Plant Hormones PDF

PLANT HORMONES What is a hormone? Greek horman = to stimulate • Biochemical molecule that regulates growth and development based on biological and environmental influences • Environment and stress responsive • Mobile throughout plant: small molecule, can easily cross membranes • Permit efficient c...

PLANT HORMONES What is a hormone? Greek horman = to stimulate • Biochemical molecule that regulates growth and development based on biological and environmental influences • Environment and stress responsive • Mobile throughout plant: small molecule, can easily cross membranes • Permit efficient communication among cells, tissues, organs à chemical messengers • Plant hormones are produced at very low concentrations Hormones in plants • Hormones can have effects on the cells that produce them and, after transport, at the target cells or tissues • Hormones can have inhibitory rather than stimulatory effects • Present in all cells at various levels • Classes of hormones work in signal cascades – Hormone-receptor interactions – Respond to a host of factors and biological needs • Abiotic – Water stress – Light – Nutrient deficiency • Biotic – Growth – Development – Herbivore stress Major plant hormones • • • • • • • • • Auxin – Greek: auxein; to grow or increase Cytokinin – cytokinesis (cell division) Abscisic acid – abscission Gibberellic acid – pathogen Gibberella Ethylene – chemical brother to ethanol Jasmonates– found in jasmine oil Brassinosteroids – derived from Brassica spp. Strigolactones Salicylic acid - induction of plant defense against a variety of biotic and abiotic stresses through morphological, physiological and biochemical mechanisms Hormone biosynthesis Made from four biosynthetic pathways: ABA Gibb Auxin Cytokinin Salicylic ac Brassinosteroid Jasmonates KEGG database: http://www.genome.jp/kegg/ Ethylene Finding plant hormones Observational: 1. Darwin stumbles on auxin – Noticed grass tips grow toward light – With tip growth responded to light – Without tip growth had no response 2. Mutation screening (aka forward genetics): – Dwarf plants are can be hormone deficient – Arabidopsis AUXIN • Greek: auxein; to grow or increase • Essential to plant growth – In almost every aspect of plant development • Hormone level very important Erland et al. 2015. Plant Signaling & Behavior 10:11 AUXIN Production: Shoot tips & developing seeds • Some known actions: – Apical dominance growth – Regulates cell elongation – Establishment of polarity of root-shoot axis during embryogenesis – Promotes cell elongation and cell division – Cell differentiation (vascular differentiation) – Lateral root formation and adventitious root formation – New leaves – Fruit formation – Phototropism, gravitropism – Germination – Delay leaf abscission Evidence for the role of auxin in apical dominance High auxin concentration Low auxin concentration Drawings depicting Coleus (Lamiaceae family) AUXIN • Although auxin affects several aspects of plant development, one of its chief functions is to stimulate the elongation of cells in young shoots – The apical meristem of a shoot is a major site of auxin synthesis – As auxin moves from the apex down to the region of cell elongation, the hormone stimulates cell growth – Auxin stimulates cell growth: concentration from about 10-8 to 10-4 M. – At higher concentrations: may inhibit cell elongation, (probably by inducing production of ethylene) • Auxin also alters gene expression rapidly, causing cells in the region of elongation to produce new proteins within minutes – Some of these proteins are short-lived transcription factors that repress or activate the expression of other genes – Auxin stimulates the sustained growth response of more cytoplasm and wall material required by elongation With synthetic auxin Adventitious roots growing from stem tissue Without synthetic auxin Evidence for the role of auxin in adventitious root formation Evidence for the role of auxin in formation of fruit and structures of similar function (e.g. receptacle in strawberry) Normal conditions All achenes removed Band of achenes removed Without seed formation, fruits do not develop. Developing seeds are a source of auxin. Auxin replacement restores normal fruit formation and can be used commercially to produce seedless fruits However, too much auxin can kill the plant and thus synthetic auxins used commercially as herbicides Fragaria (Rosaceae family) CYTOKININ • Cytokinesis (cell division), discovered in 1950’s • Accidently added degraded DNA to medium Production: primarily root tips Some known actions: • • • • • • • • • Apical dominance is reduced: lateral growth of shoots Root growth rate is reduced Shoot apical meristem produce more leaves Delay leaf senescence Organization and development of xylem tissue Response to light Open stomata Leaves with higher chl content Break bud dormancy Cytokinin delays leaf senescence (ageing and reabsorption of aged organs) Transgenic Untreated Genetic modification to increase cytokinin biosynthesis Nicotiana (Solanaceae family) Plant (b) has apical bud removed so axillary buds grow Auxin & Cytokinin ratio importance • Auxin alone = Large cells (no division) • Cytokinin alone = Cells have no change • Auxin + Cytokinin = Normal cell growth and division ABSCISIC ACID (ABA) Production: • Mature leaves, especially under stress • Roots, then transported to shoots Discovered in early 1960’s Ubiquitous hormone