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“Describe the problems generated by life on land for the green plant lineage. How have these problems been overcome?” Introduction: - Green plants are the dominant life form on the planet o Make up almost 100% biomass o Input energy into ecosystems o Provide nutrients and niches for animals, fungi and bacteria - Belong to single monophyletic group called the Embryophytes - Include: flowering plants, conifers, ferns, clubmosses, mosses, hornworts and liverworts - During transition phase, first land plants exposed to: - - o Highly desiccating environment o UV radiation o Extreme temperature fluctuations o Different conditions for reproduction and nutrient uptake. o Aerial environment – altered gas exchange o Increased gravity – increased mechanical support requirements Advantages conferred by terrestrial life: o Increased light availability o Increased CO2 concentration in air than water They have highly specialised morphological and physiological adaptations that allow them to thrive on land. Desiccation: - Bryophytes (liverworts, hornworts & mosses) descend from earliest branching events in phylogeny evolved from Charophyceae algae - Phylogenetic studies suggest that desiccation tolerance mechanisms have changed little from the earliest land plants therefore dissociation resistance = primitive trait. - Liverworts: o Maximise surface area for absorption o Hug moist ground (remain in contact with moist air near the ground) o Suspend metabolism, growth and reproduction when they dry out but recover quickly when water becomes available. This is possible because: Cellular structures stay in-tact so cellular integrity is regained upon hydration. Photosynthetic machinery is similarly protected Transcription and translation to produce rehydrins helps to stabilise membranes during rehydration. - Hornworts & all vascular plants(sporophyte generation), mosses (sporophyte and gametophyte) o Waterproof waxy cuticle covers epidermis of leaves, shoots and aerial organs. Made of cutin (a hydrophobic polymer) Forms barriers against water loss and prevent waterlogging when it rains. Prevents contamination by external water, dirt and microorganisms. - Resistant to chemical and biological decay All land plants (except liverworts) (in sporophyte generation) o Stomata (pores in the epidermis) overcome gas impermeability Allow gas exchange (including CO2 and O2 entry for photosynthesis and respiration respectively and by-product exit) Water vapour released through transpiration Stomatal aperture regulated by bordering pair of specialised guard cells Dehydrating conditions – ABA release causes stomatal closure – conserve water o Liverworts, cuticle doesn’t cover entire surface – gas exchange occurs on exposed surfaces. More vulnerable to desiccation, but have H2O stores between cell walls for back-up. More efficient water transport system: XYLEM - Liberate from the constraints of small size and constant moisture. - Specialised, unidirectional vascular transport system = XYLEM o Transport water and soluble minerals passively from roots bigger plants! o Apoptosis long continuous tubes formed from connected conducting cells. o Xylem walls strengthened with lignin (polymer of polypropanoid monomers) Antibiotic Water-resistant Withstands negative pressures in xylem – compression resistance Lateral flexibility for bending and stretching – increase light absorption. o Wide, empty cells more conductive than previous inter-cell method o Enabled transport over long distances ROOTS & PHLOEM - Only bryophytes (first terrestrial plants) didn’t have roots – able to simply absorb nutrients from surrounding water. - Huge surface area to maximise water and nutrient absorption. - Provide anchorage to substrate o Increase stability o Increase growth rate and potential plant size. - Supply of photosynthate to roots needed for growth and metabolism bc dark conditions. - Transport system = PHLOEM in vascular plants o Living tissue o Unlignified cells o Transports sucrose to all parts of the plant (esp root and stem tips – growth) o Hydrostatic pressure generates movement (achieved by phloem loading and unloading) Mycorrhizae: - Endosymbiotic association between vascular roots of early land plants and a fungus. - Host benefits: o Increased mobilisation of phosphate from the soil (conc. 2-3 orders of magnitude less than other mineral nutrients) - o Increased water and