Biochemical regulators

1. Secondary metabolites
Primary metabolites: These are metabolic intermediates of the
anabolic and catabolic pathways such as carbohydrates, amino acids,
fatty acids, cytochromes, chlorophylls.
Secondary metabolites: Plants also contain a large variety of
substances, with no apparent direct metabolic function. Certain
secondary metabolites are restricted to a few plant species,
where they fulfill specific ecological functions, such as attracting insects
to transfer pollen, or animals to consume fruits and in this way to
distribute seed, and last, but not the least, to act as natural pesticides.
Many plants protect themselves by producing toxic proteins (e.g.,
amylase or proteinase inhibitors, or lectins), which impair the digestion
of herbivores. In response to caterpillar feeding, maize plants mobilize a
protease that destroys the caterpillar intestine.

2. Microbes can be pathogens
Certain fungi and bacteria infect plants in order to utilize their resources for
their own nutritional requirements. As this often leads to plant diseases,
these infectants are called pathogens. In order to infect the plants effectively,
the pathogenic microbes produce aggressive substances, such as enzymes,
which attack the cell walls, or toxins, which damage the plant. An example is
fusicoccin, which is produced by the fungus Fusicoccum amygdalis. The
production of substances for the attack requires the presence of specific
avirulence genes, which have developed during evolution.
Plants defend themselves against pathogens by producing defense
substances that are encoded by specific resistance genes of pathogen. The
interaction of the avirulence genes and resistance genes decides the success
of attack and defense.
When a plant is susceptible and the pathogen is aggressive, it leads to
disease, and the disease is virulent. Such is termed a compatible reaction.
If, on the other hand, the infecting pathogen is killed or at least its growth is
very much retarded, this is called an incompatible reaction, and the plant is
regarded as resistant. Often just a single gene decides on compatibility and
resistance between pathogen or host.

3. Plants form phytoalexins in response to
microbial infection
Defense substances against microorganisms, especially fungi, are synthesized
mostly in response to an infection. These inducible defense substances, which
are formed within hours, are called phytoalexins.
Phytoalexins
Comprise a large number of substances with very different structures, many of
which act as antibiotics against a broad spectrum of pathogenic fungi and
bacteria.
 isoprenoids
 flavonoids
 stilbenes.

4. • Plants also produce as defense substances aggressive
oxygen compounds, such as superoxide radicals (O2-) and
H2O2, as well as nitrogen monoxide (NO), and enzymes,
such as β-glucanases, chitinases, and proteinases, which
damage the cell walls of bacteria and fungi.
• The synthesis of these various defense substances is
induced by elicitors.
• Elicitors are often proteins excreted by the pathogens to
attack plant cells (e.g., cell-degrading enzymes).
• In some cases, polysaccharide segments of the cell’s own
wall, produced by degradative enzymes of the pathogen,
function as elicitors or can be fragments from the cell wall
of the pathogen.

5. •These elicitors are bound to specific receptors on the outer surface of
the plasma membrane of the plant cell.
•The binding of the elicitor releases signal cascades, in which protein
kinases and signaling substances such as salicylic acid and jasmonic acid
participate, and which finally induce the transcription of genes for the
synthesis of phytoalexins.
•Elicitors may also cause an infected cell to die and the surrounding
cells to die with it.
•The infected cells and those surrounding them commit suicide.
•This can be caused by the infected cells producing phenols, with which
they poison not only themselves but also their surroundings. This
programmed cell death, called a hypersensitive response, serves to
protect the plant.

