BRASSINOSTEROIDS �

Dr.Ramandeep Kaur

Asstt. Prof. Botany

WHAT ARE BRASSINOSTEROIDS�

Brassinosteroids (BRs) ,represent a new sixth class of plant hormones with wide occurrence in the plant kingdom in addition to auxins, gibberellins, cytokinins, abscisic acid and ethylene. They have unique biological effects on plant growth and development. Brassinosteroids are biosynthesized inside the plant.

HISTORY AND DISCOVERY

• Brassinosteroids (BRs), a class of plant-specific steroid hormones characterized by the presence of polyhydroxylated sterol structure, were first isolated and purified from Brassica napus pollen in 1979.
• The first active compound isolated from this plant ,was named as Brassinolide.

DISTRIBUTION

• Ubiquitous in nature:
• Found in dicots(16 families), monocots(5 families), gymnosperms(3 families), a fern, and an alga.
• Since the discovery-There are 59 BRs, among them 54 unconjugated and 5 conjugated BRs, have been isolated from 58 plant species including 49 angiosperms (12 monocotyledons and 37 dicotyledons) ,6 gymnosperms, 1 pteridophyte (Equisetum arvense), 1 bryophyte (Marchantia polymorpha) and 1 chlorophyte, the alga (Hydrodictyon reticulatum).

OCCURANCE OF BRASSINOSTEROIDS IN PLANTS�

Brassinosteroids mainly occurs in plants in pollens of some plants, seeds, leaves, roots, flowers, and also in young vegetative tissues.

STRUCTURE

Brassinosteroids contain the typical steroid nucleus, with fused rings and side chains. .Both the nucleus and the side chain contain various substituent in different isomeric configurations.

The compounds can be classified as C27, C28, or C29 BRs, depending on the alkyl-substitution pattern of the side chain.

• Two most common BR’s in higher plants are:-

1. BRASSINOLIDE

2. CASTASTERONE

​

​

OTHER BRASSINOSTEROIDS�

• Brassinolide,

24-epibrassinolide

and 28 homobrassinolide

are the three biologically

active brassinosteroids,

being widely used in

physiological studies.

​

SYNTHESIS OF BRASSINOSTEROIDS�

• A major BR biosynthetic pathway has been established in arabidopsis.
• Steps regarding synthesis of brassinosteroids.

• Reduction
• Oxidation
• Hydroxylation
• epimerization

​

• Brassinolide is synthesized from campesterol via two pathways.

(Campesterol is derived essentially from the mevalonate pathway).

​

BIOSYNTHESIS�

• Initially, the double bond in the B ring of campesterol is reduced to campestanol over 3-hydro-D4,5-campesterol and 3-dehydro-campestanol.
• From campestanol, two possible routes lead to Brassinolide,the most biologically active among more than 70 natural BR’s.
• Two pathway called as:-

Late c6-oxidation pathway

Early c6-oxidation pathway

​

METHODS OF BIOASSAY

• The Rice lamina inclination test--

From etiolated seedlings of rice, segments consisting of the second leaf lamina, lamina joint and sheath were excised and floated on distilled water containing brassinosteroids. Bending of the lamina joint was observed, which was proportional to the concentration of the compounds applied. Brassinolide showed activity at a concentration of 0.005 mg/ml.

​

​

​

​

​

• The bean second internode test-

Cuttings of the second internode from seedlings of Phaseolus vulgaris ,when treated with brassinolide in lanolin paste, showed elongation, curvature and swelling at lower concentration of 0.01 mg and splitting at a higher concentration of 0.1 mg of brassinolide.

INHIBTORS INVOLVED IN BIOSYNTHESIS��

• Uniconazole-it inhibits BL biosynthesis, although it is not known whether it specifically inhibits the 22α-and/or 23α-hydroxylation catalyzed by DWF4 and CPD resp.
• Paclobutrazol- has been reported to inhibit the growth of barley seedlings and inhibition is thought to be due to a lack of sterol synthesis because it is relieved by the application of stigmasterol.
• Brassinazole-when applied at micro molar concentrations, generates phenotypes mimicking those of BR-deficient dwarf mutants. Brassinazole have been shown to inhibit cytochrome P450s.