mineral nutrient absorption (bc of increases in surface area) o Resistance to toxicity o Plant growth hormone production by fungus o Protection from pathogens o Regulation of pH Greatly enhanced plant root efficiency – they would not have been able to grow large without efficient access to fungal-liberated phosphate. - Therefore shaped evolution of land plants. Protection against predators - Link to mycorrhizae: other biotic interactions also shaped plant evolution - Plants had to cross water/land interface already transiently occupied by animals. - Had to deter predation without repelling animals involved in animal-mediated pollination and seed dispersion - whilst remaining relatively immobile! Physical defences: Trichromes: - Hairs – variable in presence and functionality among species - Dense arrangement deters consumption by herbivores - o Distasteful and sharp fragment can penetrate the soft palate of the feeder’s mouth o Causes intense and long-lasting irritation Glandular trichromes: secrete pest- or pollinator-interactive chemicals o Could impede animal movement o Drosera – they secrete proteases which digest trapped animals – αα absorption - Allows plant to survive in areas with low nitrogen availability (e.g. deserts) Stinging hairs: o When touched, fragile trichromes break and fragment o Produces a sharp edge which can penetrate the skin. o Histamine rapidly injected o Immediate itching 5-hydroxytriptamne, Ach and a Na+ channel neurotoxin Prolonged burning sensation - Reflect radiation to protect fragile tissues in hot and dry habitats - Disrupt air flow across surface: reduces evaporation in windy conditions - Formation of eddies in boundary layer increases CO2 diffusion through stomata. - Prevent frost from touching the plant surface in freezing conditions - Role in waterproofing therefore important in transition from land air Spines: - Prevents grazing by larger herbivores - Can form on almost all organs and surfaces - Particularly well-developed in water-stressed habitats (e.g. deserts) o Loss of photosynthetic area from grazing would be really damaging Chemical defences: - Deter pathogens and predators - Synthesis of toxins o Many aromatic so potential feeders can detect warning olfactory signals o 2 main classes: nitrogenous and non-nitrogenous Non-nitrogenous - Rotenone: produced in roots of a legume (Derris elliptica) o inhibits cellular respiration – disrupts electron transport chain in mitochondria o Particularly affects fish and insects (indigenous fishers and head-lice) Nitrogenous: amino acid analogues, cyanogenic glucosides, alkaloids and proteins - Amino acid analogues o Many more amino acids in plant than are used as protein precursors – some toxic! If incorporated into animal proteins – become dysfunctional (esp. enzymes) E.g. legume seeds (high protein content) protected by toxic amino acids - - (e.g. β-cyanoalanine in Vicia) Causes convulsions and death Proteins: Ricin (produced by Ricinus communis) Lethal in tiny amounts (1 microgram/kg body weight) Causes total failure of all protein synthesis Cyanide o Damage to plant tissues can cause cyanogenic glycosides (stored in vacuoles) to combine with its degradative enzyme in the cytoplasm release of HCN gas (highly toxic!) o HCN acts as terminal respiration inhibitor – rapid death of predator o Edible almonds are HCN-deficient mutants selected by humans and planted in abundance. These seeds of these trees (‘sweet almonds’) are unprotected – vulnerable to predation! - Alkyloids o Chemically characterised by nitrogen-containing rings o Found in 20% of flowering plant families o Liriodenine produced by tulip tree protect against parasitic mushrooms o Strychnine: rat and human poison, provides arrow poison and curare (muscle relaxant – used in anaesthetics) o Atropa belladonna (deadly nightshade) atropine Competitive agonist of ACh in muscarinic acetycholine receptors in parasympathetic nervous system Increases heart rate, saliva production, pupils dilate Animal hormones: - Species of yew produces ecdysone (insect hormone that induces moulting) - Quantities in plants much greater than animals Reproduction: rapid modification needed for life on land! - Plants are reproductively versatile - Generates genotypic variation needed for evolutionary change. - Can also reproduce clonally – prolong existence of single genotypes for 1,000’s of years! Two generation life-cycle: - Both generations are multicellular - Can undergo sporic meiosis o Diploid zygote divides by mitosis to generate multicellular diploid sporophyte o Sporophyte divides by meiosis to produce haploid spores Thick-walled Desiccation resistant More likely to survive in hostile conditions o Haploid spores divide by mitosis to produce haploid gametophyte o Gametophyte divides by mitosis to produce gametes o Gametes combine to produce zygote. Nourishment of zygote and young sporophyte: - In all embryophytes, eggs and embryos remain attached to gametophyte body o - In a structure called the archegonium Jackets of sterile cells used as site for production of gametes (increased protection) and to nourish the young sporophyte. - Nutrients (e.g. sucrose) move through specialised transport cells from gametophyte to sporophyte - Transport cells have extensive invaginations of cell wall to increase PM area for transport o Ensures large, well-nourished eggs o Ensure large, well-nourished zygotes and embryos following fertilisation Bryophytes (early land plants): - New sporophytes grow and remain parasitic on the gametophyte o Bc they lack functional chloroplasts - In ferns, gymnosperms and angiosperms, sporophytes are independent of and dominant to gametophytes. o Vascular plants have green, autotrophic sporophytes which are now the dominant phase of the life-cycle. - The earliest land plants probably had two equal-sizes phases of the life-cycle. - Selection against aerial sperm-producers would be very severe due to desiccation. - Water-dependent fertilisation will be most reliable in contact with the humid ground (as in many ferns) or even under the ground. - Gametophytes of all living plants are ground-hugging, small and short-lived. Clubmosses and ferns: spore producers! - Large sporophyte produces haploid spores by meiosis in sporangia - Spores are coated with sporopollenin o Decay-resistant biological material o Chemically stable o Well-preserved in soils - Spores released into the wind and germinate burrowed down in the soil. - Divide by mitosis to produce small haploid gametophyte. - Gametophyte differentiates to produce antheridia (sperm) and archegonia (ova) - Symbiotic mycorrhizal relationship established – connect it to mother for nutrition - These motile, ciliate sperm swim across moist soils to archegonia on the same or another gametophyte – CROSS-FERTILISATION if densely crowded - Fuse with ovule to produce embryo - Embryo germinates – developing diploids push up through the soil to form sporophyte - Sporophyte carries out independent photosynthesis Angiosperms and gymnosperms: - Spermatophytes (seed-producers) dominated earth ~250 mya. - First = rapid proliferation of gymnosperms. - During seed reproduction, fertilisation is truly internal. - After fertilisation, tissues surrounding the embryo harden to produce a seed from the whole ovule - Seeds are therefore: o Completely isolated from external environment o Protected from desiccation o Can enter dormancy for 10s-100s of years. o Allowed plants to inhabit previously inhospitable areas (e.g deserts) o Increased success rates of fertilised gametophytes o Endosperm = nutrient store within the developing embryo Allowed seeds to germinate more rapidly – reach size at which they can survive independently more quickly E.g. grow roots long enough to reach water table in arid environment. - True internal fertilisation of angiosperms is controlled by the sporophyte through the maternal tissue of the style – stigma and style can exercise ‘mate choice’ – discriminate between pollen grains and tubes – eugenics! - - This enables angiosperms to increase genetic diversity by: o Preventing self-fertilisation – can recognise ‘self’ and reject it o Ensuring out-crossing This is a form of sexual selection maternal tissue selects the fittest pollen grains, increasing the chances of successful fertilisation & increasing the genetic fitness of the offspring. Conclusion - Evolution of land plants has resulted in increasing levels of complexity o - From Bryophytes to complex gymnosperms and angiosperms present today They’ve evolved a diverse array of specific morphological, physiological and behavioural adaptations to allow them to tolerate hostile conditions on land. - Their colonisation of the land, and their subsequent diversification caused a series of environmental changes, resulting in the generation of different habitats and the development of entire terrestrial ecosystems.