HORMONES IN DEVELOPING SEEDS : COMPOSITION, LOCATION AND ROLES OF

SEED HORMONES

HORMONES

• Hormone- gr. to excite. Organic substances produced natuirally in small amounts that regulate and coordinate metabolism, growth and morphogenesis.

• Plant Hormones: Plant hormones (also known as phytohormones) are chemicals that regulate plant growth

Leaf primordia, young leaves, developing seeds.

Polarly (unidirectionally) and nonpolarly.

Young tissues of the shoot and developing seeds.

Xylem and phloem.

Root tips. From roots to shoots via xylem.

Most tissues in response to stress.

Diffusion from site of synthesis.

Mature leaves and roots, seeds. From leaves in phloem and from

the roots in the xylem.

PLANT HORMONES

AUXINSIn 1881, CHARLES DARWIN and his son Francis performed experiments on coleoptiles, the sheaths enclosing young leaves in germinating grass seedlings.

First of the major hormones to be discovered.

The four naturally occurring (endogenous) auxins are IAA, CIAA, Phenyl acetic acid and IBA; only these four were found to be synthesized by plants.

Major Auxins is IAA.

On the molecular level, all auxins are compounds with an aromatic ring and a carboxylic acid group.

Precursor for biosynthesis in seed tissues is TRYPTOPHAN not derived from the parent plant.

2,4-D

NAA

Dicamba

Picloram

2,4,5- Trichloro phenoxy acetic

acid

4-CI-IAA

PAA

IBA

IAA

Found in free form and bound form.IAA arabinosideIAA myo-inositolIAA myo-inositol arabinoside

Bound forms acts as precursors.Both type of Auxins can be extracted from endosperm (Maize & other cereals) and embryo (Peas and Dicots)Free IAA content in Pea is typical that in developing seeds, as its concentration rises to peak and then diminishes with relatively low amount at maturity.Variations occur as in Apple 2 peaks are observed. First peak during change in endosperm from coenocytic to cellular structure and second with formation of new endosperm cells.Final decline is due to metabolic conversion to bound forms and other products.

ABA moves UP and DOWNAuxins only downCytokinin only UPGA both directions

Auxin Provides Chemical Signals That Communicate Information Over Long Distances

Apical Dominance

Differentiation and Regeneration of Vascular

Tissue

Fruit Development

Formation of Lateral and Adventitious Roots

Role of Auxins

Prevents Abscission Herbicides Phototropism

Geotropism

Auxin produced by seeds promotes ovary tissue growth

Gibberellins (GAs) are plant hormones that regulate growth and influence various developmental processes.

Gibberellin was first recognized in 1926 by Japanese scientist, Eiichi Kurosova, studying bakane, the "foolish seedling" disease in rice. First isolated in 1935 by Teijiro Yabuta and Sumuki, from fungal strains (Gibberella fujikuroi) provided by Kurosawa. Yabuta named the isolate as “GIBBERELLIN”.

The bioactive GAs are GA1, GA3, GA4, and GA7. They are produced in stem and root apical meristems, seed

embryos, and young leaves Most bioactive GAs are located in actively growing organs on plants. GAs are usually produced from the methylerythritol phosphate

(MEP) pathway in higher plants.

GIBBERLLINS

GA1GA1 GA3GA3

• Many GA conjugates have also been identified, such as the polar, water soluble Glucopyranosides and Glucopyranosyl esterase.

• GA synthesis in cell-free systems from immature seed of Cucurbita maxima and P.sativum has following pathway

Interconversion of GAs also observed in developing seeds or in cell free embryo extracts.

2) GA12 GA15 GA24 GA9 GA51 GA51catabolite

They are produced in stem and root apical meristems, seed embryos, and young leaves.

At early stages of seed development, the major GAs are active and inactive ones are produced at the end of seed maturity due to formation of various conjugates and GA catabolites.

Peak activity of endogenous GA found at 18-22 days due to combined effect of GA9 and GA20.

GA distributed unequally in seed parts such as in testa, cotyledons and axis in different concentrations (Pea). In maize GA1 content in embryo is 40 times higher than in endosperm.

