Topic 11 - The Evolution of Plants PDF

Topic 11 The evolution of plants Learning Outcomes Identify the main plant groups based on key characteristics Place the main plant groups on a phylogenetic tree List physical, chemical and biological problems faced by plants during land colonization; and their solutions Explain th...

Topic 11 The evolution of plants Learning Outcomes Identify the main plant groups based on key characteristics Place the main plant groups on a phylogenetic tree List physical, chemical and biological problems faced by plants during land colonization; and their solutions Explain the causes and consequences of indeterminate growth Explain the expression “reduction of the gametophyte” Associate a plant structure with its ploidy level Provide arguments for the importance of vascularization in plants Explain the advantages of heterospory in plants Justify the evolutionary importance of five key innovations in plants Contrast key characteristics of gymnosperms and angiosperms Explain the process of double fertilization and its biological importance 2 https://www.wooclap.com/BIO1130 3  Topic 11 The evolution of plants 11.1 – Key innovations of plants Plants Plants (= embryophytes): 470Mya Eukaryotes Multicellular Embryo Photoautotrophs Cell walls made of cellulose Chloroplasts with chlorophylls, beta-carotenes, xantophylls Sexual reproduction but asexual reproduction is common Development of an embryo that is dependent on its parent 1st photosynthetic organisms that lived permanently on land 5 Plants ~320,000 species of plants were described Reminder: fungi are not plants! Are responsible for a large amount of atmospheric O2 First direct evidence of plant (fossil): Cooksonia  no leaves, no roots, no flowers  had vascular tissues to conduct water  was liberating spores  had cell specialized in gas exchanges 6 Colonization on land Problems of living above the water line: Dry environment Strong effect of gravity No nutrients in the atmosphere Rapid changes in temperatures Advantages of living above the water line: Brighter sunlight, unfiltered by water and phytoplankton More CO2 in the atmosphere than in the water Abundance of nutrients on the shoreline Many adaptations allowed plants to colonize land: … the protection of the spores, the gametes, the zygote and the embryo … maximizing photosynthesis … growth to compensate the lack of movement towards resources 7 Key innovations Alternation of generations (haplodiplontic life cycle): multicellular 2n individual (sporophyte) multicellular n individual (gametophyte) (2n) (n) Embryo retained in the maternal gametophyte tissues  protection and nutrition (sugars, amino acids, etc.) (n) The spores are protected by a wall made of a highly resistant polymer (sporopollenin) (2n) Spore dispersion occurs in the air (independently from water) (n) 8 Key innovations (n) The egg is non-motile but the sperm cell can often swim in water (n) Presence of waxy cuticle: protects against desiccation Presence of stomata: pore surrounded by guard cells in the epidermis of leaves and stems allows gas exchange (CO2 and O2) and water loss regulation. 9 Key innovations Plants don’t move and show indeterminate growth: growth that is not terminated, giving rise to a structure not entirely predetermined genetically.  can respond to the environment (i.e., plasticity of the architecture)  maximizes exposure to resources: Nutrients and water (roots) Sunlight and CO2 (shoot: stems, branches, leaves, flowers) Apical meristem: region of undifferentiated stem cell that divide at the tip of roots and shoots. Stem cell proliferation with differentiation (and indeterminacy) is coordinated by the CLV3-WUS signaling pathway. Growth and cell identity doesn’t depend on cell line of origin but depends on: the cells relative positioning Somssich et al. (2016) the effects of the environment (light, water, nutrients, hormones, etc.) 10 Key innovations Mutations in the CLV3 gene increases cell proliferation, stem size, flower number, and fruit size in Arabidopsis. Plants don’t move and show indeterminate growth: growth that is not terminated, giving rise to a structure not entirely predetermined genetically. Negative feedback loop between:  can respond to the environment (i.e., plasticity of the architecture) CLV3 (differentiation-promoting peptide)  maximizes exposure to resources: WUS (stem cell-promoting transcription factor). Nutrients and water (roots) Sunlight and CO2 (shoot: stems, branches, leaves, flowers) Mutations Apical meristem: in the of region WUS gene decreases undifferentiated cell cell that divide at the stem tip ofproliferation, produces irregular shoots and roots and shoots. just a few (defective) flowers in Arabidopsis. Stem cell proliferation with differentiation (and indeterminacy) is coordinated by the CLV3-WUS signaling pathway. Growth and cell identity doesn’t depend on cell line of origin but depends on: the cells relative positioning Somssich et al. (2016) the effects of the environment (light, water, nutrients, hormones, etc.) 11 Key innovations 12  Topic 11 The evolution of plants 11.2 – Non-vascular plants  lants rp cula ytes as ph on-v bryo N = Bryophytes: paraphyletic group of all non-vascular plants. They do not produce seeds or flowers. Absence of specialized tissues that conduct water and nutrients 14 Plants lants rp cula ytes as ph on-v bryo N = Marchantia polymorpha Cups contain gemmae (cell buds) that can propagate and (asexual reproduction) and grow into a new individual. 