Growth and development

Early experiments on bending responses to light (phototropism) led to discovery of hormones Experiments into tropisms (bending movements in plants in response to a stimulus) were carried out on oat coleoptiles (the sheath around a growing shoot of a germinating oat seedling that protects it from damage). Charles Darwin in the 19th century was the first to carry out such experiments. He showed that normal coleoptiles bent towards the light if it was shone from one direction only (unidirectional). However, if he chopped the tip off the coleoptile, it no longer bent. This suggested that the stimulus was received by the tip. Not only that, but covering the tip with an opaque cap also prevented the bending response, whereas using a transparent cap, or using an opaque collar did not prevent the coleoptile from bending. The stimulus was being perceived by the tip. Further experiments were carried out by Boysen-Jensen. He inserted either a gelatin block or a mica sheet into the coleoptile. The mica sheet prevented bending whereas the gelatin block did not. This was the first evidence that a transmissible chemical was involved in the bending. This work was further developed by Fritz Went in 1926. By collecting the diffusate from the tips into gelatin blocks, and applying them unevenly to the decapitated coleoptile, he showed that there was a quantitative response, and that the substance seemed to inhibit growth since coleoptiles bent away from the site of application. He termed the substance Auxin from the greek Auxein, to increase. Gelatin Block Control Opaque cap Opaque collar Mica sheet Tip removed Transparent cap

Tropisms - long term bending responses involving plant growth Phototropism - bending towards light Heliotropism - bending towards the sun Hydrotropism - bending towards water Gravitropism - bending under the influence of gravity. Thigmotropism - bending as a response to touch

Nastic movements Short lived responses involving turgor pressure of cells examples include leaf movements following the sun (e.g. Bean) and sensitivity to touch e.g. Mimosa

Plant ‘hormones’ Plant need to coordinate their growth and development and tune their development to that of their environment. In order to do this, they produce chemicals which are effective at low concentration which are moved around the plant. This definition is characteristic of animal hormones. A better phrase is Plant Growth Regulator. Since the days of these early pioneers, 5 major plant hormones have been extracted and characterised from plants.

Auxin Discovered as a result of work on tropisms One main compound, Indole -3- acetic acid (IAA) Produced in growing points (shoot and root apical meristems and young leaves) Involved in Cell division (with cytokinin) Stem elongation (with Gibberellins) Differentiation of xylem and phloem Branching (with cytokinins) Fruit development Tropisms

Cytokinin Discovered by trial and error as a component of degraded DNA. One compound - Zeatin Produced in roots Involved in: Cell division (with auxin) Apical dominance (with auxin)

Gibberellins Discovered because of ‘foolish seedling’ disease of rice (long, spindly growth) - caused by a fungus, Gibberella fujikori that secretes Gibberellins Many gibberellins known (>80) Involved in: Stem elongation (with auxins) Fruit growth (with auxins) Germination Produced in growing points (shoot and root apical meristems and young leaves)

Abscisic Acid ( ABA) Discovered because of work on leaf abscission and dormancy One compound (ABA) Produced all over the plant, especially in green tissues. Involved in: Leaf and fruit abscission Seed dormancy Embryo development plant responses to water stress

Ethylene Discovered because of work on ripening of fruits (Citrus and pineapple) One compound, the gas ethylene. Produced in all tissues, especially in response to stress. Involved in: Senescence abscission Fruit ripening

Agricultural uses of plant hormones A knowledge of the interactions of plant hormones in plant development has resulted in many agricultural applications of plant hormones, either by their direct application, or by inhibition of their action.

Agricultural uses of auxins Hormone rooting powders Prevention of fruit drop Herbicides e.g. 2,4-D Development of seedless fruits Use in plant tissue culture for cell division, somatic embryogenesis and rooting

Agricultural uses of cytokinins Use as an ‘anti-ageing’ spray to retard senescence of cut flowers Use in plant tissue culture for cell division, increasing branching, production of somatic embryos, and for adventitious shoots (2 routes to get a whole plant back from a single cell)

Agricultural applications of Gibberellins Dwarf plants have increased agricultural productivity Dwarf apple by treatment with inhibitors Enhanced stem elongation e.g. sugar cane Production of seedless grapes. Used to break dormancy of seeds

Agricultural uses of ABA ‘Stress hormone’ causing stomata to close under water stress Used in plant tissue culture to ensure that somatic embryos develop normally.

Agricultural uses of Ethylene Ethylene inhibitors (e.g silver ions) used in the cut flower trade to increase shelf life. Promotes ripening of fruits (bananas are picked green and ripen on board ship controlled by ethylene. Controlled ripening of tomatoes by use of anti-sense to ethylene Abscission of fruits to synchronise harvesting.

Circadian Rhythms Rhythms with a cycle of about a day (exact rhythm will vary, but resynchronised every day) Used to synchronise the plant responses to the day/night cycle and the seasons, especially flowering.

Flowering A critical daylength is often needed to induce flowering. 3 responses: Short day. - plants will only flower when the day length is less than a certain length. Long day - plants will only flower when the day length is more than a certain length. Day neutral. Flower all year round.

Photoperiodism ‘Short day’ plants ‘Long day’ plants Night 24 h Day There is a critical length of night (dashed line) which the plants must experience in order for the plants to flower. ‘Short day’ plants flower when the night period is longer than a certain length. ‘Long day’ plants flower when the night period is shorter than a certain length. This is mediated by the light receptor phytochrome.

Phytochrome Red Pr Pfr Far red Slow conversion in darkness Phytochrome exists in 2 forms in the plant, Pr and Pfr. The Pr form absorbs red light (plentiful in daylight) and is converted to the Pfr form. The Pfr form absorbs far red light (a larger component of the light spectrum at dawn and dusk). There is also a slow chemical conversion in the dark from Pfr to Pr. By these bioconversions the plant is able to sense the relative length of day and night. Slow conversion in darkness