Coordination and Control in Plants PDF

Coordination & Control in Plants Christharina S G [email protected] Learning Objectives By the end of this lecture, students should be able: i. To name and explain the types of plant hormones and their functions ii. To explain the comme...

Coordination & Control in Plants Christharina S G [email protected] Learning Objectives By the end of this lecture, students should be able: i. To name and explain the types of plant hormones and their functions ii. To explain the commercial application of synthetic plant hormones iii. To describe the effects of light on flowering process iv. To explain the roles of pyhtochromes and its mechanism in controlling the flowering process Plant hormones – Introduction Plant hormones/growth substances/plant growth regulator are organic chemicals Exist in very low concentrations in plant tissues Act as messengers Stimulate, inhibit or modify growth and development. Usually synthesised in a specific region of the plant such as the embryos, apical meristem of shoots and roots, young growing leaves and developing seeds.  Plant hormones vs Animal hormones PLANT HORMONES ANIMAL HORMONES No specialised glands for hormone Produced by specialised endocrine synthesis glands Hormones are transported from the Hormones are mainly transported by site of production to other parts of the bloodstream to their target the plant. cells/organs Hormones are also active at their site of production. Transport and action are generally Transport and action are relatively slow. rapid. Less specific and may affect Hormonal actions are more specific, different tissues and organs. affecting only target cells/organs Mainly involved in growth and development.  There are five major plant hormones that are recognised in modern plant biology The list includes: Auxin, Abscisic Acid, Cytokinin, Gibberellins, Ethylene These hormones are important in the growth and well being of the plant system They may act individually, may have opposing effects/antagonism (decreasing each other’s effects), or synergistic interaction (two different hormones producing greater effect than either one of the hormones in isolation) 1. AUXINS Plant hormones that influence cell division and elongation – by vacuolation and elongation Natural auxins (produced naturally by plants): Indole-3-acetic acid (IAA) https://images.nagwa.com/figures/explainers/142193981979/3.svg  Their effect depends on the target tissue E.g. Auxins produced in apical meristems result in elongation of shoots. They also induce cell division and differentiation in vascular cambium, fruit development in ovaries, and lateral root formation in roots. https://old-ib.bioninja.com.au/_Media/apical-growth_med.jpeg  Effects of auxins in plants Phototropism – plant growth toward a light source More auxin is produced on the shaded side of a shoot or coleoptile As a result, cells on the shaded side elongate faster than cells on the illuminated side https://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/898754_819540.png  Geotropism – plant growth in response to gravity  Promotes fruit growth – stimulates fruit development When IAA is applied to the stigma of a flower, it can cause fruit development without fertilisation (parthenocarphy) Without fertilisation, the ovules would not develop into seeds and thus produces seedless fruits (good market at the commercial level)  Auxins also have inhibitory effects Auxin (shoot tip) prevents the growth of lateral buds along a lengthening stem, a phenomenon called apical dominance https://o.quizlet.com/beDbW9d4CJx6YutdpG.bzw.jpg  Auxin inhibits abscission – dropping of leaves, flowers, and fruits from the plant Young leaves and fruits produce auxins In an ageing leaf or a ripened fruit, production of auxin is reduced and synthesise of abscisic acid increases This causes an abscission zone to be formed at the base of leaf petiole or fruit stalk https://resources.cdn.yaclass.in/f004b366-323b-4d5c-8756-f8f6d704ba1a/abscissionw500.png Previous studies on auxins Charles Darwin and his son Francis (1881) performed experiments on coleoptiles and concluded that some stimulus is transmitted from upper to the lower part which induced bending of the coleoptiles. Peter Boysen-Jensen (1913) demonstrated that the signal was not transfixed but mobile. Previous studies on auxins Arpad Paál (1919) demonstrated that asymmetrical placement of cut tips on coleoptiles resulted in a bending of the coleoptile away from the side onto which the tips were placed. F.W. Went (1926) successfully discovered and isolated this growth substance from Avena sativa (Oat) coleoptiles tips. From