plants are attacked by pathogens, they defend themselves with passive
and active defence mechanisms.
induced defense including hypersensitive responses.
defences involve Salicylic acid (SA), Jasmonic acid (JA) and Ethylene
gives rise to:
Reinforcement of cell walls, lignification and callose deposition.
- Production of antimicrobial metabolites such
Production of reactive oxygen species (ROS) and reactive nitrogen species
of secondary metabolism
Role of phenolics in the resistance mechanisms of plants against
phenolic synthesis induces resistance:
speed with which phenolics are synthesized determines the disease
phenolics induce resistance:
are oxidized to quinones by the
host enzymes such as polyphenoloxidase
and peroxidase. Quinones are well-known
fungitoxic substances in plants and oxidized phenolics have beeb reported
to be responsible for disease resistance.
trilingue (Fr, Ar, Eng) des Sciences de la vie. A apparaître
bientôt dans les librairies:
catechin was incubated with healty plant
extracts, high amounts of oxidized phenolics were obtained.
is an inhibitor of peroxidase and
polyphenoloxidase. When NaCN was
added to the catechin and plant extract mixture, low amounts
of oxidized phenolics were obtained and this mixture was not
inhibitory to PG enzyme activity of the pathogen. When the NaCN
was omitted, high amounts of oxidized phenolics were obtained
causing high inhibition of the fungal PG.
Phytoalexins are different from anticipins!
Anticipins are low molecular weight antimicrobial compounds present
in plants before challenge by micro-organisms or are produced after
infection solely, from pre-existing precursors.
Phytoalexins derived from the phenylpropanoid pathway or terpene biosynthetic
- Phytoalexins accumulate rapidly around the site of infection by
- Phytoalexins are frequently reported in dicotyledons but rarely
in monocotyledons and gymnosperms.
of primary metabolism
are low molecular mass secondary metabolites with antimicrobial
activity that accumulate in plants as a result of infection
Elicitors cause phytoalexin
synthesis. Phytoalexins need to be rapidly accumulated during
the early stage of the infection process at the infection
sites and in sufficient quantity in order to be considered
a resistance factor.
Phytoalexins are synthesized in response to:
Exogenous elicitors such as 1/ cell wall constituents
and metabolic products of invading microbes (potent phytoalexin
elicitors) and 2/ Glucans, chitin, chitosan, glycoproteins,
unsaturated fatty acids, organic acids
---- Endogenous elicitors (plant origin) such as Oligogalacturonides
(derived from pectin degradation)
common phytoalexins in the legume family.
- Sesquiterpenes: phytoalexins of potato,
tobacco, and tomato (Solanaceae).
- Pisatin: the first phytoalexin isolated
from pea inoculated with Monilinia fructicola by Perrin and
- Phaseollin: phytoalexin isolated
from bean inoculated with F. solani f.sp. pisi by
- Rishitin: phytoalexin isolated from
tuber of Rishiri potato following inoculation of an
incompatible race of Phytophthora infestan by Tomiyama et
Phytoalexins show antimicrobial activity
(against fungi and bacteria) causing disruption
of membrane integrity and function or/and inhibition
of electron transport system. For example, Rishitin
(from potato) lyzes chloroplasts, interacts with phospholipids
in membrane and causes an increase in membrane fluidity.
For some cases, phytoalexins could be detoxified by fungal
enzymes. For example, Nectria haematococca (anamorph
= F. solani) who causes stem and root rots of pea,
is known to detoxify pisatin by demethylating this
phytoalexin to less toxic 6 alpha-hydrozymaackiain. This detoxification
is catalyzed by a pisatin demethylase
that has been identified in Nectria haematococca as
a cytochrome P450 enzyme.
Enzymes lytiques: chitinase, glucanase.
Faire la transgénèse avec les gènes de
ses enzymes. Des gènes codant pour ses protéines
et provenant de plantes (haricot, orge, luzerne) ont été
introduits chez le tabac. Néanmoins, les résistances
obtenues par cette voie sont partielles.
proteins accumulate locally in the infected and surrounding
tissues, and also in remote uninfected tissues. Production
of PR proteins in the uninfected parts of plants can prevent
the affected plants from further infection. PR protein in
the plants was first discovered and reported in tobacco plants
infected by tobacco mosaic virus.
these proteins were found in many plants. Most PR proteins in the plant
species are acid-soluble, low molecular weight, and protease-resistant
proteins. PR proteins depending on their isoelectric points may be
acidic or basic proteins but they have similar functions.
Most acidic PR proteins are located in the intercellular spaces,
whereas, basic PR proteins are predominantly located in the vacuole.
PR-proteins were categorized into 17 families according to their properties
and functions, including ß-1,3-glucanases,
defenses, thionins, nonspecific lipid transfer proteins, oxalate oxidase,
and oxalate-oxidase-like proteins.
Among these PR proteins chitinases
and ß-1,3-glucanases are
two important hydrolytic enzymes that are abundant in many plant
species after infection by different type of pathogens. The amount
of them significantly increase and play main role of defense reaction
against fungal pathogen by degrading cell wall, because chitin and
ß-1,3-glucan is also a major structural component of the cell
walls of many pathogenic fungi. ß-1,3-glucanases
appear to be coordinately expressed along with chitinases after
fungal infection. This co-induction of the two
hydrolytic enzymes has been described in many plant species, including
pea, bean, tomato, tobacco, maize, soybean, potato, and wheat.
fungal cell wall contains a cell membrane , which have a protective
layer of chitin.
as chitosan, ethylene, ozone, wounding, polysaccharides, salicylic
acid, salt solution, and UV light can induce higher expression
of chitinases in plants.
as bacteria, insects, fungi, viruses, and fungal cell wall fragments
can also induce the expression of chitinases in plants.
of chitinases from bean, cucumber, pea, potato, sugar beet,
tomato, and tobacco have shown that the expression of chitinases
is induced dramatically after infections.
fungal cell wall contains a cell membrane with different proteins,
a protective layer of chitin as well as glucan (mostly beta)
and mannoproteins on its surface. Different fungal cell walls
contain different glucans. The cell wall of Aspergillus fumigatus
has beta-1,3- and beta-1,4-glucan and alpha-1,3-glucan. Candida
albicans contains beta-1,3- and beta-1,6-glucan.
Induction usually occurs strongly at the point of infection and drops
rapidly as the distance from the infection site
induction of chitinase can spread to adjacent tissues, resulting in
a systemic acquired resistance that may
enable the plant to protect itself from secondary infection.
AS PR PROTEIN AGAINST PATHOGENS. CASE OF MUSKMELON (MELON, البطيخ،
الشمام) CUCURBITACEAE FAMILY