17-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

17-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 17: Plant hormones and growth responses


Plant hormones

• The responses of plants to both internal and external influences involve changes at several levels: the molecular, cellular and organism

• Plant hormones are molecules that have the ability, even at very low concentrations, to affect plant growth and development

• Plant hormones may operate at some distance from their sites of synthesis, although they are more frequently produced in the same tissue in which they produce a response


Auxins• Auxins are growth-promoting hormones that

induce the bending of coleoptiles towards light, a process known as phototropism


Fig. 17.2: The Darwin’s experiment

Copyright © Grant Heilman Photography Inc. www.heilmanphoto.com

(a) (b)


Auxins—IAA

• The main auxin that occurs naturally in plants is indole-3-acetic acid (IAA)

• IAA promotes the growth of plant coleoptiles and stems by elongation of cells rather than by an increase in cell numbers

• IAA exhibits polar transport, unidirectional migration from the top to the bottom of stem segments

• In roots, IAA moves in two polar transport streams:– from the shoot to the root tip in cells adjacent to or within

the stele– from the root tip to the top of the root via epidermal and

cortical cells


Auxin, cell elongation and apical dominance

• The acid growth hypothesis proposes that the release of H+ into cell walls causes loosening of cell wall bonds, making the wall more flexible. This leads to cell expansion under turgor pressure

• Auxin may also affect plant growth by regulating gene expression

• IAA is also involved in the maintenance of apical dominance, by inhibiting the growth of lateral buds


Auxins and gravitropism

• Plant growth response to gravity is known as gravitropism

• Gravitropic bending of a root or shoot results from differential growth on upper and lower sides of the root or shoot

• Detection of gravity involves the sedimentation of plastids (statoliths), often amyloplasts, which contain starch granules


Fig. 17.7: Time-lapse photographs of a rhizoid of Chara

Copyright © Professor A Sievers & Dr K Schroter 1971, ‘Versuch einer Kausalanalyse der geotropischen Reaktionskette im Chara-Rhizoid’, Planta Journal, vol. 96, pp. 339–53


Auxin binding proteins

• The primary response to auxin is probably mediated by a receptor, possibly an auxin binding protein (ABP)

• ABPs are hydrophilic (water soluble) proteins associated largely with the endoplasmic reticulum, although small amounts are extracellular

• After binding auxins, the extracellular ABP then recognises and binds to a transmembrane protein, which activates a signalling pathway in the cell, leading to the appropriate response


Gibberellins• Gibberellins are plant hormones that promote growth,

seed germination and leaf expansion• They occur at low concentrations in vegetative

tissues but at higher concentrations in germinating seeds

• There are more than eighty different gibberellins—individual species produce only a few of these

• The active compound, gibberellic acid (GA1), is the endogenous active gibberellin that causes stem elongation in many plants


Gibberellins—stem elongation

• Gibberellins are involved in bolting, the rapid shoot elongation of rosette plants prior to flowering


Fig. 17.9: Gibberellins


Gibberellins—seed germination• Gibberellins also have a fundamental role in breaking

seed dormancy and stimulating germination• The endosperm of many seeds contains protein and

carbohydrate reserves upon which a developing embryo relies for energy and nutrition

• These reserves must be mobilised and transported to the embryo

• A range of hydrolytic and proteolytic enzymes break down endosperm starches and proteins into smaller, more easily transported molecules, such as sugars and amino acids

(cont.)


Gibberellins—seed germination (cont.)

• In barley, these enzymes are produced by cells in the outermost layer of endosperm, the aleurone layer


Fig. 17.10a: Major tissue types


Fig. 17.10b: Growth responses


Cytokinins• Cytokinins are hormones that stimulate cell division,

or cytokinesis• These hormones may also be involved in controlling

leaf senescence and the growth of lateral branches• The major sites of cytokinin synthesis include roots

and developing fruits• The most active, naturally-occurring cytokinin is

zeatin


Cytokinins—bud development• Direct application of cytokinin promotes the growth

of axillary buds• Exogenous cytokinin and auxin are thus

antagonistic in their effects on axillary bud growth


Fig. 17.12: The effect of a cytokinin on axillary bud growth

Copyright © Professor M Wilkins, University of Glasgow


Cytokinins—tissue culture

• Cytokinins are used commercially to induce growth and differentiation in tissue cultures. Leaf or stem tissue in which cell division has ceased is excised and placed on a medium containing sugar, vitamins, salts and various concentrations of auxin and cytokinin


Fig. 17.3: Effects of various levels of a synthetic cytokinin


Abscisic acid• In addition to growth promoters such as auxins,

gibberellins and cytokinins, plants also produce growth inhibitors such as abscisic acid (ABA)

• These inhibitors assist in the toleration or avoidance of adverse conditions, such as drought, salinity or low temperatures

• Plant responses to such conditions may involve changes in morphology (e.g. leaf drop or formation of dormant buds in deciduous trees), physiology (e.g. stomatal closure) or biochemistry (e.g. increase in frost resistance)


ABA—drought resistance• Abscisic acid is the key internal signal that facilitates

drought resistance in plants• Under water stress conditions, ABA accumulates in

leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss

• ABA causes the opening of efflux K+ channels in guard cell plasma membranes, leading to a huge loss of this ion from the cytoplasm

• The simultaneous osmotic loss of water leads to a decrease in guard cell turgor, with consequent closure of stomata


ABA—frost resistance• Elevated ABA levels are associated with increased

frost resistance• ABA appears to mediate a plant’s response to

environmental stresses, such as frost, by regulating gene expression

• Certain genes are switched on by ABA while others are switched off


ABA—seed dormancy• ABA plays a major role in seed dormancy• During seed maturation, ABA levels increase

dramatically. This inhibits germination and turns on the production of proteins that enable the embryo to survive dehydration during seed maturation

• As dormancy can only be broken by specific environmental cues, it ensures that a seed will germinate only under suitable conditions of moisture, light and temperature

• The breaking of dormancy is associated with a decline in the level of ABA

(cont.)


