Plant Movement, Tropisms, and Senescence (PDF)

Plants Move! TROPISMS Chapt. 18. Part 2 TROPISM Is a biological phenomenon, indicating growth or turning movement a plant in response to an environmental/external stimulus Stimulus vs Response • Stimulus – an action or condition that causes a response • Response – an action or condition that is...

Plants Move! TROPISMS Chapt. 18. Part 2 TROPISM Is a biological phenomenon, indicating growth or turning movement a plant in response to an environmental/external stimulus Stimulus vs Response • Stimulus – an action or condition that causes a response • Response – an action or condition that is a reaction to a stimulus › POSITIVE › If the plant moves TOWARD the stimulus › NEGATIVE › If the plant moves AWAY from the stimulus TROPISM Plant growth or turning in response to an environmental stimulus (“tropo” – ”turn”) . The type of the stimulus is shown by prefixes: Light • Photo 3 main types • Geo or Gravi Gravity • Thigmo Touch Water • Hydro Chemicals • Chemo Temperature • Thermo Phototropism In general, tropisms involve cell elongation or suppression of cell elongation on one side of a plant, causing the plant to grow in a particular direction. Movement of plants toward light Why? Maximize amount of sun for photosynthesis to make their food. Roots move away from the sun Phototropism https://www.youtube.com/watch?v=DhITXtENPrU Gravitropism Movement of plants in response to gravity Positive is toward gravity (roots grow down) Negative is away from gravity (shoot, stems, and leaves grow up) Why? Allows plants to grow properly and get nutrients, water and sunlight Auxin is responsible for geotropism: inhibits cell elongation in roots stimulates cell elongation in shoots https://www.youtube.com/watch?v=eDA8rmUP5ZM Gravitropism in shoots vs roots • SHOOTS will grow against gravity (upwards) • ROOTS will grow with gravity (downwards) • Why is this gravitropism in shoots and not • How does a root “know” which way is phototropism? down? – Auxin is transported in cell-to-cell fashion. • Plastids, particularly amyloplasts – Process requires metabolic energy: sensitive to (Statoliths), in the root cap cell tend to O2 deprivation, sucrose depletion, metabolic settle on the bottom side of the cell. This inhibitors stimulates the release of auxins Thigmotropism Plants moving in response to touch. Positive is toward touch (vines wrap around structures) Negative is away from touch (some plants close up when touched) Why? To support leaves as they grow higher to reach the sun to make more food (photosynthesis). Roots: move around obstacles https://www.youtube.com/watch?v=dTljaIVseTc Hydrotropism Movement by plants toward water. Why? Roots search for and grow toward water, because it is needed for photosynthesis and to support cell structure. Chemotropism Movement caused by chemical stimuli Positive: toward high nutrient soil/pollen germination Negative: away from contaminated soil Why? Helps control and regulate growth and development of plants Thermotropism Temperature Plant grows in the direction of/away from a source of heat or cold – Example: In the first picture, this tree has a positive response to the cold. – Example: A plant called a Rhododendron curls its leaves away when it experiences cold weather. Differences between tropisms & nastic responses • Tropisms are a directional response, the stimulus (eg. sunlight through a window) determines the direction the plant grows. • Nastic responses are not the result of a stimulus from a certain direction. Are rapid and reversible. These include responses to changes in temperature (thermonasty), light intensity (photonasty), humidity (hydronasty) and touch (thigmonasty) • Nastic responses can be fast, whereas tropisms are generally slow growth changes. Examples of nastic responses • The leaves of the Venus Fly Trap and the Mimosa plants both close up in response to touch – a thigmonastic response. • The flowers and leaves of many plants close up when the light intensity decreases (it gets dark) – a photonastic response. https://www.youtube.com/watch?v=7mSBTkKqqOU Nastic response Turgor pressure Leaf movement is an osmotic effect: (K+) is released into the leaf tissues and makes the cells on one surface of the leaves wilt (water is leaving the cells) à this makes the wilting surface slightly smaller than the unwilted, opposing leaf surface. The leaf curls towards the wilted side. One remarkable feature of rapid leaf movement is that signals are transmitted from leaflet to leaflet via action potentials (AP). • Traveling at ± 1 cm/s through the leaf: resemble nervous-system messages in animals, although they are thousands of times slower • AP as a form of internal communication • Venus flytrap: stimulation of sensory hairs in trap à AP travels to cells that close trap. • Electrical potentials across cell membranes, like those in our nerve cells, signal plant cells at the base of the Mimosa leaf to rapidly lose water. This causes the leaf to drop. Movies • Sensitive Plant: http://www.youtube.com/watch?v=BVU1YuDjwd8 • Venus Fly Trap: http://www.youtube.com/watch?v=ktIGVtKdgwo&featur e=related • http://www.youtube.com/watch?v=BLTcVNyOhUc • http://www.youtube.com/watch?v=PRo4rg07_gg&fea ture=fvw PLANT SENESCENCE & CELL DEATH Chapter 22 CONCEPTS • Senescence: energy-dependent, autolytic process controlled by environmental factors + genetically regulated developmental program • Necrosis: death caused by physical damage, poisons, other external agents • Abscission: separation cell layers. Occurrs at the base of leaves, floral parts, fruits 3 types of plant senescence • Programmed cell death (PCD) • Organ senescence • Whole plant senescence Processes differ with respect to size, cell number, complexity of senescing units and with respect to developmental/environmental factors that trigger them PCD • Genetically regulated death of individual cells – Protoplasm (sometimes cell wall) à autolysis – Part of normal plant development – Can be induced in response to biotic/abiotic stress • It’s a common feature of all 3 types of senescence • All eukaryotes have evolved cellular suicide – Needed for normal growth, removal of unwanted, damaged or infected cells PCD • Initiated by specific developmental signals or potentially lethal events (pathogens, errors in DNA replication) – ANIMALS: apoptosis à apoptotic bodies that are phagocytosed – PLANTS: autolysis à similar to animals but more variable. Plant use many proteases. 2 types: Vacuolar & Hypersensitive Vacuole ruptures releasing hydrolases Vacuolar PCD: occurs normal development. Vacuole: repository of enzymes • Hypersensitive reponse-type PCD: plant defense mechanism against microbial attack – Cells around the infection commit suicide à deprive pathogen of nutrients – Not initiated by vacuolar rupture but shrinkage followed by DNA degradation. Autophagy • Protects cells form harmful or lethal factors • Recycling of cellular components • Two types: Macro & microautophagy. In plants à macro – Macroautophagy: specialized organelles enclose cytoplasmic components and fuse with vacuole. – Micro: invagination of tonoplast, formation of small vesicles, degradation by lytic enzymes Macroautophagy Organ senescence • Whole leaves, branches, flowers, fruits – Usually includes abscission of the senescing organ – Leaf senescence: strongly influenced by photoperiod and temperature All leaves eventually senesce: age-dependent factors, environmental cues, biotic/abiotic stress LEAF SENESCENCE SYNDROME • Specialized form of PCD: nutrients from leaf to growing sinks via phloem • Changes in cell structure and metabolism – Breakdown of chloroplast: 70% leaf protein – Chl, proteins and macromolecules: exported à reused – Contributes to overall fitness of the plant – Many minerals are also mobilized • Can serve as survival mechanism during abiotic adverse conditions (drought, T) but also occurs in normal conditions (older leaves) • Senescence + Abscission LEAF SENESCENCE Sequential, seasonal or stress-induced • Sequential: older leaves at the base • Seasonal: all leaves at the same time in response to shorter days and cooler temps These two: developmental senescence. Normal growth conditions. Vacuolar PCD Nutrient retention during senescence In deciduous tree species ~ 60 – 70% N, 60 – 70% P, 30% K, 25% Mg, and 15% Ca are withdrawn from leaves prior to them being shed. Storage in bark and elements are re-mobilized in spring Adaptation that keeps deciduous trees from desiccating during winter when roots cannot absorb water from the frozen ground Changing leaf color Yellow and orange carotenoids and xanthophylls, always present within the leaf, begin to show. Water and nutrients are drawn into the stems and from the leaves. Senescing cells also produce other chemicals, particularly anthocyanins, responsible for red and purple colors. Some species, particularly oaks, contain high quantities of tannins in the leaves which are responsible for brown colors. LEAF SENESCENCE • Stress-induced: premature. UV-B, drought, mineral deficiency, ozone, T, high light, darkness, herbivory, pathogens. Hypersensitive PCD à induced. Targets specific sites on leaf. Not coordinated at the whole leaf level Developmental leaf senescence Seasonal or sequential 3 phases: • Initiation: nutrient sink to source. Photosynthesis, signaling of events • Degenerative: dismantling cellular constituents. Macromolecule degradation • Terminal: