Hello everyone! It’s me Shanty and you are
watching Biology Nowadays. In this lecture, we will see what phytohormones are, their
main functions and characteristics, and also the biosynthetic precursors of different classes
of phytohormones. Finally, we will have a short look on the synthetic plant growth regulators.
From a single cell zygote, begins the life of a plant. It does not simply increase it’s
mass and volume as it grows, but it differentiates, develops and takes shape, forming a variety
of cells, tissues and organs. This beautiful time lapse video shows the germination of
a pea seed. Watch the full video by clicking the link given in the description box below.
Plants cannot move around like animals. So the plant growth depends on the interaction
of a number of external and internal factors. Examples for external factors are light, temperature,
day length and gravity. The main internal factors are chemicals called plant hormones,
or phytohormones. For many years, it was considered that there were only 5 groups of plant hormones-
auxins, cytokinins, gibberellins,ethylene and abscisic acid. They are still the ‘classical’
or ‘traditional’ phytohormones. But during the last three or four decades, some more
groups were added to this list. They are brassinosteroids, jasmonates, salicylic acid, strigolactones,
peptide hormones and finally the phytohormone-like polyamines. We will see each of them in detail.
First of all, auxins. They are indole compounds. Indole-3 Acetic Acid or IAA is the most common
auxin. Auxins are produced in the shoot and root apical meristems, or tips, of a plant.
They are also found in the immature leaves and seeds. Auxins promote stem elongation.
They also promote apical dominance. Apical dominance occurs when the shoot apex inhibits
the growth of lateral buds so that the plant may grow vertically. This kind of growth will
help the plant to get more light to conduct photosynthesis. In this plant, there is an
active apical bud and it is in this bud, the auxins are produced. Eventhough auxins promote
stem elongation, they inhibit growth of the lateral buds. As a result, the lateral buds
will not grow into side shoots. Now, if I remove the apical bud, the auxin concentration
lowers and because of this the lateral buds will grow into side shoots. Auxins promote
the activity of vascular cambium. Vascular cambium produces secondary xylem as well as
phloem, and result in the thickening of stem. Auxins help in root initiation. They also
help in fruit growth. As I explained in the case of apical dominance auxins inhibit lateral
bud growth. They also inhibit abscission, or in other words dropping of leaves and fruits.
Now, have a look at these Arabidopsis plants. Arabidopsis plants are used in various plant
research studies. Here is a normal Arabidopsis plant with normal levels of auxin, but this
genetically modified plant cannot produce auxin. See the difference in growth shown
by these two plants. So, from this we can conclude that auxins are promoters of plant
growth. Now let’s see what are the functions of cytokinins. They are adenine derivatives.
Zeatin is the most abundant cytokinin. Cytokinins are produced mainly in root tips. Small amounts
of cytokinins are also produced in actively growing plant parts such as developing shoot
buds, young fruits etc. Cytokinins are called so because they promote cytokinesis or in
other words cell division. They also promote cell differentiation i.e, specialization of
cells. They play a major role in promoting shoot growth. Cytokinins also promote opening
of stomata and chloroplast development thereby promote photosynthesis rate. They also cause
movement of nutrients to tissues where their concentration is higher. They inhibit apical
dominance caused by auxin which means that they help the lateral buds grow into side
shoots. They delay leaf senescence. Leaf senescence means ageing and gradual death of leaves.
Here, these two tobacco plants were grown under drought conditions for the same period
of time. The plant that has less number of green leaves is a normal plant. There is no
surprise that it’s leaves have become dry because that’s how a plant reacts to a drought
condition. The plant with lots of healthy green leaves is a genetically modified plant.
The speciality of this plant is that during drought stress it can produce more cytokinins
and since cytokinins reduce ageing of leaves, the leaves are greener and healthier than
the normal plant. However cytokinins inhibit root growth. But overall cytokinins are promoters
of plant growth. Now what about gibberellins? They are terpenoids. Gibberellic acid-3 or
GA3 is the best known. Gibberellins got their name because they were first isolated from
a fungus Gibberella fujikuroi. Gibberellins are produced mainly in developing seeds. Also
in shoot and root tips, and young leaves. One of the main functions of gibberellins
is that they promote seed germination. They help in mobilization of food stored in the
endosperm to the growing embryo. Another function is, promoting intermodal elongation. Internode
means the part of the stem between two nodes. Can you identify these tall plants in this
picture? Then be ready for a shock! They are cabbage plants. Yes! Cabbage plants treated
with gibberellins. These two cabbage plants are the normal ones. In the gibberellin treated
cabbage plants you can see the intermodal elongation caused by gibberellins compared
to the normal cabbage plants. Gibberellins promote flower formation and promote the production
of male flowers in the place of female flowers in plants like Cucurbits. They also promote
pollen tube growth and fruit development. Gibberellins inhibit seed dormancy. Overall
gibberellins are also promoters of plant growth. Next phytohormone is ethylene, which is the
only gaseous hormone. Of all the phytohormones, ethylene has the simplest chemical structure.