15 Life cycle of a moss (bryophyte) Bryophytes have life cycles that are dominated by gametophytes. Spore germination and sperm cell swimming depend on water. No vascular system (stem/roots) Bryum argenteum Rhizoids: filament that attaches to the substrate.  no absorption of water/minerals Bryum argenteum 16  Topic 11 The evolution of plants 11.3 – Vascularization  ts plan es r yt cula oph s e Va ach tr = Tracheophytes: monophyletic group of all vascular plants  life-cycle dominated by the sporophyte (larger and more complex).  The gametophyte is reduced in size! 18 Vascularization Vascularization: presence of lignified tissues that transport water, nutrients andsugars through the plant. On land, sunlight, CO2, nutrients and H2O are not in the same location.  need a transport system to complete photosynthesis Two specialized transport tissues: Xylem: water and minerals to the leaves Phloem: sugars, the products of photosynthesis Lignin: polymer in cell walls (impermeable to water and structural support for gravity)  Plants now have the support to grow tall, disperse farther and compete for light First forests (Devonian ~380Mya) 19 Production of spores Leaves can be specialized to produce spores… … only 1 type (homosporous) producing a bisexual gametophyte:  most seedless vascular plants Fern … or 2 types (heterosporous) producing either a ♀ gametophyte or a ♂ gametophyte: Microsporangium  all seed plants Megasporangium Advantages of heterospory: for each spore: specific selection with specific functions a separate ♀ gametophyte can better nourish the embryo (no energy spent producing ♂ gametes) ♂ and ♀ gametophytes can mature at different times (no self-fertilization = higher genetic diversity) 20  Topic 11 The evolution of plants 11.4 – Seed and flower plants Spermatophytes: seed plants lants yte p h ed top Se rma pe =s Seed: embryo surrounded by nutritive substances and a protective coat 22 Five key innovations of seed plants 1. An extremely reduced gametophyte (often microscopic) is protected from environmental stresses, from UV and from desiccation and is directly nourished from the sporophyte. = Pollen grain 23 Five key innovations of seed plants 2. Ovule: structure containing the megaspore  Fertilization without requiring water from the environment 3. Seed plants are heterosporous: Microspore  ♂ gametophyte (n) which can disperse farther. Megaspore  ♀ gametophyte (n) and nourishes of the developing embryo. 4. Pollen grain: ♂ gametophyte (n) enclosed within a pollen wall.  Can disperse very far (wind, animals, etc.). 24 Five key innovations of seed plants 5. Production of a seed: ↑ survival of plants during reproduction. embryo is nourished and can resist drought or low temperatures  Seed germination during favourable conditions.  Adaptations to many new environments. E.g., Jack pine: germination only after periodic fires Pinus banksiana (Jack pine) 25 Svalbard global seed vault (Norway) Long-term storage of duplicates of seeds collected around the world.  Loss of seeds due to mismanagement, accident, equipment failures, funding cuts, and natural disasters. -18°C, low oxygen Indigenous communities have deposited seeds duplicates: Parque de la Papa in Peru deposited 750 samples of potatoes (2015) The Cherokee Nation became the first US tribe to deposit 9 samples of heirloom food crops which predate European colonization. Cherokee Nation Principal Chief Chuck Hoskin Jr. and Secretary of Natural Resources Chad Harsha As of 2021… 1,081,026 distinct crop samples (>13,000 years of agricultural history) 26 Angiosperms g erin ow nts on-fl pla N eed s Gymnosperms produce seeds (not enclosed in chambers) but no flowers 27 Gymnosperms Gymnosperms (gymno: naked, sperm: seed)  the seed is exposed on sporophylls and can survive years before germination. Seed after fertilization Pollen and seeds = key terrestrial adaptations. by pollen Evolved as the climate became much dryer (outcompeted many vascular seedless plants) Most sperm cells from gymnosperms are not flagellated (exception: Ginkgo) Sequoia sempervirens Pinus halepensis Sequoia Aleppo pine 28 Angiosperms r lowe F nts pla Angiosperms flowering plants, produce seeds (enclosed in chambers: ovaries)  90% of all living plant species 29 Flower plants Angiosperms (angio: receptacle, sperm: seed) ~250,000 species  seed enclosed in a chamber (carpel) that matures into a fruit Flower: modified leaves (sporophylls) specialized in reproduction  carpel (megasporophyll) produces the ♀ gametophyte Flower Seed after fertilization by pollen  stamen (microsporophyll) produces the ♂ gametophyte Coevolution with animal species who participate in pollination Fruit: mature ovary of a flower, that helps seed dispersal Wind (anemochory) Animals (zoochory) 30 Flower plants The transfer of pollen (pollination) to the female egg is independent of water (non-motile sperm). Cross-pollination (between individual plants) Double fertilization: one sperm cell fertilizes the egg  zygote the other sperm cell fuses with two nuclei of the central cell  endosperm (tissue that nourishes the developing embryo) The ovary matures into a fruit The ovules mature into seeds 31