the experimental result, it can be concluded that: i. The chemical messenger is produced in the tip of the coleoptile and moves down into the growing region ii. The bending towards light is due to asymmetrical distribution of the chemical messenger (auxin) Commercial uses of auxins Common synthetic auxins: Indolebutyric acid (IBA) and naphthaleneacetic acid (NAA) Induce root formation in stem cuttings, promote seedless fruit production before pollination, keep mature fruit on trees until harvest time, widely used as herbicides against broadleaf weeds in agriculture 2. GIBBERELLINS Plant hormones that promote cell elongation and growth in stem length Synthesised in meristems of apical buds, roots, young leaves and embryos in seeds Gibberellins have a terpene structure, a group of plant chemicals related to lipids  Gibberellins stimulate growth of stems by interact with IAA to promote cell elongation and growth Sometimes substituted for red light, to promote flowering in long-day plants and inhibit it in short-day plants Effects of gibberellins in plants Promotes fruit growth and parthenocarpy https://www.researchgate.net/publication/324312270/figure/fig4/AS:960350887354394@1605976902848/Physical-appearance-of-different-stages-as-influenced-by-GA3-application.gif  Breaking seed dormancy  Inhibits root initiation Stimulates leaf growth https://assets.newatlas.com/dims4/default/5f91141/2147483647/strip/true/crop/1620x1080+0+0/resize/1200x800!/quality/90/?url=http%3A%2F%2Fnewatlas-brightspot.s3.amazonaws.com%2Farchive%2Fartificial-leaf-realworld-2.jpg Previous studies on gibberellins The discovery of gibberellins was done by E. Kurosawa in the late 1920s. He isolated chemicals from a fungus Gibberella fujikoroi that parasitizes rice plants causing them to grow tall and thin. These were found to have the same effect on the plants. The chemicals were called gibberellins. Commercial uses of gibberellins Increase fruit size Delay citrus fruit ripening 3. CYTOKININS Plant hormones that promote cell division and differentiation in the presence of auxin Produced in actively growing tissues such as embryos in seeds, fruits and roots Transported in the xylem to various parts of the plant  Effects of cytokinins in plants Promote lateral bud growth (inhibition of apical dominance) https://www.simply.science/images/content/biology/plant_form_and_function/plant_responses/conceptmap/Cytokinins.gif  Delay senescence (ageing) of plant cells It promotes maintenance of normal levels of nucleic acids, stimulates protein synthesis, delays breakdown of chlorophyll and large food molecules In one experiment, the leaves removed from a plant remained green and active when cytokinins were added Cytokinins signal to shoots that roots are healthy and active. When roots stop growing, they stop producing cytokinins, so shoot growth slows and leaves begin to deteriorate. https://images-provider.frontiersin.org/api/ipx/w=1200&f=png/https://www.frontiersin.org/files/Articles/394629/fpls-09-01039-HTML/image_m/fpls-09-01039-g001.jpg  Previous studies on cytokinins During the 1940s and 1950s scientist found that coconut water contained an active ingredient that could induce the cells to divide and differentiate in tissue culture. The active substance was named kinin. In 1956, it was discovered that a stable sample or autoclaved fresh DNA sample also contained an active substance that showed similar activity. It was called kinetin. Kinin and kinetin were later renamed cytokinin because it induced cytokinesis or cell division. In 1963, the first naturally occurring cytokinin was isolated from young maize (Zea mays) grains and was named zeatin. The structures are similar to purine and adenine, and closely related to nucleic acid (tRNA) synthesis. https://cdn.britannica.com/36/167236-050-BF90337E/Ears-corn.jpg https://upload.wikimedia.org/wikipedia/commons/thumb/b/bb/Zeatin.png/800px-Zeatin.png Commercial uses of cytokinins Tissue culture propagation Prolong shelf life of cut flowers 4. ABSCISIC ACID (ABA) Plant hormone that slows down metabolism and inhibits growth in most plant parts Works antagonistically to auxins, gibberellins and cytokinins Synthesised in leaves, stems, fruits, and seeds Mainly translocated in the phloem, and can diffuse from the root cap to the root cells  Effects of abscisic acid in plants Causes the closure of stomata – during water stress condition, it stimulates the removal of K+ ions from the guard cells https://images.nagwa.com/figures/explainers/142193981979/7.svg  Causes dormancy in some seeds E.g. The seeds of some desert plants contain high concentration of abscisic acid