Dr Zoë Popper Room 319, Martin Ryan Institute [email protected]
Botany and Plant Science How did plants evolve?
Earth “The Blue Planet” Mars
Green plants Green photosynthetic organisms include all green algae & land plants, including mosses, liverworts, ferns, gymnosperms & flowering plants
Green plants Green plants are monophyletic which means they all originate from a single common ancestor Last common ancestor of all land plants A member of the Charophycean Green Algae Zoë A Popper Current Opinion in Plant Biology 2008, 11:286–292ë A Popper Current Opinion in Plant Biology 2008, 11:286–292 Zo
Green plants Plants exhibit tremendous diversity Some plants are less than 1 mm in width & some can grow to more than 100 m tall Wolffia sp. Redwoods ZA Popper
Green plants Some plants complete their life cycles in a few weeks Others may live up to 10,000 years Arabidopsis Bristlecone pine Swedish spruce
Green plants share specific characteristics: • are eukaryotic • starch storage • photosynthesise • lack phycobilins (found in cyanobacteria & red algae) • possess chlorophyll a and b • DNA sequence data • DNA structural features — transfer of some genes from chloroplast to nucleus • cell walls composed of cellulose • cellulose synthase enzymes arranged in rosettes within the plasma membrane
Charophytes Land plants originated from charophycean green algae ~500 million years ago & share characteristics including: • phragmoplast formation during cell division • asymmetric arrangement of their sperm flagellae (in Charales & Coleochaetales) • features of cellulose synthesis
Charophycean green algae (CGA) The CGA are morphologically diverse and it is currently being Popper et al., 2011. Annual Review of Plant Biology investigated which is most closely related to land plants
Land plants Cooksonia sp. Major innovation: move to land The movement to land from water required plants to adapt to: • scarcity of water • changes in climate And was driven by initial lack of competition and predation as at that time there were few plants and herbivores on land
Land plants Water was vital to the first land plants and essential for: • photosynthesis • the mechanisms of fertilisation
Land plants On land water may be scare due to lack of water or water may present but in an unusable form
Land plants The temperature or climate on land is more unpredictable & harsher than in water Air temperature changes more rapidly than water Higher air temperature means greater water loss by evaporation
Land plants Members of the most early diverging land plant groups grow near water and in humid environments Susan Prediger Fascination of Plants Day photo competition winner 2012
Land plants Major innovation: cuticle • Some plants have developed thick waxy cuticles to help slow water loss by evaporation
Land plants Major innovation: stomata •The cuticle keeps water in but CO out 2 • CO is essential for photosynthesis 2 • Plants that developed pores (stomata) in their surface, allowing gas exchange, were able to survive • Guard cells either side of the stomata control when they open and close Guard cell Stomata
Land plants Living on land offered advantages: • more exposure to sunlight for photosynthesis • increased carbon dioxide levels • greater supply of inorganic nutrients ZA Popper
Land plants Land plants are monophyletic which means they all originate from a single common ancestor Last common ancestor of all land plants Zo Zoë A Popper Current Opinion in Plant Biology 2008, 11:286–292ë A Popper Current Opinion in Plant Biology 2008, 11:286–292
Land plants – alternation of generations All plants have a life cycle that involves two phases 1. the first phase consists of a haploid gametophyte that produces eggs & sperm (by mitosis) 2. the second phase – a diploid sporophyte that produces spores (by meiosis)
Mitosis Mitosis occurs only in vegetative cells and gives rise to two identical cells Each daughter cell is diploid It is involved in plant growth
Meiosis Meiosis occurs in sex cells It gives rise to four genetically different daughter cells Each daughter cell is haploid It is involved in reproduction It results in offspring that are genetically different to their parents
Meiosis Meiosis forms haploid spores (pollen & eggs) in sexually reproducing plants There are 2 nuclear divisions so that 4 nuclei are formed from a cell which undergoes meiosis
Bryophytes ZA Popper Bryophytes all have a dominant gametophyte generation which is green & photosynthetic Bryophytes = mosses + liverworts + hornworts
Bryophytes — hornworts K. S. Renzaglia There are ~6,000 Hornwort sporophyte species of hornwort epidermal tissues have true stomata They are less common than mosses & The cells have large liverworts central chloroplasts with pyrenoids
