BIOL 211 Lecture 20: Land Plants Part 1

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BIOL 211
Bruno Pernet

Lecture 20 Notes: Land Plants Part 1 Major Transitions in the History of Life • Origin of life and prokaryotic cells: ~3.5 bya • Photosynthesis: ~ 2.7 bya • Eukaryotic cells: ~1.8 bya • Multicellular organisms: ~1.2 bya Phylogenetic Context • Land plants (embryophytes) are Archaeplastida, which is defined by plastids that arose by primary endosymbiosis • Common ancestor (an alga) of the clade had chlorophyll a, phycoerythrin, and cellulose in the cell walls • In the lineage leading to the green algae, two things happened: loss of phycoerythrin and gain of chlorophyll b • Charophytes – clade of multicellular “green algae” that includes the land plants o Paraphyletic clade; embryophytes are excluded • A number of synapomorphies link embryophytes to charophytes o Lots of DNA sequence homologies o Structure of flagellated sperm (in land plants that have flagellated sperm) o Rings of cellulose-synthesizing proteins (the “cellulose synthase compelx”) in the plasma membrane o Phragmoplast – microtubules that run between daughter cell nuclei and are formed during mitosis • Common ancestor of the charophyte/land plant clade very likely had chlorophylls a and b (but no phycoerythrin), cellulose in the cell wall, formation of a phragmoplast during mitosis, and the ability to produce sporopollenin What are Charophytes? • Green algae that live in shallow bodies of freshwater; they are distributed worldwide • Often at risk of drying out • Charophytes coat their zygotes with a layer of sporopollenin, a very strong polymer that helps protect zygotes from dessication Derived Traits of Embryophytes • Plants first appeared on land ~470 mya (Paleozoic era) • Plants needed derived traits that allow them to succeed in dry and low-density terrestrial environments (rather than aquatic environments) o Plants tend to dry out and flagellated gametes have nothing to swim through o Flotation doesn’t support their bodies; need structural support against gravity • Adaptations present in most land plants, but not in charophytes o Cuticle – layer of waxy lipids that covers the plant body; reduces water loss by evaporation o Stomata – small openings in leaves and stems used to regulate gas exchange and evaporation of water (present in all land plants except liverworts) o Apical meristems – growth zones that allow a plant to grow taller (toward light 2nd CO ) and down into the soil (toward water and nutrients) o Mutualisms with soil fungi called mycorrhizae – help plants take up nutrients from the soil o Alternation of generations life cycle: all land plants have this life cycle (but charophytes are haplontic) o Multicellular gametangia: gamophytes produce gametes in multicellular organs called gametangia ▪ Archaegonium produce female gametangium ▪ Antheridium produce male gametangium ▪ Eggs are fertilized and develop into embryos in the archaegonium o Protected, dependent embryos: embryos are retained inside the tissues of the female gametophyte, where they are protected from dessication; mother can transfer nutrients to them during development o Spores with sporopollenin-rich walls; produced in sporangia ▪ Sporangia – structures produced by diploid sporophytes; inside, meiosis occurs and produces haploid spores that have a thick sporopollenin wall (making them resistant to dessication) Overview of Land Plant Diversity • Land plants are categorized by whether or not they have vascular tissue or seeds • Vascular tissue – cells that form tubes in the plant body to transport water up from the soil, and photosynthetic products down from the photosynthetic structures (leaves) o Tracheids – type of cell included in vascular cells • Seeds – embryos packaged with a supply of nutrients in a protective coat o Can only be found in vascular plants • Trend from nonvascular  vascular non-seed  vascular seeded: reduction of gametophyte generation • Nonvascular plants (arose earlier): gametophyte is larger, long-lived, and more self-sufficient than the sporophyte o Gametophyte is photosynthetic o Sporophyte can be photosynthetic but is always nutritionally dependent on the gametophyte and permanently attached to it • Vascular plants (arose later): sporophyte is larger, longer-lived, and more self-sufficient than the gametophyte Nonvascular Plants (All Seedless) • Liverworts, mosses, and hornworts • Informally called the “bryophytes”, but bryophytes are paraphyletic • Common ancestor arose ~470 mya and was probably similar to a liverwort (fossil evidence) • Shared traits of the three taxa o Restricted to moist habitats: sperm are flagellated and require water to swim o Usually very short: all parts of the plant must be close to the soil since they have no vascular system to get higher up nutrients (must get nutrients from the soil) o No “woody” tissue to support tall stems o Simple alternation of generations life cycle with the gametophyte generation dominant • Byrophyta: mosses (~15000 species) o Haploid spores are released from the sporangium o When they land in a moist place, they germinate and undergo mitosis to form multicellular threads called protonemata ▪ Form photosynthetic tissue as well as rootlike filaments (rhizoids) that anchor gametophytes to the ground o Gametangia develop on the gametophytes; each gametangium contains sperm (anteridia) or eggs (archaegonia) o Sperm are released from the antheridium and must swim to the egg in the archegonium o Egg or archaegonium usually releases chemicals that sperm are attracted to o Water is necessary for fertilization o Embryo develops into a sporophyte that is nutritionally dependent on the gametophyte for sugars, amino acids and minerals o In the capsule (sporangium), meiosis occurs and forms spores covered by sporopollenin coat o Spore dispersal in mosses can be explosive o Gametophyte (1N stage) is long-lived and photosynthetic o Sporophyte (2N stage) is shorter-lived and nutritionally dependent on the gametophyte o C
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