ABA—seed dormancy (cont.)

• Dormancy may be broken by a period of exposure to low temperature, a process known as stratification

• This is advantageous for many alpine species, which germinate under more favourable spring conditions

(cont.)


Fig. 17.15: Celmisia aseriifolia (snow daisy)


ABA—seed dormancy (cont.)• Some species are photodormant; that is, they will

only germinate when exposed to appropriate levels of red light

• This ensures that such seeds will not germinate if they are buried too deeply in the soil, covered by litter or beneath too dense a canopy

• Many arid and semi-arid taxa require heavy rains to flush ABA from their seeds

• The seedlings of such species have an increased likelihood of survival under conditions of higher soil moisture


Ethylene• Ethylene is the only gaseous plant hormone (C2H4)

• It is produced naturally by higher plants and is able to diffuse readily, via intercellular spaces, throughout the entire plant body

• Ethylene is involved primarily in plant responses to environmental stresses such as flooding and drought, and in response to infection, wounding and mechanical pressure

• It also influences a wide range of developmental processes, including shoot elongation, flowering, seed germination, fruit ripening and leaf abscission and senescence


Ethylene—signal transduction• Several transmembrane proteins have been

identified that bind to ethylene at the cell surface and function as signal transducers


Fig. 17.16: Signal transduction chain


Ethylene—fruit ripening• Under natural conditions, fruits undergo a series of

changes, including changes in colour, declines in organic acid content and increases in sugar content

• In many fruits, these metabolic processes often coincide with a period of increased respiration, the respiratory climacteric

• During the climacteric there is also a dramatic increase in ethylene production

• Ethylene can initiate the climacteric in a number of fruits and is used commercially to ripen tomatoes, avocados, melons, kiwi fruit and bananas


Ethylene—shoot growth• Applied ethylene has the capacity to influence shoot

growth• Application of ethylene to dark-grown seedlings can

cause reduced elongation of the stem, bending of the stem and swelling of the epicotyl or hypocotyl

• The combination of these responses is known as the triple response, a growth manoeuvre observed in a seedling that must circumvent an obstacle, or where seedlings are grown together in a confined space


Ethylene—flowering• The ability of ethylene to influence flowering in

pineapples has important commercial applications• Ethylene also promotes flower senescence (ageing)

in plants such as petunias, carnations and peas


Fig. 17.19: Senescence in carnations

(a) (b)


Brassinosteroids• Brassinosteroids (BRs) are plant steroid hormones

that have a similar structure to animal steroid hormones

• They have multiple developmental effects on plants, including promotion of cell elongation, cell division and xylem differentiation, and delaying of leaf abscission

• BR-deficient mutants exhibit dramatic growth defects, including dwarfism, reduced apical dominance and male fertility, as well as delayed senescence and flowering

(cont.)


Brassinosteroids (cont.)

• Brassinosteroids switch on specific genes by inactivating a protein that otherwise indirectly blocks transcription of those genes


Fig. 17.20: Signal transduction chain for the response to brassinosteroids


Photoperiodism• Diurnal cycles of light and dark provide a constant

stimulus that regulates the growth and development of many plants

• Response to the length of light and dark periods in a 17-hour cycle, photoperiodism, allows plants to reproduce synchronously in the appropriate season

(cont.)


Photoperiodism (cont.)• There are two types of responses to photoperiod

1. Short-day plants flower when the photoperiod is less than the critical day length (usually between 12–14 hours), and thus are typically autumn-flowering plants

2. Long-day plants flower when the photoperiod exceeds a critical day length and typically include many spring and early summer flowering plants of temperate origin

• Plants that do not show a photoperiod response for flower initiation are day-neutral plants

• The length of the dark period, rather than the length of the light period, determines when flowering will occur

(cont.)


Photoperiodism (cont.)• In some species that have a wide geographic range,

different ecotypes have evolved that are suited to local environmental conditions

• Leaves detect changes in photoperiod. Phytochrome pigments, which enable plants to detect light and darkness, interact with an internal clock mechanism to measure the length of the dark period


Vernalisation• The induction of flowering in plants exposed to low

temperature is known as vernalisation• Vernalisation-inducing temperatures range from

–1C to 9C, and are usually required for at least 4 weeks, although vernalisation can be reversed by short periods of high temperatures


Monocarpic senescence• The death of an entire annual plant, once flowering

and fruiting are complete, is termed monocarpic senescence

• Seed development appears crucial for the onset of senescence, since it is delayed by fruit removal

17-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

Documents

growth of plant coleoptiles

differential growth

cell elongation

growth of lateral budsauxins

cell walls

cell numbersiaa

cell expansion

root tip