Gibberellin Plays Multiple Roles in

1) Breaking Seed Dormancy

2) In Germination

3) Gibberellin (GA) causes dwarf mutants to grow tall.

4) Gibberellin causes hyper-elongation of shoots by stimulating cell division and elongation.

5) Gibberellin Can Cause Bolting

6) Gibberellin Affects Fruit Development

CYTOKININS• Cytokinins (CK) are a class of plant growth substances

that promote cell division, or cytokinesis, in plant roots and shoots.

• Kinins are widely distributed in plants, especially in seed. Kinins isolated from immature maize seed.

• They are produced in the roots and transported throughout the plant via the xylem.

• They are involved primarily in cell growth and differentiation, but also affect apical dominance, auxiliary bud growth, and leaf senescence.

• There are two types of cytokinins: 1. Adenine-type represented by kinetin, zeatin, and BAM. 2. Phenylurea-type like Diphenylurea and Thidiazuron

(TDZ).

Derivatives of Cytokinins can be present in glycosylated forms containing ribose, Glucose or both sugars.

Amount of cytokinins increases during seed development, particularly while seed tissues are growing and then declines at maturity.

Cytokinins regulates a range of plant activities including seed germination. They are active in all stages of germination.

Also affect the activities of meristemic cells in roots and shoots, as well as leaf senescence. In addition, they are effective in nodule formation

Adenosine phosphate-isopentenyl transferase (IPT) catalyses the first reaction in the biosynthesis of isoprene Cytokinins. It may use ATP, ADP, or AMP as substrates and may use dimethyl allyl diphosphate (DMAPP) or hydroxy methyl butenyl diphosphate (HMBDP) as prenyl donors.

Cytokinins can also be produced by recycled tRNAs in plants and bacteria.

Auxin is known to regulate the biosynthesis of Cytokinins.

Recent years kinins have attracted the attention since they participate in the biosynthesis of proteins, chlorophyll and other vital important compounds.

Role of CytokininsPromote Cell Division Delay Leaf Senescence

Promote the Growth of Lateral Buds

Cytokinins are also able to enhance seed germination by the alleviation of stresses such as salinity, drought, heavy metals and oxidative stress. They can be inactivated by the enzyme cytokinin oxidase/ dehydrogenase catalyzing the cleavage of their unsaturated bond. (Galuszka et al.,2001 )

Cytokinins are also able to enhance seed germination by the alleviation of stresses such as salinity, drought, heavy metals and oxidative stress. They can be inactivated by the enzyme cytokinin oxidase/ dehydrogenase catalyzing the cleavage of their unsaturated bond. (Galuszka et al.,2001 )

Somatic embryos formation

Cytokinins encourage the growth of lateral shoots.

Cytokinins encourage the growth of lateral shoots.

ABSCISIC ACID (ABA)Abscisic acid (ABA), also known as abscisin II and dormin. ABA functions in many plant developmental processes, including bud dormancy

Abscisic acid is a growth-inhibiting hormone largely responsible for seed dormancy.

ABA can be isolated from immature seeds of many species.

Free from of inhibitor can occur at relatively high concentrations especially in legumes.

Bound forms like Glucosyl ester and glucoside are also common.

Both free and bound forms can be located in various parts of seed like embryo, endosperm and the enclosing tissues.

ABA is an isoprenoid, which is synthesized in the plastidal 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway;

ABA rises in its concentration during seed development, reaches one or two peaks and generally then declines rapidly at the time of drying.

Location and timing of ABA biosynthesis

1. Released during desiccation of the vegetative tissues and when roots encounter soil compaction.

2. Synthesized in green fruits at the beginning of the winter period.

3. Synthesized in maturing seeds, establishing dormancy

4. Mobile within the leaf and can be rapidly translocated from the roots to the leaves by the transpiration stream in the xylem.

5. Produced in response to environmental stress, such as heat stress, water stress and salt stress.

6. Synthesized in all plant parts, e.g., roots, flowers, leaves and stems

Role of ABA1) It inhibits precocious germination and viviparity.2) Adversely affects the process of seed germination. For ex: Conc. of 1–10 µM can inhibit seed germination in

plants like Arabidopsis thaliana. However, other hormones including GA, ethylene, cytokinins, and brassinosteroids, as well as their negative interaction with ABA, can positively regulate the process of germination. (Kucera et al., 2005; Muller et al.,2006)

3) Promotes seed storage reserve accumulation and descication tolerance.