in vascular plants Some known actions: • Originally implicated in leaf and fruit abscission • Involved in leaf senescence • Maintains seed/bud dormancy (opposed to GA) • Response to stress (water stress): Desiccation tolerance – Involved in stomata regulation (closes) Seed storage proteins Prevents preharvest sprouting (vivipary) Embriogenesis Regulates salinity, dehydration, temperature stress response • Responses to pathogens • Heterophylly • • • • http://www.rikenresearch.riken.jp/en g/research/6121 • One major affect of ABA on plants is seed dormancy – Levels of ABA may increase 100-fold during seed maturation à inhibition of germination and the production of special proteins that help seeds withstand the extreme dehydration that accompanies maturation – Seed dormancy has great survival value à ensures the seed will germinate only when there are optimal conditions of light, temperature, and moisture Without abscisic acid the mutant corn seed sprouts Solutes (e.g. potassium and chloride ions) accumulate in guard cells causing water to accumulate in guard cells, making them turgid ABA induces stomatal closure ABA is one signal that causes guard cells to release solutes and thus release water, making them flaccid and closing the stoma (pore) GIBBERELLIC ACID / GIBBERELLINS • Originally found in Gibberella (rice pathogen); 1926 Japan – Responsible for ‘foolish seedling’ phenomenon – Uninhibited growth until breaking • Involved in cell division & elongation Roots and leaves are major sites of gibberellin production Some known actions: • Promotes stem growth: more cell elongation • Hypothesis: gibberellins stimulate cell wall loosening enzymes that facilitate penetration of expansin • Stimulate seed germination • Stimulate flowering: Floral induction and development • Stimulate fruit development: fruit maturation • Leaf expansion • Apical dominance • Flowering and seed germination Commercial use of GA Without GA With GA Larger fruits that are easier to clean are attractive in markets Fruits in absence of seeds Thompson seedless grapes (Vitis (Vitaceae family) ETHYLENE • A hydrocarbon gas produced in most tissues under stress, senescence, or fruit ripening; discovered in 1930’s • Inhibition of growth in dark conditions Some known actions: • • • • • • • • • • Fruit ripening Leaf and flower senescence Leaf and fruit abscission Sex shifts in flowers, promote female Altered geotropism in roots and stems Promotes seed germination Induction of disease resistance factors Initiation of roots Bud dormancy release Lateral cell expansion Both are 100 days after picking Mutated ethylene receptor Normal ethylene receptor Lycopersicon (Solanaceae) Experimenting with plant response to ethylene commercial uses Mutated ethylene receptor Normal ethylene receptor levels 8 days after pollination Petunia (Solanaceae family) Conversational Plants? • Ethylene production increases during stress – Drought – Heat • Perceived by neighboring plants • Unstressed plants induce stress pathways JASMONIC ACID • First identified in jasmine oil • Response to biotic stress Plant-insect co-evolution – Wounding induces JA biosynthesis – Microbial and fungal invasion • Plant growth effects similar to auxin – Specialty growth structures The first physiological effects of JAME, however, and its free acid (JA) were only observed two decades later (1980’s). Degenhardt (2009) Plant Physiology 149:96-102 http://pubchem.ncbi.nlm.nih.gov/ Tri-trophic interactions Jasmonic acid: Pathogen response http://park.itc.u-tokyo.ac.jp/biotec-res-ctr/kampo/eng/research_plant.html BRASSINOSTEROIDS Steroid plant hormone. Are structurally very similar to androgens, estrogens and corticoids • Stress responses • • • • • • • • – Switchgrass suspension cells have minor amounts of lignin. Addition of brassinolide induces normal lignin formation and composition Like auxins and gibberellins: predate the evolution of land plants Stem elongation Seed germination Pollen tube growth Brassinolide Cell division, elongation, differentiation Germination Leaf senescence Stress response Discovered in 1979 http://pubchem.ncbi.nlm.nih.gov/ Gibberellins and Brassinosteroids interact with phytochrome to promote cell elongation in the dark STRIGOLACTONES • In ~ 80% of plant species; discovered 1966, understood many decades later • Promote symbiotic relationships – Arbuscular mycorrhizae • Suppress shoot branching • Stimulate cambial activity & 2ary growth • Reduce adventitious roots and lateral root formation • Promote root hair growth Hormones (PGR) pathways • PGRs work in complex cascades to produce signals Environment al input – Synergistic – Antagonistic Hormone synthesis OR RESPONSE TERMINATION Transport to site of action • Variety of actions in cell – Gene transcription – Protein degradation Programmed development Signal transduction Response Receptor Feedback Threshold Compartmentation /reversible conjugation Catabolism Efflux • Produced at VERY low concentrations: Signal transduction pathways amplify the hormonal signal many fold and connect it to a cell’s specific responses à altering gene expression, affecting enzyme activity, or changing the properties of membranes • Response to a hormone usually depends not so much on its absolute concentration as on its relative concentration compared to other hormones – It is hormonal balance, rather than hormones acting in isolation, that may control growth and development of the plants