatives)– Pterophytes (ferns and their relatives)

– Seedless vascular plants are paraphyletic, and are of the same level of biological organization, or grade

• First plants to grow tall


Seedless vascular plantsGymnospermsAngiosperms


Diphasiastrum- a lycophyte

Athyrium- a monilophyte


Origins and Traits of Vascular Plants

• Fossils of the forerunners of vascular plants date back about 425 million years

• These early tiny plants had independent, branching sporophytes

• Living vascular plants are characterized by Life cycles with dominant sporophytes Vascular tissues called xylem and phloem Well-developed roots and leaves


1 m

Figure 29.12

Sporangia


Life Cycles with Dominant Sporophytes

• In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in familiar ferns

• The gametophytes are tiny plants that grow on or below the soil surface


Animation: Fern Life Cycle Right-click slide / select “Play”


1 m

Figure 29.13-1

Key

Haploid (n)Diploid (2n)

MEIOSISSporedispersal

Maturesporophyte(2n)

Fiddlehead (young leaf)

Sporangium

Sorus

Sporangium


1 m

Figure 29.13-2

Key



Spore(n)

Younggametophyte

Rhizoid

Undersideof maturegametophyte(n)

Antheridium

Sperm

Archegonium

Egg

FERTILIZATION



Sporangium

Sorus

Sporangium


1 m

Figure 29.13-3

Key



Spore(n)

Younggametophyte

Rhizoid

Undersideof maturegametophyte(n)

Antheridium

Sperm

Archegonium

Egg

FERTILIZATIONZygote(2n)

Gametophyte

Newsporophyte



Sporangium

Sorus

Sporangium


Transport in Xylem and Phloem

• Vascular plants have two types of vascular tissue: xylem and phloem

• Xylem conducts most of the water and minerals and includes dead cells called tracheids

• Water-conducting cells are strengthened by lignin and provide structural support

• Phloem consists of living cells and distributes sugars, amino acids, and other organic products

• Vascular tissue allowed for increased height, which provided an evolutionary advantage


Evolution of Roots

• Roots are organs that anchor vascular plants• They enable vascular plants to absorb water and

nutrients from the soil• Roots may have evolved from subterranean stems


Evolution of Leaves

• Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis

• Leaves are categorized by two types Microphylls, leaves with a single vein

According to one model of evolution, microphylls evolved as outgrowths of stems

Megaphylls, leaves with a highly branched vascular system– Megaphylls may have evolved as webbing between flattened

branches


Vascular tissue Sporangia Microphyll

(a) Microphylls (b) Megaphylls

Overtoppinggrowth

Megaphyll

Otherstemsbecomereducedandflattened.

Webbingdevelops.

SEEDS AND POLLEN GRAINS ARE KEY ADAPTATIONS FOR LIFE ON LAND


Overview: Transforming the World

• Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems

• Seed plants originated about 360 million years ago• A seed consists of an embryo and nutrients

surrounded by a protective coat• Domestication of seed plants had begun by 8,000

years ago and allowed for permanent settlements


Advantages of Reduced Gametophytes

• The gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte


Heterospory: The Rule Among Seed Plants

• The ancestors of seed plants were likely homosporous, while seed plants are heterosporous

• Megasporangia produce megaspores that give rise to female gametophytes

• Microsporangia produce microspores that give rise to male gametophytes


Ovules and Production of Eggs

• An ovule consists of a megasporangium, megaspore, and one or more protective integuments

• Gymnosperm megaspores have one integument• Angiosperm megaspores usually have two

integuments

Figure 30.3-1

Immatureovulate cone

Integument (2n)

Spore wall

Megaspore (n)

(a) Unfertilized ovule

Megasporangium(2n)

Pollen grain (n)Micropyle


Pollen and Production of Sperm

• Microspores develop into pollen grains, which contain the male gametophytes

• Pollination is the transfer of pollen to the part of a seed plant containing the ovules

• Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals

• If a pollen grain germinates, it gives rise to a pollen tube that discharges sperm into the female gametophyte within the ovule

Figure 30.3-2


Integument (2n)

Spore wall

Megaspore (n)

Femalegametophyte (n)

Egg nucleus(n)

Dischargedsperm nucleus(n)

Pollen tubeMale gametophyte (n)


Megasporangium(2n)


(b) Fertilized ovule


The Evolutionary Advantage of Seeds

• A seed develops from the whole ovule• A seed is a sporophyte embryo, along with its food

supply, packaged in a protective coat


Integument (2n)

Spore wall

Megaspore (n)

Femalegametophyte (n)

Egg nucleus(n)

Dischargedsperm nucleus(n)

Pollen tubeMale gametophyte (n)


Megasporangium(2n)


(b) Fertilized ovule (c) Gymnosperm seed

Seedcoat

Sporewall

Foodsupply (n)

Embryo (2n)


• Seeds provide some evolutionary advantages over spores

– They may remain dormant for days to years, until conditions are favorable for germination

– Seeds have a supply of stored food– They may be transported long distances by wind or

animals


A seed is an embryo and nutrients surrounded by a protective coat

• Seed plants form a clade and can be divided into further clades

– Gymnosperms, the “naked seed” plants, including the conifers

– Angiosperms, the flowering plants


Gymnosperms bear “naked” seeds, typically on cones

• Gymnosperms means “naked seeds”• The seeds are exposed on sporophylls that form

cones• Angiosperm seeds are found in fruits, which are

mature ovaries



Gymnosperms

Angiosperms


Gymnosperm Evolution

• Fossil evidence reveals that by the late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants


Living seed plants can be divided into two clades: gymnosperms and angiosperms

• Gymnosperms appear early in the fossil record about 305 million years ago and dominated Mesozoic (251–65 million years ago) terrestrial ecosystems– Gymnosperms were better suited than nonvascular plants to

drier conditions– Today, cone-bearing gymnosperms called conifers still

dominate in the northern latitudes

• Angiosperms began to replace gymnosperms near the end of the Mesozoic

• Angiosperms now dominate more terrestrial ecosystems


• The gymnosperms consist of four phyla– Cycadophyta (cycads)– Gingkophyta (one living species: Ginkgo biloba)– Gnetophyta (three genera: Gnetum, Ephedra,

Welwitschia)– Coniferophyta (conifers, such as pine, fir, and

redwood)



Phylum Cycadophyta• Individuals have large cones and palmlike leaves• These thrived during the Mesozoic, but relatively few

species exist today


Phylum Ginkgophyta• This phylum consists of a single living species,

Ginkgo biloba• It has a high tolerance to air pollution and is a

popular ornamental tree


Phylum Gnetophyta• This phylum comprises three genera• Species vary in appearance, and some are tropical

whereas others live in deserts

Ovulate cones Gnetum

Ephedra

Welwitschia


Phylum Coniferophyta• This phylum is by far the largest of the gymnosperm

phyla• Most conifers are “evergreens” and can carry out

photosynthesis year round

Figure 30.5e

Douglas fir

Common juniper

European larch

Sequoia

Wollemi pine Bristlecone pine


The reproductive adaptations of angiosperms include flowers and fruits• Angiosperms are seed plants with reproductive

structures called flowers and fruits• They are the most widespread and diverse of all

plants



Gymnosperms

Angiosperms

Figure 30.UN02



Gymnosperms

Angiosperms


Characteristics of Angiosperms

• All angiosperms are classified in a single phylum, Anthophyta, from the Greek anthos for flower

• Angiosperms have two key adaptations– Flowers– Fruits



Flowers

• The flower is an angiosperm structure specialized for sexual reproduction

• Many species are pollinated by insects or animals, while some species are wind-pollinated



• A flower is a specialized shoot with up to four types of modified leaves

– Sepals, which enclose the flower – Petals, which are brightly colored and attract

pollinators– Stamens, which produce pollen– Carpels, which produce ovules



• A stamen consists of a stalk called a filament, with a sac called an anther where the pollen is produced

• A carpel consists of an ovary at the base and a style leading up to a stigma, where pollen is received


© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.

