Plant tax II week 1

Definition of 'plants' (broadest sense) in this lecture

Oxygenic photosynthetic organisms (and close relatives)

algae = ?

plants minus land plants

About how many groups of 'plants' (broad sense) are there?

About 10

Name the ca. 10 groups of 'plants'

Cyanobacteria, Glaucophytes, Rhodophytes (red algae), Green plants, Cryptophytes, Haptophytes, Euglenids, Chlorarachnids, Dinophytes, Heterokontophytes

Green plants = ? (taxon name and rank)

Green algae + land plants | Chloroplastida / subkingdom Viridiplantae (viridi = green)

Size range of green plants

Ecological importance of green plants

Very important primary producers | probably ca. 50% of total primary production on Earth | land plants directly support human life (food, materials)

Plastid: definition

Collective term for organelles derived from the endosymbiotic cyanobacterium. Usually photosynthetic (chloroplast) but sometimes not (leucoplast, amyloplast etc.)

Green plant plastids: type and pigments

Mostly chloroplasts | chlorophylls a + b | green color not masked by other pigments (e.g. phycobilins)

Why are green plants green?

Chlorophyll color is not masked by other pigments (in red algae it is masked by phycoerythrin, in heterokontophytes by fucoxanthin)

Pigment masking green in red algae

Phycoerythrin (reddish)

Pigment masking green in heterokontophytes

Fucoxanthin (yellowish)

Is chlorophyll a + b unique to green plants?

No - euglenids and some other algae also have green chloroplasts with chl a + b

Number of membranes around green plant plastids

Two

Thylakoid arrangement in green plants

Stacked to form lamellae (not single)

Thylakoids in red algae

Single (separated) | plastid has 2 membranes

Plastid membrane number: Rhodophytes / Heterokontophytes / Euglenids / Green plants

2 / 4 / 3 / 2

Storage polysaccharide of green plants and where it is stored

Starch (α-glucan | α-1,4/1,6) within the plastid stroma - UNIQUE to green plants

Other algae: where do they store starch/glucan?

Starch outside plastid stroma (e.g. cytosol: glaucophytes, rhodophytes, cryptophytes, dinophytes) | or β-glucan in cytoplasmic vesicles (heterokontophytes, haptophytes, chlorarachnids, euglenids)

Mitochondrial cristae of green plants

Flat (plate-like) cristae | also in animals, fungi, red algae (not unique)

Three cristae shapes in eukaryotes and examples

Flat (animals, fungi, red algae, green plants) | discoid (Euglenozoa) | tubular (Amoebozoa, stramenopiles)

Flagella type of green plants

Usually isokont (nearly same structure, length and motion)

Isokont vs anisokont

Isokont = nearly same structure, length, motion | anisokont (= heterokont) = different. Isokont is relatively rare in eukaryotes (also haptophytes) | some green algae have anisokont flagella

Flagellar feature unique to green plants

Stellate structure in the flagellar transition region (between flagellum and basal body/kinetosome)

Stellate structure contains what protein? Function?

Centrin - Ca2+-mediated contraction, probably related to flagellar severing (but centrin itself is universal in eukaryotes)

Green plants monophyletic or not?

Monophyletic (a clade) in eukaryotes

Green algae: mono-, para- or polyphyletic?

Paraphyletic (grade)

Land plants: mono-, para- or polyphyletic?

Monophyletic (clade)

Monophyly

Group with the common ancestor and ALL its descendants (clade) | defined by synapomorphy

Paraphyly

Group with the common ancestor and PART of its descendants (grade) | defined by symplesiomorphy

Polyphyly

Group with NO common ancestor | shared feature is homoplasy (convergent/parallel evolution)

Synapomorphy

Shared derived character (black circles) | defines monophyletic groups

Symplesiomorphy

Shared ancestral character (white circles) | defines paraphyletic groups

Homoplasy

Similar feature by convergent/parallel evolution (black triangles) | shared feature in polyphyly | analogous not homologous

Homology = ?

Synapomorphy + symplesiomorphy (shared features due to common ancestry)

First 'plants' (first oxygenic photosynthetic organisms)

Cyanobacteria (blue-green algae), probably ca. 2.7 billion years ago

Cyanobacteria are...

