Plants L10-14
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Plants L10-14 - Marcador
Plants L10-14 - Detalles
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| Three Distinct Genetic Components Form the Seed | ENDOSPERM 3N (TRIPLOID), EMBRYO 2N (DIPLOID), SEED COAT 2N (DIPLOID) |
| Developmental plasticity exhibited by embryo | It can undergo: Androgenesis, zygotic embryogenesis, gynogenesis, maternal/paternal apomixis, somatic embryogenesis. |
| Embryogenesis in Arabidopsis | First zygotic asymmetric division. Apical cell: gives rise to the embryo proper which contributes most of the embryo structure. Basal cell: develops into the suspensor and contributes to part of the root apex. Their embryo patterning is tightly regulated by a defined transcriptional network, by surrounding tissues. |
| Evolution of Seeds | Seeds of flowering plants contain three distinct genetic components. Angiosperms are the flowering seed plants. |
| Paternal vs maternal regulation | Early embryo development is regulated by a novel signalling pathway involving maternal extra-embryonic peptides and paternal zygotic factors. |
| Endosperm Function | Required for embryogenesis and seed germination. Acts as an intermediary between the embryo and surrounding maternal sporophyte. Post-fertilisation barrier by preventing wide-hybridisations. |
| Endosperm development | Life-cycle is confined to the seed stage. Different modes of development. Relative contribution to mature seed varies. Syncytial Phase: mitotic synchrony and nuclear migration |
| Signalling within the embryo sac | In Cdc2a mutants of Arabidopsis, fertilisation of the egg triggers proliferation of the central cell despite the absence of a second fertilisation event. Signalling from zygote to central cell / endosperm within the embryo sac. |
| Reciprocal signalling between the ovule integuments and the endosperm | The ovule integuments can restrict growth of the endosperm (ttg2 mutants in Arabidopsis). The endosperm can restrict integument elongation (iku1/2 mutants). Both Endosperm and Female Sporophyte (integuments) control seed size. |
| Embryo and Endosperm have very different genetic requirements for their correct development | Embryo: - follows normal Mendelien genetics, - does not require the genomic contribution of both parental genomes (eg. Haploidy, apomixis). Endosperm: -requires unique parental contribution, -deviation from 2 Maternal:1 Paternal (2M:1P) genomic ratio has dramatic effects on endosperm development. |
| Summary of seed phenotypes from reciprocal interploidy crosses in Arabidopsis (maternal vs paternal excess) | Maternal excess: small peripheral endosperm, early cellularisation, little or no development of chalazal, endosperm past binucleate stage and Small embryo. Paternal excess: large peripheral endosperm, late cellularisation, overgrown chalazal, endosperm and Viable: large embryo. |
| Parental tug-of-war | Paternally derived genomes promote growth. Maternally derived genomes inhibit growth. |
| Genomic imbalance in the plant endosperm often leads to eventual seed abortion. (2 explanations) | 1. Dosage compensation mechanisms altered (2M:1P genomic contribution in triploid endosperm). 2. The inherited parental alleles do not contribute equally (Imprinting). Maternally and paternally derived genomes are not functionally equivalent in the endosperm, but somehow have been ‘finely tuned’. |
| Imprinting (only affects endosperms) | Differential expression of maternally or paternally inherited genes. Imprinted genes are genes whose expression is determined by the parent that contributed them. Imprinted genes violate the usual rule of inheritance that both alleles in a heterozygote are equally expressed. |
| Fertilisation Independent Seed (FIS) mutations on imprinted genes | Fis mutant: exhibit extensive nuclear proliferation in the unfertilised ‘endosperm’. FIS genes are imprinted and regulate endosperm growth and development. |
| Genome-wide transcriptomic analysis to identify imprinted genes in plant endosperm | Discover SNPs in parental genomes. Measure allelic expression level by Next Generation Sequencing. Computational analysis to identify candidates genome-wide. MEGs are preferentially expressed in endosperm, however, PEGs are expressed in a wide range of vegetative and reproductive tissues. |
| Embryo vs endosperm development | The main difference between embryo and endosperm is that the embryo is the conceptus of fertilization whereas the endosperm is the nutritive tissue of the seed. Furthermore, the fusion of the egg cell with a sperm results in an embryo while t he fusion of a sperm with the binucleate central cell results in an endosperm. |
