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Plant bio
Plant hormones I
| Question | Answer |
|---|---|
| What are phytohormones? | - chemical messengers produced at low and tightly controlled concentrations ie. auxins, gibberellins and cytokinins |
| Vegetative development | • Auxin • Cytokinins • Strigolactones • Gibberellins • Brassinosteroids |
| Control of reproduction | • Ethylene • Abscisic acid |
| Response to distress | • Salicylates • Jasmonates |
| How are hormones regulated? | • There are several check-points involved in hormonal regulation • Synthesis and conjugation • Transport • Receptor-binding and signal transduction • Down-stream effects |
| Conjugation | Conjugation can temporarily store a hormone in an inert form, lead to breakdown, or be the means for producing the active hormone. |
| Hormone mobility | Hormones can move: • through the xylem or phloem • across cellular membranes • through regulated transport proteins They can be membrane bound or soluble |
| Hormone signals | Hormonal signals are transduced in diverse ways. Common methods are protein phosphorylation and proteolysis |
| Downstream effects | Downstream effects can involve changes in gene transcription and changes in other cellular activities like ion transport |
| Auxin | •Growth •Phototropism and gravitropism •Branching •Embryonic patterning •Stem cell maintenance •Organ initiation |
| Cutting off or covering the coleoptile tip interferes with the response | - untreated coleoptile bends - Coleoptiles with tips shielded from light or removed do not bend |
| Auxin synthesis | Auxin is synthesized from tryptophan - reversible conjugation - auxin active form IAA - IBA: storage/transport form |
| Polar auxin transport | Auxin moves long distances through the phloem • Auxin also moves via auxin transport proteins |
| Polar auxin transport | Auxin normally moves from the tip of the shoot towards the tip of the root. At the root tip, auxin changes direction and moves short distances up the root again (basipetally) |
| Auxin moves through efflux and influx carrier proteins | The AUX1/LAX influx carriers contribute to movement of IAAH into the cytoplasm The PIN family of proteins contributes to directional movement of auxin out of the cell The ABCB transporters contribute to auxin transport in a diverse ways |
| The PIN proteins are named for the pin-formed mutant | pin-formed, which has a mutation in the PIN1 gene, makes some abnormal leaves and then a bare inflorescence |
| Auxin’s effects are mediated through at least two types of receptors | - ABP1 action occurs at the cell surface - Auxin perception by SCFTIR1, which occurs in the nucleus |
| Auxin acts like a molecular glue that holds coreceptor proteins together | - TIR1 is an F-box protein, part of the SCFTIR1 ubiquitin ligase complex - It forms a coreceptor complex with Aux/IAA proteins |
| Aux/IAA and ARFs | Aux/IAA genes are induced immediately following auxin application Aux/IAA proteins, shortlived, nuclear proteins that are repressors of auxinresponses Auxin Response Factors (ARFs) an act as repressors or activators |
| The auxin signaling pathway | - At low auxin levels, Aux/IAA proteins and ARF proteins associate and interfere with ARF action - Auxin promotes the association of Aux/IAA proteins and the SCFTIR1 ubiquitin ligase complex |
| The auxin signalling pathway | - Elimination of Aux/IAA proteins stops them from interfering with ARF proteins, which can activate or repress transcription |
| Auxin controls phototropism | Increased auxin promotes cell elongation on the shaded side, causing bending toward the light |
| Gravitropism is a response to a change in orientation relative to gravity | - Roots are positively gravitropic – they grow towards the center of gravity - Shoots are negatively gravitropic – they grow away from the center of gravity |
| Auxin regulates plant development | - Lateral organ initiation at the shoot apical meristem - Maintain stem cell fate at the root apical meristem - Patterning and vascular development - Inhibit branching in the shoot - Promote branching in the root |
| Cytokinins | •Cell division •Control of leaf senescence •Control of nutrient allocation •Root nodule development •Stem cell maintenance •Regulate auxin action |
| Cytokinin biosynthesis | The main enzymes involved in CK biosynthesis (that we know of) are: • ISOPENTENYL TRANSFERASE (IPT) • LONELY GUY (LOG) enzymes |
| Cytokinins are transported throughout the plant | Various transporters can transport CK via the xylem (downwards) or phloem (upwards) |