loss of celullar integrity. Cell death. Leaf abscission Chl and its products: extremely reactive and potentially lethal 1. Chloroplasts are converted into gerontoplasts: loss of grana, thylakoid membranes, accumulation of lipids. This can be reversed up to a point – Not all chloroplasts senesce at same time. Guard cells: last to degrade – Nucleus and mitochondria remain intact until later stages 2. Degrading of chloroplast using enzymes (proteases). Rubisco breakdown à deliver content to vacuole. • ROS, especially H2O2 play key role during leaf senescence – ROS cause oxidative damage to DNA, proteins, lipids – By-products of respiration, photosynthesis. • ROS: act as signals that activate the genetic pathways à cell death • Sugars accumulate during senescence. May act as signals too. High sugar = less photosynthesis – Important when there’s low N availability HORMONE ROLE Positive regulators • Ethylene: senescence and abscission. Not essential to onset of senescence. Accelerates • ABA: enhancer but not triggering. ABA and water stress are coupled during leaf senescence. Stomata stay open as leaves senescence -> dehydrate and leaf falls • JA: not essential for initiation or progression. More important in flower than leaf senescence • Brassinosteroids: global regulators of leaf development rather than senescence • Salicylic acid: plays a role in the onset as well as progression HORMONE ROLE Negative regulators • Cytokinins: senescence-repressing role. Represses senescence: regulating nutrient mobilization and source-sink • Auxin: plays too many roles, and high auxin stimulates ethylene production. • GA: concentration decreases in leaves as they age. LEAF ABSCISSION • Shedding of leaves, fruits, flowers, other organs • Abscission zone: base of petiole – Morphologically and biochemically differentiated way before organ separation • Prior to abscission: separation layer forms within the zone – Dissolution of walls between cells of separation layer Protective layer Abscission layer Stem Petiole • Parenchyma cells here have very thin walls, and there are no fiber cells around the vascular tissue • The abscission layer is weakened when enzymes hydrolyze polysaccharides in cell walls • The weight of the leaf (+ wind) à separation within the abscission layer Vascular tissue Leaf Abscission layer Developing leaf scar Axilliary bud Stem Balance of ethylene and auxin controls abscission. Ethylene plays a role in activation of events leading to separation 3 phases: • Leaf maintenance: Leaves healthy and functional. Auxin flows normally • Abscission induction: Reduction auxin flow. Sensitivity to ethylene • Abscission: Cell wall degrading enzymes à cell separation Whole plant • Death of the entire plant • Different from animals – From days to > 4000 years (more for clonal) – Plants could potentially live forever • Life cycles: annual, biennial, perennial – Differences in longevity • Monocarpic vs polycarpic • Complex function of plant’s genetic program, nutrient and water availability, age and other factors Whole plant senescence • Death of plant: different from death of cells, organs – Evolutionary terms? – Is it similar to aging in animals? – Role of reproduction? – Why meristems stop dividing? – Single vs clonal plants? – Size of plant and life span? Aging • Animals: gradual deterioration • Plants: senescence is accelerated form of aging – Organs fail or become deficient. Plants that live long have mechanisms to protect themselves against deteriorative effects of time • Two types of time-based cellular damage: – Mutational load: not always correlative to death. Plants have high tolerance for genetic mosaicism: robust mechanisms for purging deleterious mutant cells – Telomere shortening: this has been shown in animals but no robust evidence for plants Unclear why plants and animals have differences with respect to aging!!! Aging Plants have indeterminate growth: apical meristem • Determinate (monocarpic) vs indeterminate (polycarpic) Monocarpic: determinate à floral apices. Whole plant dies (supression of axillary buds) – Redistribution of vital nutrients via phloem – Source-sink – Flowers: higher nutrient demand than vegetative tissues – Shift in global hormonal/nutrient balance – Increase in C:N ratio à induces vacuolar PCD Taller trees: rate of growth decreases • Over certain height: issues with vascular system delivery of sugars, water, nutrients • Resources become limiting à photosynthetic productivity is reduced • Growth efficiency declines with increasing tree size • WPS: caused by massive organ failure over a relatively short period of time, rather than by slow decline due to aging • What causes it (abiotic/biotic or internal factors)? poorly understood