which inhibits germination. When there is heavy rainfall, the water washes out the abscisic acid and the seeds germinate and complete the life cycles rapidly. Seed dormancy enables the plant to adapt to adverse surrounding conditions such as dry desert conditions, drought or cold winter months. https://media.licdn.com/dms/image/v2/C4D12AQGYdriqylBkFw/article-cover_image-shrink_600_2000/article-cover_image- shrink_600_2000/0/1628764392413?e=2147483647&v=beta&t=1boAZlAyaY59lrNBBB8YPOVzeahaem4Matw9UoDiBrc  Inhibits growth in most plant parts – by slowing down metabolism High level of abscisic acid promote abscission of fruits and leaves https://thegreenthumb20.wordpress.com/wp-content/uploads/2012/11/012.jpg Previous studies on abscisic acid In 1963, an extract from young cotton fruit was shown to induce abscission and was called abscisin II. In 1964, an active substance was isolated and crystallized from sycamore leaves. It was called Dormin. Dormin was later found to be identical to abscisin II and the extract isolated from birch leaves. In 1967, it was agreed to call all these active substances abscisic acid (ABA). https://cdn.pixabay.com/photo/2016/10/07/10/01/cotton-1721144_960_720.jpg https://peiinvasives.com/wp-content/uploads/2021/07/sycamore-maple-leaf.jpg Commercial uses of abscisic acid Induces nursery stock to enter dormancy before shipment to minimize damage during handling 5. ETHYLENE Plant hormone in a form of gas A metabolic by-product in most plant organs especially ripening fruits and ageing leaves  Effects of ethylene in plants Increased level of ethylene triggers fruit ripening This triggers the release of even more ethylene (positive feedback)  Hydrolytic enzymes are released which break down cell wall components, chlorophyll and soften the fruits The enzymes also convert the starches and fruit acids to sugars The bright colours, scents and sugars produced during fruit ripening help to attract animals to eat the fruits and disperse the seeds Commercial uses of ethylene Allows shipping of green, still-hard fruit (minimizes bruises and rotting) Carbon dioxide application stops ripening of fruit in transit to market, then ethylene is applied to ripen distributed fruit quickly OTHER HORMONES Brassinosteroids First discovered in Brassica pollen Stimulate cell division and elongation Broad spectrum of physiological effects — promote apical dominance, promote leaf senescence, enhance seed germination, enhance gravitropism increase the production of ethylene, inhibit root growth, inhibit the formation of stomata, promote the formation of xylem OTHER HORMONES Oligosaccharins Oligosaccharides with hormone-like functions Released from the cell wall by enzymes secreted by pathogens Signal defence responses, such as hypersensitive response Summary of the major plant hormones The effect of light on flowering – Introduction Most organism have a biological clock, which is an internal mechanism that governs the timing of rhythmic cycles of activity Light is vital in directing plant growth and development Sunlight resets biological clocks in plants by activating and inactivating photoreceptors called phytochromes  PHYTOCHROMES Phytochromes are photoreceptors for red light responses, present in very low concentration in many plant organs e.g. in the leaves. Each phytochrome is a homodimer consisting of two identical protein molecules each covalently linked to a small nonprotein light-absorbing part called chromophore.  When the chromophore absorbs light, there is a slight change in its structure which then causes a change in the conformation of the protein molecule Phytochromes therefore exist in two photo-interconvertible isomeric forms  Phytochromes is a pale-blue, light sensitive protein that exists in two forms: Active form Absorbs red light (light with a wavelength of 660 nm) Inactive form Absorbs far-red light (light with a wavelength of 730 nm) The two forms readily convert from one form to the other after the absorption of light of specific wavelengths  Phytochrome is synthesised in its Pr form Normal sunlight contains more red than far red light, hence Pr is rapidly converted to Pfr Pfr (physiologically active form) accumulates and predominant during the day  At night or in the dark, Pfr is spontaneously and gradually converted back to Pr  Phytochrome (red light) sensing system includes: Photoperiodic induction of flowering Circadian rhythm Seed germination Leaf senescence Abscission of leaves E.g. Circadian rhythm Rhythmic leaf movements of a bean plant, a cycle of biological activity that starts anew every 24 hours. Sunlight resets biological clock in plants