| Cytokinin control of gene expression in Arabidopsis | - Cytokinins bind to membrane bound receptors - Arabidopsis response regulators (ARRs) are transcription factors that can either promote or inhibit CK |
| Cytokinins act antagonistically to auxins: CK | Promote stem cell fate at the shoot apical meristem Promote branching in the shoot Inhibit branching in the root Promote differentiation at the root apical meristem |
| Cytokinins act antagonistically to auxins: Auxins | Promote lateral organ initiation at the shoot apical meristem Maintain stem cell fate at the root apical meristem Inhibit branching in the shoot Promote branching in the root |
| Auxin and cytokinin regulate each other’s function at the root apex | Through effects on each other’s synthesis, transport and response, auxin and cytokinin establish two mutually exclusive domains that coordinate cellular activities at the root apex |
| Auxin, cytokinin and strigolactones control branching | Shoot branches are promoted by CK and inhibited by auxin and strigolactones Lateral roots, are promoted by auxin and inhibited by CK Branching controls every aspect of plant productivity from nutrient uptake to crop yields |
| Strigalactones | •Inhibit shoot branching •Promotes associations with arbuscular mycorrhizal (AM) fungi •promote germination of parasitic plants |
| Strigolactones contribute to a devastating form of plant parasitism | Striga are parasitic plants that are the single largest biotic cause of reduced crop yields throughout Africa (> $10 billion per year in Striga germination is induced by strigolactones produced by host plant roots yield losses) |
| SLs are synthesized in the plastids of roots and shoots | |
| SLs have a similar receptor complex as auxin | and inhibit shoot branching |
| SL synthesis in root or shoot is sufficient to control shoot branching | Reciprocal grafts, in which wildtype tissue is either the root or scion, have normal phenotypes; this says that the branch-controlling signal can be made in either tissue, and can move from root to shoot |
| Strigolactones promote beneficial symbiotic interactions | SLs promotes the symbiotic association with AM fungi. This symbiosis occurs in 80% of land plants and helps them assimilate nutrients from the soil |
| Strigolactones (SLs) regulate seemingly distinct events | - inhibit shoot branching - promote AM fungi association - promote germination |
| Giberelins | •Growth •Seed germination •Promote flowering •Promote sex determination in some species •Promote fruit growth |
| Gibberellins are a family of compounds | Only some GAs are biologically active. The major bioactive gibberellins are shown here • GA4 is the major active GA in Arabidopsis |
| The GA biosynthetic pathway is complex and spatially separated | Stage 1 - proplastid Stage 2 - endomembranes Stage 3 - cytoplasm |
| GA intermediates are mobile | GA intermediates move in the phloem to the site of activity where they are further modified to active GA |
| GA signalling pathway | - DELLA proteins inhibit growth, in part through blocking transcription - GA triggers DELLA protein proteolysis |
| Genes controlling GA synthesis are important “green revolution” genes | |
| Brassinosteroids | •Cell elongation •Pollen tube growth •Seed germination •Differentiation of vascular tissues and root hairs •Stress tolerance - steroids like some animal hormones |
| BRs are transported cell-to-cell; BR receptors are facing the extracellular space | - wildtype: cell elongation - BR mutant: less cell elongation |
| BRs are not transported directionally over long-distances | |
| BR signalling | Without BR, the receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) is bound to an inhibitor BRI1 Kinase Inhibitor 1 (BKI1) The active BR-INSENSITIVE 2 (BIN2) kinase phosphorylates and inactivates transcription factors |
| BR signalling | BR-binding causes BRI1 Associated receptor Kinase 1 (BAK1) and the BRI1 receptor to phosphorylate each other and BSK (Kinases) |
| BR signalling | BSKs phosphorylate and active BSU1 phosphatase, which inactivates BIN2. When BIN2 is inactive, its target transcription factors are dephosphorylated and active |
| Brassinosteroid (BR) mutants are dwarfed | - BRs promote cell elongation in part by loosening cell walls - Lowered resistance to internal turgor pressure; cell expansion |
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rose.coo
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