| Why seeds are so important? | Seeds are the start and end of the plant life cycle. They are the only mobile phase and therefore determine when and where the plant exists. Conservation: remain viable for 1000 years in dry storage. They are time capsules containing the full genetic information for the survival of a species. They are the major currency of the biotechnology revolution. |
| Seed longevity is predictable | Seeds deteriorate in storage and so viability changes with time at a rate determined by temperature and water content. Longevity in terms of temperature and water content are predictable and this is essential for conservation (gene banks) and agriculture (seed trade). |
| Germination and establishment is supported by seed reserves | Storage reserves (Oil and protein). TAG lipase is broken down, generally the oil turns to fatty acids and glycerol and undergo a process to become sucrose. |
| Seed priming | 1. Inhibition 2. Activation 3. Growth is blocked 4. Dehydration 5. variable period of storage 6. Growth. The regulation of germination by water availability led directly to commercial seed improvement treatments. Seed progress to germination at different rates and uniformly. |
| Seed Dormancy | A dormant seed does not have the capacity to germinate in a specified period of time under any combination of normal physical. Dormancy is not a constant state, it cycles in response to the environment. Seeds act as environmental sensors: select season and time of the year and suitable conditions for germination. |
| How do seeds determine if the conditions are suitable for germination | Light indicates soil disturbance or a position at the surface. Spectral quality of light informs the seed about the presence of competing plants. Alternating temperature can inform about time of year, but also depth in the soil and presence of competing plants. Nitrate levels in the soil can also inform the seed about the presence of competing plants. Increased sensitivity to nitrate enhances the germination response to light at any given temperature. |
| Germination is regulated a complex relationship between two plant hormones | ABA and Gibberellin levels are regulated by environmental cues. ABA to dormant and Gibberellins to germinate. |
| The structure and organisation of the root makes it an ideal dynamic sensing and responding system | Outside cells:- sense environment (lateral root cap), - nutrient/water uptake (epidermis; root hairs). Inside cells: - transport nutrients/water (cortex & endodermis), - regenerate themselves (quiescent centre), - new organ formation (pericycle). |
| Signals are sensed and responded to at many levels | Genetic level (regulate gene expression), molecular level (how many molecules,where they are; metabolism), cellular level (signalling pathways), physiological level (change how cells and organs operate), developmental level (modify which organs grow, when and how they grow) |
| Reception of stimulus, signal transduction and response | Reception: Internal and external signals detected by specific receptors (proteins that change in response to specific stimuli). Transduction: Second messengers transfer and amplify signals from receptors to proteins that cause specific responses. Response: Regulation of one or more cellular activities. In many cases this involves the increased activity of certain enzymes. |
| Signalling in different parts of the plant has to be connected | Plants forage for resources in heterogeneous environments. Modules sense local resource availability and respond (local plasticity). This response is modified by systemic signals that indicate whole plant resource status. |
| Influence of nutrition on root branching e.g. nitrogen | Antagonistic interplay between local detection/response and its modification by systemic signals. Need to forage for soil nutrients in competition with other plants. But foraging for nitrate when internal N-status is high is non-productive, better to invest carbon in leaves. |
| Split root experiments: local vs systemic signalling | Different parts of the root system talk to each other - revealed using a split root experimental system = local & systemic signalling. Signals go up through the shoot and down to each part of the root system. |
| The root can also signal conditions to the shoot | Stomata often respond to mild soil water deficits before there is a detectable reduction in shoot water status. Roots are able to sense soil water deficit, and signal stomatal closure (feed-forward signalling). Abscisic acid is required for the signalling (hormones mediate). |
| Definitions of abiotic environmental stress | Abiotic stress, Stress dynamics, Stress resistance, Effect on ‘performance’. |