by activating and inactivating phytochromes. https://plantsinmotion.bio.indiana.edu/plantmotion/MovieFiles/beanleaves48hr.m4v  Activated phytochromes also control important process such as photoperiodic induction of flowering https://media.gettyimages.com/id/1334633694/video/farm-produce-strawberry-flower-blooming-gyeonggi-do-south-korea.mp4?s=mp4-640x640-gi&k=20&c=uOV_rn1m0WZSFUNtyH2Dymx0nvzzPrkmdY8UwISJIHU= Photoperiodism – response of a plant to the relative lengths of daylight and darkness  Many plants flower at specific times of the year. The differences in the time of flowering are mainly related to photoperiod and temperature. Flowering plants can be classified according to the photoperiod in which they flower: short-day plants, long-day plants, day-neutral plants Roles of phytochromes in controlling flowering The photoreceptor in photoperiodism is phytochromes The balance between the two forms of phytochrome – Pr and Pfr – controls flowering in short-day and long-day plants Pfr (active form of phytochrome) triggers or inhibits flowering  A build-up or high amount of Pfr would: Stimulates flowering in long-day plants Inhibits flowering in short-day plants Florigen, is a chemical messenger that transmits information from leaves to flower buds It is secreted in response to the relative amounts of the two forms of phytochromes (Pr and Pfr) in the leaves and then carried in the vascular system to the flower buds Short-day plants (SD) Inactive florigen Short-day plants – plants which flower when the dark period exceeds a Dark certain critical length Absence of Pfr E.g. Chrysanthemum, strawberry, soya bean Active florigen Decreased level of Pfr stimulate Florigen travels from conversion of inactive hormone precursor leaves to flower buds to florigen Flowering This induces flowering in short-day plants Long-day plants (LD) Inactive florigen Long-day plants – plants which flower when the dark period is less than a Light certain critical length Presence of Pfr E.g. Spinach, lettuce Active florigen Increased level of Pfr stimulate conversion of inactive hormone precursor Florigen travels from to florigen leaves to flower buds This induces flowering in long-day plants Flowering  Day-neutral plants – plants which flowering is unaffected by photoperiodism. E.g. tomatoes and cucumbers Phototropic control of flowering is important to plants because it helps to synchronise flowering with favourable environmental conditions When plants of similar species flower at the same time, this makes it possible for cross-pollination and cross-fertilisation to take place A horticulturist can make use of knowledge of photoperiodic control (using suitable light or dark periods) to induce flowering in SD and LD plants when it is required  Experiment 1 Short-day plants Long-day plants No flowering – dark a. Flowering When the dark period is shorter than a critical length: period is not long enough No flowering – dark Flowering – the short dark b. period is not long enough period is the critical factor, If a dark period is introduced into the light period: inducing flowering c. No flowering – dark period is Flowering When the dark period is longer than a critical length: not short enough d. Flowering – each dark period No flowering – no single If the dark period is interrupted by a light flash: long dark period is short enough to induce flowering 0 6 12 18 24 Time (hours) Light Dark Conclusion: It is the length of uninterrupted darkness at night that controls flowering, rather than the duration of exposure to light  Experiment 2 Short-day plants Long-day plants No flowering – dark period is Flowering – dark period is a. not long enough short enough When the dark period is shorter than a critical length: b. Flowering – dark period is No flowering – dark period is When the dark period is longer than a critical length: long enough not short enough c. No flowering – dark period is Flowering – red light shortens When a flash of red light interrupts the dark period: interrupted the dark period d. Flowering – effects of red No flowering – dark period is When a subsequent flash of far-red light cancels the light is cancelled not short enough effect of red light: e. No flowering – dark period is Flowering – dark period is When a red flash follows the far-red flash, the effect interrupted short enough of the far-red light is cancelled: f. If a plant is exposed to red and far-red lights alternately, the plant will respond to the last flash of light Critical dark period 0 4 8 12 16 20 24 Light Dark Red light Far-red light Time (hours) Short-day vs Long-day plants  Thank you! Best of luck for your final exam