Bryophytes — liverworts Thalloid Leafy ZA Popper Liverworts can be either leafy or thalloid
Bryophytes — liverworts (thalloid) ZA Popper Thalloid liverworts have pores for gaseous exchange but they are always open Liverworts need to have an abundant supply of water to prevent them drying out
Bryophytes — liverworts (leafy) Leafy liverworts look similar to mosses but: 1. 3 rows of leaves (microphylls) 2. They do not have pores
Bryophytes — mosses ZA Popper Mosses look similar to leafy liverworts, differences are: 1. Leaves are 1 cell thick 2. Leaves are in pairs
Bryophytes ZA Popper Moss and leafy liverwort leaves are only one cell thick allowing them to absorb water directly from the air
Bryophytes Bryophytes have rhizoids (not roots) they: • do not have a have vascular tissue (do not take water up) • anchor the bryophyte to the soil, wall, etc
Early land plants Flagellated antherozoids Pollen The earliest land plants needed water for reproduction as sperm swam through water to fertilise the non-motile egg Later structures such as spores & pollen evolved — these helped to prevent reproductive cells from drying out
Ferns & fern allies Major innovation: dominant sporophyte generation Ferns & fern allies (and the seed plants that evolved later) have a dominant sporophyte generation A horsetail A fern ZA Popper
Ferns & fern allies Similarly to bryophytes ferns have green, photosynthetic gametophytes The gametophyte generation is short-lived
Ferns & fern allies Ferns and fern allies produce spores
Ferns & fern allies Fern spores are produced in large numbers in sporangia The sporangia are clustered in sori The sori may be covered by a protective covering known as an indusium Fern spores may be dispersed by wind
Ferns & fern allies Major innovation: vascular system • Ferns & their allies (and the seed plants) have a vascular system enabling water transport throughout the plant • This makes them less dependant on water ferns + gymnosperms + angiosperms = vascular plants
Seed plants Major innovation: seeds • Seeds are more efficient at dispersal than spores • Seeds are embryos + a food source in a protective shell • Some co-evolved with animals that can disperse the seeds Gymnosperms + flowering plants = seed plants
Seed plants Gymnosperms & Angiosperms reproduce using seeds Seeds are reproductive units formed from a fertilized ovule & consist of an embryo, food store & protective coat Gymnosperm means naked seeds
Seed plants ZA Popper ZA Popper Gymnosperms & Angiosperms do not require an external source of water for fertilisation The gametophyte generation is very small, non- photosynthetic and is parasitic on the sporophyte generation
Angiosperms = flowering plants ZA Popper Gymnosperm cone Angiosperm cone! Major innovation: flowers • Angiosperm flowers evolved from Gymnosperm cones
Angiosperms = flowering plants Long cone/flower axis Exposed seed ZA Popper Angiosperm cone! In more recently evolved flowers the floral axis is reduced and ovules and seeds are protected
Angiosperms = flowering plants ZA Popper ZA Popper ZA Popper ZA Popper ZA Popper • There are many shapes and forms of flowers • They are all modified cones • They are adapted to optimise pollination in different ways • many co-evolved with animal pollinators
Diversity exists at many levels Including: • Ecosystem • Habitat • Species •Population ‘Survival of the fittest’ has driven plant evolution through adaptation to their environment
Habitat ZA Popper ZA Popper Plants live in communities consisting of many species together with the animals and micro-organisms they constitute an ecosystem
Extinction and Biodiversity Extinction is a natural process BUT, humans are driving extinction leading to a biodiversity crisis………
Genetic diversity As well as conserving communities and plant species it is essential also to conserve genetic diversity within a species
Genetic diversity Wheat evolution Important to conserve crop wild relatives
Threats to biodiversity? Threats: • harvesting medicinal plants • habitat loss • climate change • deforestation • grazing • competition • lack of knowledge — we need to know plant diversity before we can conserve it (many plants not yet recorded and categorised) e.g. The bee orchid (Ophrys apifera) is rare in Ireland Threats include sand dune erosion due to recreational activities © J R Crellin 2005
Why conserve biodiversity? Conservation of biodiversity is important: ZA Popper ZA Popper • biophilia (affinity of humans for the natural environment = plants make people happier!) • economy — ecotourism, food, medicines…. • plants created the environment and make it habitable • animals are dependant on plants for food, fuel, fibre... • each plant species is a unique evolutionary point…… ZA Popper
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