The sole group of oxygenic photosynthetic prokaryotes | origin of all oxygenic photosynthesis (including that of green plants)

How did eukaryotes acquire photosynthesis from cyanobacteria?

Primary endosymbiosis

Primary endosymbiosis: process

Cyanobacterium engulfed by a colorless eukaryote | symbiont enslaved via gene transfer/loss and host control | becomes organelle = chloroplast (plastid) surrounded by 2 membranes

When did primary endosymbiosis occur?

ca. 2-1.5 billion years ago

Primary plants = ?

Photosynthetic eukaryotes with plastids from primary endosymbiosis

Three groups of primary plants

Glaucophytes, Rhodophytes (red algae), Green plants (Viridiplantae)

How many times did primary endosymbiosis occur?

Only once (single primary endosymbiosis)

Evidence for single primary endosymbiosis

Plastid-gene phylogeny (50 plastid genes, Rodriguez-Ezpeleta 2005): all plastids form a monophyletic clade within cyanobacteria | all plastid DNA share unique gene arrangements not found in cyanobacteria

Host phylogeny: do primary plants form a clade?

Probably yes (nuclear genes, e.g. 317 genes, Schön 2021) | not fully confirmed

Archaeplastida

Clade of the three primary plant groups | also treated as Kingdom Plantae (sensu Cavalier-Smith). Primary endosymbiosis occurred in its common ancestor

Synapomorphies of green plants vs glaucophytes/rhodophytes

Chlorophyll b | thylakoid lamellae (stacked) | starch in plastid | loss of phycobilins | stellate structure in flagellar transition region

Comparison table: Plastid membranes (Glauco / Rhodo / Viridi)

2 / 2 / 2

Peptidoglycan (wall) in plastid: Glauco / Rhodo / Viridi

Present / Absent / Absent (?)

LHC (light harvesting complex): Glauco / Rhodo / Viridi

Absent / Present / Present

Chlorophyll: Glauco / Rhodo / Viridi

a / a / a, b

Phycobilins: Glauco / Rhodo / Viridi

Present / Present / Absent

Thylakoids: Glauco / Rhodo / Viridi

Single / Single / Stacked

Storage polysaccharide: Glauco / Rhodo / Viridi

Starch in cytosol / Starch in cytosol / Starch in plastid

Mitochondria cristae: Glauco / Rhodo / Viridi

Flat / Flat / Flat

Flagellate cell: Glauco / Rhodo / Viridi

Anisokont / Absent / Aniso- to isokont

Which two primary plant groups share LHC and lack peptidoglycan wall in plastid?

Red algae and green plants

Which non-green-plant organisms have chlorophyll b?

Chlorarachnids, photosynthetic euglenids, some dinoflagellates (green dinoflagellates), prochlorophytes

Chlorarachnids, euglenids, green dinoflagellates: how did they get chlorophyll b plastids?

Secondary endosymbiosis with a green alga

How many times did secondary endosymbiosis with green algae occur?

At least three times, independently

Supergroup placement: euglenids / chlorarachnids / green dinoflagellates

Excavata / Rhizaria / Alveolata

Plastid-protein phylogeny: where do plastids of green dinoflagellates, chlorarachnids and euglenids fall?

Within green plants (Viridiplantae) - because the plastids came from green algae, though hosts are distantly related

Prochlorophytes

Cyanobacteria (Bacteria) with chlorophyll b | once thought to be the symbiont that produced green plants

How did prochlorophytes get chlorophyll b?

Probably lateral (horizontal) gene transfer of chl b synthetase gene from green plants (or vice versa) | molecular data show they are distantly related to green plant plastids

Organization levels in green plants

Unicellular (flagellate, coccoid), colonial (flagellate, palmelloid, sarcinoid, coenobium), multicellular (filamentous, parenchymatous, siphonocladous), siphonous

Coccoid

Non-motile unicellular

Coenobium

Colony with fixed number and arrangement of cells

Palmelloid / sarcinoid

Colonial forms: cells in mucilage (palmelloid) | cells in packets (sarcinoid)

Siphonocladous

Multinucleate multicellular

Siphonous (coenocytic)

Macroscopic giant single cell with many nuclei (multinucleate)

