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PhotomorphogenesisMorphogenesis: development of plant organs –not directional.e.g. photomorphogenesis: developmental effects mediated by light
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Photomorphogenesis vs. Phototropism
Morphogenesis: development of plant organs – not directional, e.g. photomorphogenetic effects mediated by phytochrome on seed germination and flowering.Tropism: directional growth response of plant organs to an environmental stimulus; results from differential growth (usually differential cell elongation)
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TropismsGravitropism: growth directed by the gravitational fieldThigmotropism: growth response to touch Phototropism: growth response to light
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Phototropism• Phototropism: growth toward (or away from) lightLight is perceived by blue lightphotoreceptors (phototropins)
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Introduction • What is a plant hormone?
Plant hormone can be defined as a small organic molecule that elicits a physiological response at very low concentrations.
1. Synthesized by plants and broadly distributed within the plant kingdom.
2. Show specific biological activity at very low concentrations.
3. Display multiple functions in plants.
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Plant Hormones •Plant hormones classification:•The five classical hormones: 1.Auxin 2.Cytokinin 3.Gibberellin 4.Abscisic acid 5.Ethylene
Additional substances considered as plant Hormones: 1.Polyamines 2.Jasmonates 3.Salicylic acid 4.Brassinosteroids5.Signal peptides
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Major functions of auxin1- Promotes elongation growth in shoots (phototropism)2- Inhibits elongation growth in roots (gravitropism)3- Promotes lateral bud dormancy (“apical dominance”)4- Controls (with cytokinins) organogenesis in tissue culture
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Actions of Auxin: Phototropism and Gravitropism
Phototropism is mediated by the lateral redistribution of auxin.
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Developmental effects of Auxin–Auxin regulates apical
dominance.
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Developmental effects of Auxin–Auxin delays the onset of leaf abscission.
–Auxin transported from the blade normally prevents abscission.
–Abscission is triggered during leaf senescence, when auxin is no longer being produced.
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Developmental effects of Auxin–Auxin promotes fruit
development.
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Developmental effects of Auxin– Auxin promotes the formation of lateral and adventitious roots.
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Figure 39: Effect of 6.0 mg/l TDZ treatment on in vitro multiplication, shoot length and number of leaves/explant of coffee (Coffea arabica L. ) (Oudayni cultivar).
)Odayni. cultivar.(
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CytokininsOppose apical dominance – promote outgrowth of lateral budsRegulate organogenesis in tissue cultureStimulate cell division in young shoots and leavesDelay leaf senescence
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Cell Division and Plant Development
–Crown gall is dramatic natural evidence of the mitotic potential of mature plant cells.
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Biosynthesis, Metabolism and Transport of Cytokinins –Crown gall cells have acquired a gene for
cytokinin synthesis
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The Biological Roles of Cytokinins –The auxin: cytokinin ratio regulates
morphogenesis in cultured tissues .
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The Biological Roles of Cytokinins –The auxin: cytokinin ratio regulates
morphogenesis in cultured tissues .
IBA IBA+Zeatin
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The Biological Roles of Cytokinins –Cytokinins promote chloroplast development.
Dark Dark + Cytokinin
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Figure 39: Effect of 6.0 mg/l TDZ treatment on in vitro multiplication, shoot length and number of leaves/explant of coffee (Coffea arabica L. ) (Oudayni cultivar).
)Odayni. cultivar.(
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Figure 34: Effect of 8.0 mg/l BA treatment on in vitro multiplication, shoot length and number of leaves/explant of coffee (Coffea arabica L.) (Dawaeiry cultivar).
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Effects of Ethylene–Leaf epinasty results when ACC from the root is
transported to the shoot.
Fruit ripening
+ Ethylene
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Developmental and Physiological Effects of ABA –ABA closes stomata in response to water stress.
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–GA biosynthesis occurs in developing seeds.
Biosynthesis and Catabolism of Gibberellins
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–GAs are known to be regulators of many phases of higher plant development, including seed germination, stem growth, induction of
flowering, pollen development and fruit growth .
Gibberellins: Their Discovery and Chemistry
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–From the time of their discovery, GAs were known to be effective in promoting stem elongation.
–Only few of the 136 known GAs have intrinsic biological activity. These include GA1, GA3, GA4, GA5, GA6 and GA7.
Gibberellins: Their Discovery and Chemistry
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–GAs can stimulate stem growth in genetically dwarf mutants by promoting internode elongation.
Effects of Gibberellins on Growth and Development
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–GA induces the synthesis of α-amylase in aleurone layer.–Cereal grains can be divided into three parts: the embryo,
the endosperm and the testa-pericarp. The endosperm is surrounded by aleurone layer .
Gibberellin Responses
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