BRASSINOSTEROIDS & mutants

• Several screening and testing have led to the identification of several mutants that are blocked in the synthesis of campesterol.
• Mutants can be artificially made or naturally made.
• All mutants are dwarfs, which are restored

to wild type when supplied with exogenous

BRs-this leads to wider acceptance

as growth hormones.

Importance-

• For studying pathways.
• Studying the different genes regulating

the pathway and their regulation.

• For studying BRs effects on plant growth

and development.

CHARACTERS OF MUTANTS

• Mostly they are dark-green dwarfs with reduced internode, petiole, and leaf elongation, giving them a cabbage-like appearance. The mutants have reduced apical dominance, inflorescence stems are reduced in length, and flowers are small with reduced fertility. Roots are also shorter than those of wild-type plants.
• For e.g.;-In Arabidopsis plant mutants.
• If BR’s present then wild plants are produced., and if BR’s absent then dwarf varieties are produced.

​

ROLE OF BRASSINOSTEROIDS

Role in plant development-

• Promote seed germination
• Promote cell expansion
• Promote cell elongation

Role in organ development-

• Promotes vascular differentiation
• Promotes xylem development

Responses to various biotic and abiotic stresses

including drought, extreme temperatures, heavy metals, herbicidal injury and salinity.

ROLE IN SEED GERMINATION

• BR is a positive regulator of germination.
• BR blocks the activity of ABA which is a negative regulator of seed germination.

ROLE IN CELL ELONGATION

• Genetic and molecular studies revealed that the BR-promoted cell elongation largely depends on the expression of xyloglucan endotransglycosylases (XETs), which function is to incorporate new xyloglucan into the growing cell wall.
• BRU1(BR-upregulated) in soybean and TCH4 in Arabidopsis, both encoding proteins, are highly and rapidly induced by BR treatment during early stages of elongation .

​

ROLE IN SHOOT DEVELOPMENT

• Reported in no. of plants like Arabidopsis, bean, azuki bean, pea, rice and tomato.

How?- by promoting expressions of genes responsible for cell elongation and wall stability.

e.g.:-

​

ROLE IN LEAF DEVELOPMENT

• Firstly found when it was observed that the cell number per leaf blade in det2 and dwarf1 is lower than in wild type.
• It was observed that stomata no. was increased in the BR deficient and BR-perceptional mutants, such as bul-1, cpd, and bin2-1 with unknown mechanism. This indicates that BRs have inhibitory effect on the stomata formation.
• In monocots, such as rice, BRs have a specific role in regulating lamina joint inclination .

ROLE IN FLOWERING

• BR-deficient and BR-insensitive mutants exhibit late flowering phenotype.
• Late flowering in bri1 mutant was studied and it was due to enhanced expression of FLC i.e. a floral repressor.
• In grape fruits, spraying of brassinosteroids in autumn increased the number of flowers, while such application in late winter reduced flower production.
• Application of brassinolide to the staminate inflorescence of Luffa cylindrica induced bisexual and pistillate flowers.

​

​

ROLE IN SENESCENCE

• Delayed senescence in BR-deficient and BR-insensitive mutants in Arabidopsis and early senescence in the bes1-D mutant had been observed.
• HOW?-BRs regulate senescence via ‘activated oxygen’, because the activities of peroxidase, superoxide dismutase and catalase were altered. And malondialdehyde was significantly increased by BR treatment.

ROLE IN STRESS RESPONSE

• Brassinosteroid-treated tomato and rice plants grew better than control plants under low-temperature conditions. –it occurs because BRs promote plant growth and maintains chlorophyll contents.
• Brassinosteroids improved the tolerance of maize and cucumber seedlings against low-temperature stress.
• Brassinosteroids were found to be involved in increasing resistance to chilling in brome grass and rice. In rice, 24-epibrassinolide increased the resistance against chilling stress (1–5°C) -the tolerance was associated with increased ATP, proline levels and SOD activity, thus Indicating brassinosteroid involvement in membrane stability and osmoregulation.