Video: Flower Blooming (time lapse)

Stamen Anther

Filament

StigmaCarpel

Style

Ovary

Petal

Sepal

Ovule

Figure 30.7


Fruits

• A fruit typically consists of a mature ovary but can also include other flower parts– Fruits protect seeds and aid in their dispersal– Mature fruits can be either fleshy or dry – Various fruit adaptations help disperse seeds

• Seeds can be carried by wind, water, or animals to new locations

TomatoRuby grapefruit

Hazelnut

Nectarine

MilkweedSom

e Ty

pes

of F

ruit

Wings

Seeds within berries

Barbs

See

d D

ispe

rsal


Angiosperm Evolution

• Darwin called the origin of angiosperms an “abominable mystery”

• Angiosperms originated at least 140 million years ago• During the late Mesozoic, the major branches of the

clade diverged from their common ancestor• Scientists are studying fossils, refining phylogenies,

and investigating developmental patterns to resolve the mystery


Fossil Angiosperms

• Chinese fossils of 125-million-year-old angiosperms share some traits with living angiosperms but lack others

• Archaefructus sinensis, for example, has anthers and seeds but lacks petals and sepals

(a) Archaefructus sinensis, a 125- million-year-old fossil

(b) Artist’s reconstruction of Archaefructus sinensis

Carpel

Stamen

5 cm

Figure 30.11


Angiosperm Phylogeny

• The ancestors of angiosperms and gymnosperms diverged about 305 million years ago

• Angiosperms may be closely related to Bennettitales, extinct seed plants with flowerlike structures

• Amborella and water lilies are likely descended from two of the most ancient angiosperm lineages

Microsporangia(containmicrospores)

Ovules

Most recent common ancestorof all living angiosperms

300 250 200 150 100 50 0

Eudicots

Monocots

Magnoliids

Star aniseand relatives

Water lilies

Amborella

Bennettitales

Livinggymnosperms

Millions of years ago

(b) Angiosperm phylogeny

(a) A possible ancestor of the angiosperms?

Figure 30.12

LAND PLANTS AND FUNGI FUNDAMENTALLY CHANGED CHEMICAL CYCLING AND BIOTIC INTERACTIONS


Fern

Lycophyte Tree

Horsetail

Tree trunk with small leaves

Lycophyte tree reproductive structures

Carboniferous Forest


Lichen • Symbiotic association between fungus and

photosynthetic microorgamism• Pioneer species for primary succession • Provide requirements for soil formation



Biotic Interactions• Plants produce most of the usable energy

within every terrestrial ecosystem


Chemical Cycling and Plants

• Plants absorb nutrients from the physical environment– Those nutrients pass to the primary consumers

• Plants also alter the composition of Earth’s atmosphere– Oxygen in the form of O2 is released as a by-product of

photosynthesis• Fungal decomposers break down the bodies of dead

organisms returning the chemicals back to the soil– This completes the cycle


Fungi as Mutualists• Aborb nutrients from a

host, but reciprocate– Mycorrhiazae– Endophytes

• Fungi that live inside leaves without causing harm

• Many produce toxins that deter herbivores

• Increase fundamental niche of host


Fungi as Parasites

• Absorb nutrients from a host without reciprocation – Ringworm is an example


Fungi as Pathogens

• Some fungi cause diseases

• Rye can get ergots– May have been

the cause for the Salem Witch Trials



Over the past several hundred years, nearly half of Earth’s tropical forests

have been cut down

Documents

BIF 26: The Colonization of Land by Plants and Fungi