4) Promotes maturation and dormancy.5) When seed has highest ABA level, seed acumulates storage

compounds that will support seedling growth and germination subsequently.

6) Synthesis of storage proteins, lipids and LEA proteins.

Role as a Root-to-Shoot Signal Viviparity Stomatal Closure

7) Antitranspirant - Induces stomata closure, decreasing transpiration to prevent water loss.

8) Down regulates enzymes needed for photosynthesis.9) Acts on endodermis to prevent growth of roots when

exposed to salty conditions

ETHYLENEEthylene is also an important natural plant hormone, used in agriculture to force the ripening of fruits.

It is a hydrocarbon with the formula C2H4 or H2C=CH2.

It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves.

Ethylene biosynthesis in plants

Ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seeds; regulated by various developmental and environmental factors.

Its production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers.

Its production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators.

Ethylene is biosynthesized from the amino acid methionine to S-adenosyl-L-methionine (SAM, also called Adomet) by the enzyme Met Adenosyl transferase.

SAM is then converted to Alpha amino cyclopropene-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS).

The final step involves the action of the enzyme ACC-oxidase (ACO), formerly known as the ethylene forming enzyme (EFE).

Induced by endogenous or exogenous.

ACC synthesis increases with high levels of auxins, especially IAA and cytokinins.

1. Seed Germination : The amount of ethylene increases during the germination of many plant seeds including wheat, corn, soybean and rice, affecting the rate of seed germination (Pennazio and Roggero, 1991; Zapata et al., 2004). ACC can enhance seed radicle emergence through the production of ethylene, produced in the radicle.

2. Inhibits the adverse effects of ABA and releases seed dormancy.

3. Breaks bud dormancy in Potato tubers.

4. Thickening and shortening of hypocotyls with pronounced apical hook.

5. Induces root hair growth — increasing the efficiency of water and mineral absorption.

6. Sex expression in monoecious species (ratio of to )♀ ♂7. Thigmomorphogenesis (reduced stem elongation in some

environments)

Role of Ethylene

Fruit Ripening Thigmomorphism

POSSIBLE ROLE OF SEED HORMONES

Endogenous GR play important roles in the regulation of certain aspects of seed

1.Seed growth and Development

2.Germination and Growth

3.Fruit growth and development

4.Other effects of seed hormones.

Seed growth and DevelopmentMost of the studies shown that GR with seed development comes from correlations between regulator content and embryo growth.In case of dwarf pea seeds highest concentration of GA (GA9 & GA20) occur during maximum growth of developing embryo.GA deficient mutants of Pea Mutants show seed abortion where as no effect observed in mutants of Arabidopsis and tomato.During cell division and enlargement highest concentration of Cytokinins are observed.CK in liquid endosperm participate in the mobilization of assimilates.ABA associated with arrest of embryo growth. In case of Barley and rapeseed normal embryo development can occur but germination and growth cannot.

ABA content higher in young, non-germinable seeds than older ones. (Several cereals & Legumes)Suppression of pre-mature germination.

Ex: Most viviparous mutants of Maize have 20-25% of ABA conc. Than normal wild non-viviparous type.Relative insensitivity to ABA found in Mangroves where vivipary is common occurance.Involved in regulation of storage protein synthesis. (Soybean & P.vulgaris)Involved in accumulation of sugars enhancement in Grapes and grain filling in Wheat.Arabidopsis mutants shown reduced concentration of certain storage proteins and TAG. But this is not found in Tomato mutants.

Auxin by itself is not a necessary hormone for seed germination. However, according to the analyses regarding the expression of auxin related genes, auxin is present in the seed radicle tip during and after seed germination.The growth and development of different plant parts, including the embryo, leaf and root is believed to be controlled by auxin transport. (Popko, J. Et al.,2010)ABA/GA ratio affects embryo dormancy. Dormancy occur when ABA sensitivity is higher than GA sensitivity.Seed germination occurs at low ABA/GA ratio.ABA control embryo dormancy and GA controls embryo germination.