​

​

EFFECT ON PLANT RESPONSE

ROLE IN AGRICULTURE

• Foliar spray of brassinolide substantially improved the yields of wheat and mustard, rice, corn and tobacco.
• Brassinosteroids were also found to increase the growth and yield of sugar beet, legumes and rape seed.
• Application of 24-epibrassinolide increased yields of corn, tobacco, watermelon and cucumber.
• Exogenous application of brassinosteroids considerably improved the growth and economic yield of radish.
• ‘The role of brassinosteroids in protecting the plants against environmental stresses will be an important research theme and may contribute greatly to the usage of brassinosteroids in agricultural production’.

​

SOME OTHER FUNCTIONS

• Application of Brassinosteroids to cucumbers was demonstrated to increase the metabolism and removal of pesticides, which could be beneficial for reducing the human ingestion of residual pesticides from non organically grown vegetables.
• 24-Epibrassinolide(EBL), a brassinosteroid isolated from Aegle marmelos correa(Rutaceae),was further evaluated for the antigenotoxicity against maleic hydrazide(MH)-induced genotoxicity in Allium cepa chromosomal aberration assay. It was shown that the percentage of chromosomal aberrations induced by maleic hydrazide(0.01%) declined significantly with 24-epibrassinolide treatment.

​

BRASSINOSTEROIDS AND OTHER� HORMONES

• Compared to auxin, the response in cell elongation to BRs is much slower. Auxin begins to promote cell elongation within 10–15 min and reaches its maximum rate within 30–45 min in several plant species . In contrast, BRs show a lag time of cell elongation onset of at least 45 min and can continually promote elongation for several hours in soybean.
• Cytokinins, abscisic, ethylene show inhibitory effects on BR- induced elongation of stem tissue.
• As BRs induces ethylene synthesis, thus they inhibits root elongation.
• Brassinosteroids can stimulate cell division by atleast 50% in the presence of auxin and cytokinin. E.g.- in parenchyma cells of Helianthus tuberosus, in petunia protoplasts.

​

BRASSINOSTEROIDS AND HUMANS�

​

​

​

​

​

Homobrassinolide, which is found in the mustard plant, produces an anabolic effect, increases appetite and muscle mass along with the number and size of muscle fibres.

​

​

​

RESEARCH ON BRASSINOSTEROIDS�

• Research is being going on to predict the genes involved in controlling cell division by BRs.
• Studies on the effect of phosphorylation/ dephosphorylation states on BR signal transduction using available inhibitors of kinases and phosphotases.
• Recent work confirmed the inhibition of BR-induced elongation in stem tissue by inhibitors of cellulose biosynthesis or microtubule orientation.
• The receptor for BRs has also been shown to bind the plant peptide hormone systemin. Systemin is associated with the wound response and plants that over-express this are more resistant to insect herbivorary. Projects in elucidating the mechanism of systemin’s interaction in plants are also being carried out in the Bishop Lab, imperial college, London.

​

RESEACH WORK AT INTERNATIONAL LEVEL

• Department of horticulture, Zhejiang university Hangzhou, china.
• Japan international research center for agricultural sciences, Tsukuba, Ibaraki, Japan.
• Gene discovery research group, plant science center, RIKEN yokhoma institute, japan.

​

CONCLUSION

• After a long period of neglect by plant scientists, Brassinosteroids are now receiving a great deal of international attention. Future researches may bring into light many more significant roles to this group of steroid hormones. New discoveries of the physiological properties of Brassinosteroids allow us to consider them as highly promising, environment- friendly natural substances suitable for wide application in plant protection and yield promotion in agriculture.

​

THANKS TO ALL