PLANT PHYSIOLOGY
Ethylene, abscisic acid and
brassinosteroids
Overview
1. Ethylene: the gaseous hormone
2. Effects of ethylene on plant growth and development
3. Abscisic acid (ABA): a seed maturation and stress-response hormone
4. Developmental and physiological effects of
ABA
1. Ethylene: the gaseous hormone
1.1. Ethylene can be produced by almost all parts of higher plants
1.2. Its production increases during fruit ripening, leaf abscision, and flower senescence
1.3. The amino acid methionine is the precursor of ethylene
1.4. Ethylene biosynthesis is promoted by several factors 1.5. The primary steps in ethylene action are likely similar:
binding to a receptor, followed by activation of signal transduction pathways
Ethylene response of etiolated pea seedlings
(left – untreated, right – treated with 10 ppm ethylene)
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 651.
Ethylene biosynthetic pathway
Model for ethylene receptor action: ethylene binding inactivates the receptor, allowing the response to occur
2. Effects of ethylene on plant growth and development
2.1. Ethylene affects the transcription of numerous genes via specific transcription factors
2.2. The hormone promotes the ripening of some fruits 2.3. Ethylene inhibits hypocotyl elongation
2.4. It regulates flowering, sex determination, and defence responses in some species
2.5. Ethylene is active in leaf and flower senescence and in leaf abscision
Ethylene production and respiration during banana ripening
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 657.
A single mutant seedling has constitutively activated ethylene response:
its hypocotyl elongation is inhibited among wild-type seedlings
Inhibition of flower senescence by inhibition of ethylene action
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 666.
Schematic view of the roles of auxin and ethylene during leaf abscision
3. Abscisic acid (ABA): a seed maturation and stress-response hormone
3.1. ABA has been detected in every major organ or living plant tissue
3.2. It is synthesized from a carotenoid intermediate in almost all cells that contain chloroplasts or amyloplasts 3.3. ABA short-term responses frequently involve
alterations in the fluxes of ion across membranes
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 674.
Structures of active and inactive ABAs
Simplified diagram of ABA biosynthesis via the terpenoid pathway
4. Developmental and physiological effects of ABA
4.1. In seed development, ABA promotes the synthesis of storage proteins and lipids, as well as special proteins
4.2. Seed dormancy and germination are controlled by the ratio of ABA to gibberellic acid (GA)
4.3. In germinating seeds, ABA inhibits the GA induced synthesis of hydrolitic enzymes
4.4. ABA promotes root growth and inhibits shoot growth at low water potentials
Germinating of ABA-deficient seeds in the fruit while still attached to the plant
4. Developmental and physiological effects of ABA
4.5. ABA greatly accelerates the senescence of leaves, thereby increasing ethylene formation and stimulating abscision
4.6. ABA accumulates in dormant buds, inhibiting their growth; it may interact with growth-promoting hormones 4.7. Abscisic acid closes stomata in response to water stress
Changes in water potential, stomatal resistance, and ABA content in corn in response to water stress
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 678.
Redistribution of ABA in the leaf resulting from alkalinization of the xylem sap during water stress
Simplified model for ABA signaling in stomatal guard cells
5. Brassinosteroids (BRs): regulators of cell expansion and development
5.1. BRs cause dramatic changes in growth and differentiation at very low concentration
5.2. BRs have been detected in all tissues examined with greatest activity in the apical shoot
5.3. Brassinosteroids are synthesized from campesterol 5.4. BRs act near their sites of synthesis and do not
undergo long-distance transport
Simplified pathways for brassinolide (BL) biosynthesis and catabolism
5. Brassinosteroids (BRs): regulators of cell expansion and development
5.5. BRs promote both cell proliferation and cell elongation
5.6. BRs promote root growth at low concentrations and inhibit root growth at high concentration
5.7. BRs promote differentiation of the xylem and supress that of the phloem
5.8. BRs promote seed germination by interacting with ther hormones, such as GA and ABA
Bean second-internode bioassay for brassinosteroids
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 711.
The kinetics of BR stimulation of soybean epicotyl elongation
BR is required for a normal vascular development in stem: the wild type plant has higher xylem-to-phloem ratio then the BR-deficient mutant
Source: Taiz L., Zeiger E. (2010): Plant Physiology. p. 714.
BR stimulates germination of Arabidopsis seeds