Germination and Growth

ABA affects testa or seed coat growth characteristics such as thickness and affects the GA-mediated embryo growth potential.

Endosperm is composed of living tissue that actively responds to hormones generated by embryo. Endosperm often acts as barrier for seed germination.

Living cells respond to and also affect ABA/GA ratio and mediate cellular sensitivity; GA thus increases the embryo growth potential and weakens endosperm.

GA and Cytokinins broke seed dormancy of certain light sensitive seeds such as lettuce and tobacco.

GA involved in α-Amylase synthesis in aleuronic layer.

GA stimulates germination in pine, grape, mustard, cabbage etc.,

Cytokinins affect the activities of meristematic cells in roots and shoots.

Cytokinins are also able to enhance seed germination by the alleviation of stresses such as salinity, drought, heavy metals and oxidative stress. (Peleg, Z., Blumwald, E. 2011)

Inhibitory effect of far red light treatment on germination is overcome by Kinetin treatment.

Parasitic weed Striga germination can be induced by treating with Cytokinins even in absence of host.

Fruit set and pod elongation in pea are normally dependent on the presence of seeds. Developing seeds produce signal molecules that regulate cell division and expansion of surrounding fruit tissues.

Interaction between GA and Auxin present in seeds and pericarps of pea are responsible for cell division and expansion in developing fruits of pea.

Growth of fresh fruits is linked to the activity of the developing seed. Eg: Fruit size in cucurbits positively correlated to seed number.

Auxin in developing seeds promotes flesh growth in strawberry. In deseeded strawberry auxin application restores flesh growth.

Fruit Growth and Development

Treatment of pea seedlings with NAA or GA prevents formation of abscission layer.

Abscission of fruits and leaves promoted by ABA from developing seeds.

Apple seeds rich in GA and can cause flower bud suppression.

Other Effects

• The high level of auxin present during all the seed development phases suggests that this hormone has a key role throughout the entire program of seed formation.

• The pattern of CK accumulation is the opposite with respect to auxin. CKs have a prominent role during the phase that involves cell divisions, decreasing progressively during the maturation phase, when cell expansion prevails.

• The BR follow the same pattern of CKs. The highest concentration of BRs is found at the beginning of seed development, and is detected in the maternally derived tissues (i.e., integuments). Their levels decrease at the end of maturation.

• The pattern of accumulation of GA is characteristic, showing two peaks corresponding to specific phases of seed development: the stage of embryo differentiation, when the GAs promote cell growth and expansion, and the end of the maturation phase.

• ABA shows an accumulation pattern complementary to the GAs, being the main hormone that inhibits all the processes induced by GAs.

Additional Chemical Signals

• Brassinosteroids- required for normal growth of most plant tissue. BRs are plant steroid hormones involved in several developmental programs, including seed development.

• They function in the pathway that regulates ovule number and seed size and shape, in some cases complementing CKs and auxins.

• They also participate in the regulation of seed germination, by antagonizing the inhibitor effect of ABA (Zhang et al., 2009), and being synergic to gibberellins (Leubner-Metzger, 2001).

• Salicylic acid- signal in defense responses to plant pathogens.

• Jasmonates- plant growth regulation and defense.

• Polyamines- growth and development; mitosis and meiosis.

• Systemin- long-distance signal that activates chemical defenses against herbivores.

• Nitric oxide- signal in hormonal and defense responses.

11/26/14 48Dept. of GPB

REFERENCES1. Plant hormones and seed germination

Mohammad Miransari, D.L. Smith “Environmental and Experimental Botany” 99 (2014) 110–121.

2. Seeds : Physiology of Development and Germination. J.Derek Bewely and Michael Black. Second edition, Plenum Publishers.

3. http://en.wikipedia.org/wiki/Plant_hormone.

4. Current perspectives on the hormonal control of seed development in Arabidopsis and maize: a focus on auxin. Antonella Locascio, Irma Roig-Villanova, Jamila Bernardi and Serena Varotto. Frontiers in Plant Science Plant Evolution and Development August 2014, Volume 5, Article 412.