Parenchymatous

Body formed by three-dimensional cell division (note: used with slightly different meaning in land plants)

Cell covering types in green plants

Naked (e.g. Micromonas, Dunaliella) | scales (many prasinophytes - ancestral condition) | cell wall (incl. lorica, theca) | mucilage | calcified (e.g. Charales, some ulvophyceans)

Ancestral cell covering in green plants

Organic scales (prasinophytes)

Cell wall composition in green plants

Usually fibrous polysaccharide (usually cellulose) in an amorphous matrix | mannan or xylan in some ulvophyceans | glycoprotein in Chlamydomonas and relatives

Calcium carbonate forms in calcified green plants

Calcite or aragonite

Gamete

A cell that fuses with another cell in sexual reproduction

Isogamy

Both gametes identical in appearance (+ and -) | e.g. Chlamydomonas, Pediastrum

Anisogamy (heterogamy)

Gametes of different appearance (large female, small male) | e.g. Codium, Bryopsis, Eudorina

Oogamy

Special form of anisogamy: large non-motile female gamete (egg) + small motile male gamete (sperm) | e.g. Volvox, land plants

Which gamete types occur in green plants?

All (isogamy, anisogamy, oogamy) | isogamy to oogamy evolved multiple times independently

Life cycle (definition)

A series of changes that an organism returns to the same state

What a sexual life cycle must include

Change of ploidy: n (haploid) and 2n (diploid) stages | gamete fusion (n→2n) | meiosis (2n→n)

Three basic life cycle types

Haplontic, diplontic, haplodiplontic - all found in green plants (distinguished by timing of meiosis)

Haplontic life cycle

Zygotic meiosis | most of the cycle is haploid (n) | zygote is the only diploid cell

Diplontic life cycle

Gametic meiosis (gametes produced via meiosis) | most of cycle is diploid (2n)

Haplodiplontic life cycle

Sporic meiosis | alternation of generations: haploid gametophyte makes gametes by mitosis | diploid sporophyte makes haploid spores by meiosis

Isomorphic vs heteromorphic alternation of generations

Isomorphic: gametophyte and sporophyte look the same | heteromorphic: different appearance (e.g. ferns: gametophyte very small vs sporophyte)

Photosynthetic pigments in green plant chloroplasts

Chlorophylls a + b | carotenoids: lutein, neoxanthin, zeaxanthin, β-carotene

Unique carotenoids in some green algae

Prasinoxanthin, siphonaxanthin - absorb blue-green light (main light in deep water) so allow photosynthesis in deep water

Chloroplast shapes (examples)

Cup-shaped, stellate, sheet-like, ribbon-like, reticulate, discoid (many)

Pyrenoid

Structure in chloroplast composed mainly of RuBisCO | probably associated with CO2 concentrating mechanism (CCM) | in many green algae and one group of land plants (hornworts) | also in other algae

Pyrenoid ultrastructure

Diverse (thylakoids/starch/cytosol arrangements) | important taxonomic character

Nutrition modes in green plants

Mostly photoautotrophic | some mixotrophic (use organic matter, mostly by osmosis, not phagocytosis) | some completely heterotrophic

Mixotrophic green plant examples

Chlorella, Chlamydomonas, Marchantia (liverwort)

Completely heterotrophic green plant examples

Prototheca, Polytoma (green algae) | Rafflesia, Monotropastrum (land plants)

Habitats of green plants

Almost all illuminated habitats (except air): marine, freshwater, snow, glaciers, hypersaline lakes, land, and in symbiosis with other organisms

Main producers in terrestrial ecosystems

Land plants

Green algae as symbionts

Important in lichens (e.g. Trebouxia) | also in ciliates, amoebae, sponges, hydra, sea anemones, insects, salamander eggs

Traditional view of green algae phylogeny

Based on organization level (flagellate, coccoid, filamentous, siphonous...) from a Chlamydomonas-like ancestor | land plants thought to evolve from multicellular filamentous green algae

Traditional orders (organization-based)

Volvocales (flagellates), Tetrasporales, Chlorococcales (coccoid), Ulotricales (filamentous), Ulvales, Cladophorales, Siphonales, Zygnematales, Charales