It is produced during seed germination, ripening of fruits, abscission of leaves and senescence
of flowers. Ethylene promotes ageing and senescence of plant organs. It causes leaf epinasty or
drooping of leaves. In this picture, among these two tomato plants, the one on the right
side was exposed to ethylene. After some time the leaves showed epinasty or drooping down
of leaves. Ethylene also promotes fruit ripening. One of the main functions of ethylene is promoting
abscission of leaves, flowers and fruits. Ethylene is the primary and most important
chemical involved in the process of abscission. It inhibits stem elongation. Overall ethylene
is a growth inhibitor. Next phytohormone is abscisic acid or ABA. They are carotenoid
derivatives produced in the mature leaves, dormant buds and seeds. The name abscisic
acid was given because researchers at first believed that it was this hormone that stimulated
abscission or falling down of plant parts. Today this name abscisic acid is a little
misleading because it is now proven that, eventhough abscisic acid promote abscission,
it does not play a primary role. As I said earlier, it is ethylene which plays the primary
role in abscission. Abscisic acid promotes dormancy in seeds and buds. It also promotes
closure of stomata under drought conditions so that less water is lost through stomata
and helps the plant survive in such unfavourable conditions. In this picture these two plants
were under drought stress for the same period of time. Among these two plants, the plant
treated with more abscisic acid analog shows better tolerance towards drought. As I mentioned
earlier, together with ethylene, ABA promotes abscission of leaves. Abscisic acid is a general
inhibitor of plant growth. It also inhibits seed germination. So overall, abscisic acid
is an inhibitor of growth. Eventhough when I say that ethylene and ABA are growth inhibitors,
under certain unfavourable conditions, like drought stress, it is necessary to slow down
or inhibit the plant growth to make sure that the plant uses less water, and survive the
drought conditions. Because under drought the strategy is, to survive rather than grow.
So growth inhibitors are also actually good and necessary for the plant growth and survival.
The sixth class of hormones is the brassinosteroids. They are steroid compounds. The most abundant
among the brassinosteroids is the brassinolide. It was first isolated from the pollen of Brassica
napus, the rape plant from which we get the canola oil. Brassinosteroids are found mainly
in pollen and immature seeds. They promote cell enlargement and cell elongation, cell
division, pollen tube growth, seed germination, xylem differentiation which means specialization
of xylem cell types, and resistance to stress. Among these two Cucumber plants, the small
plant is a genetically modified plant which produces less brassinosteroids than the normal
plant. From the difference in growth exhibited by these two plants, I hope you can understand
how important brassinosteroids are for the normal plant growth and development. So brassinosteroids
are promoters of plant growth. Next group is the jasmonates. They are lipid derived
cyclopentanone compounds. A member of this group is methyl jasmonate. The isolation of
methyl jasmonate or the methyl ester of jasmonic acid, from jasmine oil derived from Jasminum
grandiflorum led to the discovery of the molecular structure of jasmonates and so their name.
The scent of jasmine mainly comes from methyl jasmonate. Jasmonates are produced by plants
under stress. Jasmonates promote initiation of defence reactions against pathogen. In
a recent collaborative study conducted at Utrecht University and University of Amsterdam
in the Netherlands showed that, when two Arabidopsis plants infected with a pathogenic fungus,
the plant with more jasmonic acid exhibited less disease symptoms. However, it showed
less growth than the normal plant. This is because of the growth inhibitory effects of
jasmonates. Jasmonates promote senescence of leaves. They inhibit seed germination and
root growth. Next is salicylic acid. It is a phenolic acid. It was first isolated from
the bark of Willow tree. The word salicylic acid was derived from the Latin word ‘Salix’
which means Willow tree. Salicylic acid promotes resistance to pathogen by inducing the production
of pathogenesis-related proteins i.e., proteins produced in plants during a pathogen attack.