| What is stress resistance? | Stress avoidance: “homeostatic adaptations at the organism level that prevent stress in the cellular environment from developing”. Includes stress escape. Stress tolerance: “protective adaptations at the cellular level that prevent damage when the homeostatic balance of the cellular environment is disturbed”. |
| Phenotypic plasticity | The capacity of a genotype to produce different phenotypes in different environmental conditions. The reaction norm is the set of phenotypes produced by a genotype over a range of environments. |
| Identifying the genetic components of plasticity | Flooding response in rice. Elongation occurs in the internodes to raise the plant above the water's surface. Plasticity of the phenotype involves the genes SNORKEL1 and SNORKEL2. |
| Three distinct mechanisms of drought resistance | Drought escape: avoid drought stress by altering developmental timing. Dehydration avoidance: ability to avoid cellular dehydration. Dehydration tolerance: ability to withstand cellular dehydration. |
| Symbionts | Most plants: Mycorrhizal fungi. Some plants: Nitrogen-fixing bacteria. Fungal surface facilitates nutrient and water update. |
| Main steps leading to nodulation | Pathway between perception and nodulation goes through a set of SYM genes and involves Ca2+ oscillations. The common SYM pathway (indicated in red) transduces information from the plasma membrane to the nucleus. 1. Communication 2. Root hair curling 3. Infection thread formation 4. Infection thread grows towards cortex 5. Nodule expressing ledgehomoglobin. |
| Signalling between AM fungi and plant (Arbuscular mycorrhizal) | 1. Plant root produces strigolactones & other chemicals that initiate branching in fungal hyphae. 2. Fungi produce a Myc factor, identifying it as a symbiont. 3. Plant prepares to permit the fungi to penetrate its cells. |
| Transporters accumulate at the periarbuscular membrane | Facilitates nutrient exchange: - P & N compounds in from the fungus, - Sugars out from plant cell to fungus |
| Autoregulation Of Nodulation (AON) balances supply & demand | Inoculation of one half of a root system can inhibit nodulation in the other half; perception of this signal generates a signal to the root to inhibit further nodulation. Infection with rhizobia generates a mobile signal that travels to the shoot. |
| Hypernodulating mutants revealed a role of a CLV-like receptor in AON | CLV-like receptor: acts in shoots. Har1: required in shoots suggesting that an inhibitory signal moves from shoot to root. |
| Autoregulation of nodulation involves shoot and root-derived signals | The root-derived signal may be a CLE-like peptide: - CLV1 is a CLE-peptide receptor, - Several CLE peptides are induced by rhizobium infection and systemically reduce nodule number |
| Nitrate represses nodulation locally via root-localised CLV-like receptors | Nitrate-induced CLE peptides interact with CLV-like receptor in the root to repress nodulation. Grafting studies show that CLV-like receptors in the root are necessary suppress nodulation in response to nitrate. |
| Strategies of pathogens | A virulent pathogen is one that a plant has little specific defence against. An avirulent pathogen is one that may harm but not kill the host plant. Necrotrophs, Biotrophs, Hemibiotrophs |
| How are pathogens recognised? Pattern recognition receptors (PRRs) | Many PRRs have an extracellular leucine-rich repeat domain and an intracellular kinase domain that recognizes conserved microbial elements. 1. PRRs recognize PAMPs, 2. recognition triggers defence responses |
| Mechanism of recognition and response | 1. Recognition 2. Ca2+ influx 3. Reactive O2 production 4. Transcriptional responses to nucleus 5. Defence responses. |
| Fungal and oomycete biotrophs usually make haustoria | Haustoria remain outside the plant plasma membrane, and are specialized for nutrient and signal exchange. |
| Pathogens produce effectors that enhance their virulence | Microbial effectors suppress the plant’s immune response and / or contribute to the pathogen’s viability. Many bacterial effectors are introduced into plant cells through Type-III secretion systems. Some effectors alter plant behaviour and development. |
| Resistance proteins – intracellular immune receptors-Effector- triggered immunity (ETI) | ETI is faster, stronger and more prolonged than PTI. R proteins recognize effectors intracellularly This is a biotroph-specific type of resistance. ETI response: pore formation, Ca2+ release, hypersensitive response, transcriptional defences. PTI defences: ROS production, Callose deposition, transcriptional defences. |
| R protein some details TIRs vs CCs (steps) | 1. MAMP recognition 2. PTI signalling 3. PTI suppression by effectors 4. Effector recognition by NLRs 4a. CC-mediated ETI 4b. TIR-mediated-ETI. |