6. http://park.itc.u-tokyo.ac.jp/biotec-res-ctr/kampo/eng/research_plant.html
Jasmonic acid: Pathogen response

7. Receptor like Protein Kinases Transduce Signals from
Peptides
Receptor like kinases (RLKs) have a single helical segment in the plasma membrane
that connects a receptor domain on the outside with a protein Ser/Thr kinase on the
cytoplasmic side. This type of receptor participates in the defense mechanism
triggered by infection with a bacterial pathogen.
The signal to turn on the genes needed for
defense against infection is a peptide
(flg22) released by breakdown of flagellin,
the major protein of the bacterial
flagellum. Binding of flg22 to the FLS2
receptor of Arabidopsis induces receptor
dimerization and auto phosphorylation on
Ser and Thr residues, and the downstream
effect is activation of a MAPK cascade and
activates a specific transcription factor,
triggering synthesis of the proteins that
defend against the bacterial infection. The
steps between receptor phosphorylation
and the MAPK cascade are not yet known.

8. Plant defense substances can also be a risk for humans
A number of plant constituents that are harmful to humans [e.g.
proteins such as lectins, amylase inhibitors, proteinase inhibitors, and
cyanogenic glycosides or glucosinolates decompose when cooked. But
most secondary metabolites are not destroyed in this way. In higher
concentrations, many plant secondary metabolites are carcinogenic. It
has been estimated that in industrialized countries more than 99% of
all carcinogenic substances that humans normally consume with their
diet are plant secondary metabolites that are natural constituents of
the food. However, human metabolism usually provides sufficient
protection against these harmful natural substances.

9. Amino acids as precursor for some Alkaloids
Present in hemlock. Socrates
died when he was forced to
drink
Nicotine synthesized in roots of tobacco plants and is carried to leaves.
Cocaine present in deadly nightshade (Atropa belladonna). In low doses, it dilates the pupils
of the eye and is therefore used in medicine for eye examination.

10. Antimalarial drug
Painkiller and is also a
precursor for the
synthesis of heroin
Stimulant of coffee

11. Some plants emit prussic acid when wounded by animals
Since prussic acid (HCN) inhibits cytochrome oxidase, which is the final step of the
respiratory chain, it is a very potent poison.
The consumption of peach kernels, for instance, or bitter almonds can have fatal
results for humans.
As also plants possess a mitochondrial
respiratory chain and, in order not to
poison themselves, they contain prussic
acid in the bound form as cyanogenic
glycoside and store in vacuole.

12. Some wounded plants emit volatile mustard oils
Glucosinolates, also called mustard oil glycosides, have a similar protective function against
herbivores.

13. False amino acids
Many plants contain unusual amino acids with a structure very similar to
that of protein building amino acids [e.g., canavanine from Jack bean
(Canavalia ensiformis), a structural analogue of arginine.
Herbivores take up canavanine with their food. During protein
biosynthesis, the arginine-transfer RNAs of animals cannot distinguish
between arginine and canavanine and incorporate canavanine instead
of arginine into proteins.
This exchange can alter the three-dimensional
structure of proteins, which then lose their
biological function partially or even completely.
Therefore canavanine is toxic for herbivores.
Plants contain a large variety of false amino
acids, which are toxic for herbivores in an
analogous way to canavanine.

14. Phytohormones
Hormones diffuse throughout the plant to promote growth and development.

15. Hormone biosynthesis
Made from four biosynthetic pathways:
– Terpenoids
• AMP + IPP (cytokinins)
• Carotenoid breakdown (abscisic acid)
• Diterpene (gibberellic acid)
– Fatty acids (jasmonic acid)
– Tryptophan (auxins)
– Methionine (ethylene)

16. Auxin
Role of
Hormone
Cell elongation (increase cell size)
Site of
Production
Shoot Tips, Young leaves, Developing
fruits and seeds
Effect of
Hormone
Growth of plant in response to the
environment, production of roots.
Greek: auxein; to grow or increase
Tropism: a plant’s response to environment
Phototropism- response to light
Geotropism-response to gravity
Thigmotropism-response to touch

17. Auxin Growth Effects
Stimulates Adventitious Root
Formation.
Adventitious roots grow from stems or
leaves rather than from the original root
system of the plants.
This is especially useful when cutting and
transplanting plants.