It promotes protection to various environmental stresses. For example, these two plants were
grown at a temperature of 40 degree Celsius. The plant on the right side was treated with
salicylic acid and this plant showed better tolerance by growing better than the untreated
plant. Salicylic acid promotes the levels of chlorophyll and carotenoid pigments and
as a result promotes photosynthesis. It also promotes flowering. Now, the next group of
phytohormones- the Strigolactones. They are terpenoid lactones. Strigol was the first
isolated strigolactone. Strigolactones are produced in roots. They promote seed germination
of parasitic weed for eg. Striga, and because of this role these phytohormones got their
name. They also promote recognition of the plant by arbuscular mycorrhizal fungi and
promote branching in the arbuscular mycorrhizal fungi. They promote plant height and secondary
growth. Strigolactones promote the formation of root hairs. They also promote senescence
of plant organs. They inhibit branching. They also inhibit lateral or side root formation.
Next group is the peptide hormones. They regulate cell division, reproduction, nodulation, senescence,
defence and environmental responses in plants. Some examples for peptide hormones are Systemins,
Phytosulfokines, CLV peptides and PLS peptide. The last group is the phytohormone-like polyamines.
They play a role in the modulation of senescence of plant organs and in the regulation of programmed
cell death in plants. Examples for polyamines are Putrescine, Spermidine and Spermine. Now,
let’s see the biosynthetic precursors of the main phytohormones. Eventhough the phytohormones
have diverse chemical structures, most of them are derived from three main types of
metabolic precursors: amino acids, isoprenoid compounds, and lipids. The amino acid tryptophan
is the precursor for auxins, methionine serves as precursor for ethylene and phenylalanine
is the precursor of salicylic acid. The isoprenoid pathway gives rise to five classes of plant
hormones. Here, isopentenyl pyrophosphate or IPP serves as the main precursor. Cytokinins,
gibberellins, abscisic acid, brassinosteroids, and strigolactones are the 5 classes of phytohormones
synthesised through isoprenoid pathway. Jasmonates are synthesized from a lipid precursor which
is alpha-Linolenic acid. Now, I hope you got a clear picture on the functions and biosynthetic
precursors of different classes of phytohormones. Actually the word hormone comes from Greek
word ‘hormaein’ which means to excite or to stimulate because at first people thought
that hormones always stimulate or promote growth. But now we know that hormones can
promote as well as inhibit growth. Let’s see some of the characteristics of phytohormones.
They are naturally produced in plants. They are present in extremely small quantities.
For example, in a pineapple plant there will be only 6 micrograms of Indole-3 acetic acid
per kilogram of plant material. I know it is very difficult to have a feel of such a
minute quantity. So let me put it in another way. It is like a big Volvo bus of 20 metric
tonnes carrying a small needle in it. That’s interesting. right? The third feature of phytohormones
is that the site of synthesis and the site of their action can be different. For example,
as we saw earlier cytokinins are mainly produced in the root tips. But besides affecting root
growth, these cytokinins are transported to shoot where they affect shoot growth also.
Next characteristic of phytohormones is that they often interact or do crosstalk with each
other. Let me explain what this means. This is a summary of what we discussed about the
main functions of the 5 classical hormones-auxins, cytokinins, gibberellins, abscisic acid and
ethylene in the 6 main plant processes- seed germination, general growth, flowering, fruit
development, abscission and seed dormancy. Now, consider the case of growth. Here auxins,
cytokinins, and gibberellins together help to bring about the effect of growth in plants.
Such kind of acting together to promote each other’s activity like….. friends, is called
synergetic interaction. Now let’s take the case of seed germination and seed dormancy.
Here gibberellins promote seed germination and inhibit seed dormancy while abscisic acid
inhibits seed germination and promotes seed dormancy. So such an interaction where two
phytohormones have opposite effects on same process is called antagonistic interaction.
Finally, let’s have a short look on the synthetic plant growth regulators. Firstly,
synthetic auxins- as the name indicates they exhibit auxin or growth promoting activity.
For example, 2, 4-Dichlorophenoxy acetic acid. It is commonly called as 2, 4-D. and Indole-3
butyric acid or IBA. There are some synthetic plant growth regulators that come under the
category of growth retardants or growth inhibitors. First group is morphactins, meaning morphologically
active substances. They have an inhibitory effect on plant growth, then antigibberellins-they
act by blocking the main steps in gibberellin biosynthesis in plants. Examples for antigibberellins
are Phosfon-D, Cycocel and Amo-1618. The next synthetic growth retardant is Maleic Hydrazide
which acts by inhibition of cell division. And that’s all for this video. Please help
us to improve the quality of this channel by giving your valuable suggestions in the
comment section below. Thank you for being with me and stay tuned.