18. Auxin Growth Effects
Thigmotropism-touch
Phototropism-light
Geotropism-gravity
Tropisms-How a plant grows in
response to the environment

19. Auxin
How does this hormone stimulate tropism?
Cell Elongation
Auxin travels away from the
sunlight and expands the cells

20. Biosynthesis of auxins
• Tryptophan dependant pathway
• Tryptophan is synthesized by shikimate
pathway by the help of Tryptophan Synthase
• This is used in the synthesis of Indole-3-
Pyruvic acid.

22. Cytokinin
Role of
Hormone
Cell division (increase number of cells)
Site of
Production
Root Tips
Effect of
Hormone
Mitosis of new cells;
Stimulates seed germination and new
shoot growth
Adding cytokinins to young cotton increase drought-resistance

24. Biosynthesis of cytokinin

25. AUXIN stimulates the
production of roots.
CYTOKININ stimulates
the production of shoots.
Auxin and cytokinin ratio importance
• Auxin alone = Large cells (no division)
• Cytokinin alone = Cells have no change
• Auxin + Cytokinin = Normal cell growth and
division
• Auxin + >Cytokinin = Shoot growth
• >Auxin + Cytokinin = Root growth

26. Gibberellin
Role of
Hormone
Internode Elongation (height)
Site of
Production
Root and Shoot Tips
Effect of
Hormone
Controls yearly cycles (flowering/bolting,
seeding and dormancy exiting)
Rapid growth of stems and seeds.
gibberellin in a plant recognizes seasonal
changes
Photoperiod: a plant’s recognition of
daylight length in a 24 hour period.
As daylight increases in the spring, the plant
recognizes a longer photoperiod. Gibberellin
triggers the plant to exit dormancy.

28. Abscisic Acid
Role of
Hormone
Dormancy (a period of no growth)
Site of
Production
Chloroplasts
Effect of
Hormone
Enters dormancy: (leaves drop off trees, seeds
fall, the stomata close to
reduce water loss during drought stress)
As daylight decreases in the winter, the chloroplasts in the plant recognize a shorter
photoperiod.
Abscisic Acid triggers the plant to enter dormancy.
The flowers, seeds and leaves fall from the trees

31. Ethylene
Role of
Hormone
Ripening and Death
Site of
Production
Ripening fruits, aging flowers,
germinating seeds and wounded tissues
Effect of
Hormone
Stimulates fruits to ripen, flowers to
enter senescence (to grow old and die)
Ethylene is the only plant hormone that exists in a gas
form.
It can be synthesized from anywhere in the plant. It can
even diffuse outside the origin plant and affect another
plant nearby.
Examples: flowers or fruit that are not “ripe” need
ethylene to reach their peak

33. Jasmonic acid
• First identified in jasmine oil
• Response to biotic stress
– Wounding induces JA biosynthesis
– Microbial and fungal invasion
• Plant growth affects similar to auxin
– Speciality growth structures

35. Signal Transduction in plants

37. http://www.nsf.gov/news/news_images.jsp?cntn_id=104205

39. Plants Detect Ethylene through a Two-Component System and a
MAPK Cascade
The gaseous plant hormone ethylene (CH2PCH2), which stimulates the ripening of fruits acts
through receptors that are related in primary sequence to the receptor His kinases of the
bacterial two-component systems and probably evolved from them.
In Arabidopsis, it contained a single integral
membrane protein of the endoplasmic
reticulum (not the plasma membrane).
In plants, in the absence of ethylene, the
CTR1 kinase is active and inhibits the MAPK
cascade, preventing transcription of
ethylene responsive genes. Exposure to
ethylene inactivates CTR1 kinase, thereby
activating the MAPK cascade that leads to
activation of transcription factor EIN3.
Active EIN3 stimulates the synthesis of a
second transcription factor (ERF1), which in
turn activates transcription of ethylene-
responsive genes; the gene products affect
processes ranging from seedling
development to fruit ripening.