Plant defense mechanisms

Whatever concerns plant defense or resistance, it is controlled by genes of the plant and pathogen

Nonhost resistance The most common case: Apple tree cannot be infected by pathogens specific to tomato and vice versa

Partial, polygenic, quantitative or horizontal resistance Result of more or many minor genes Present in all plants Different levels against different pathogens Basically against necrotrophic or semibiotrophic pathogens as Botrytis, fusarium, sclerotinia etc.

Race-specific, monogenic or vertical resistance Result of one major gen Usually against biotrophic pathogens as downy mildews, powdery mildews and rusts Gen to gen theory

Preexisting structural defense Wax, cuticle, structure of cell wall, stomata and lenticels, presence of thick-walled tissues (sclerenchyma) etc.

Preexisting chemical defense Released into environment: Fungitoxic exudates, phenolic compounds (eg. Red and yellow onion cannot be infected by smudge, whereas white one can be infected because of lacking catechol), coumarins

Preexisting chemical defense Inhibitors in plant cells before infection: tannins, catechin, saponins (eg. Tomatine, avenacine), low molecular proteins – phytocystatins, lectins, glucanases and chitinases contained in surface cells

Defense through lack of essential factors Lack of recognition between host and pathogen Lack of host receptors and sensitive sites for toxins Lack of essential substances for the pathogen

Induced structural and biochemical defenses

Induced structural and biochemical defenses Recognition of the pathogens by the host plant: - Pathogen elicitors Host plant receptors => Mobilization of defenses and transmission of alarm signal

Induced structural defenses Cytoplasmic defense reaction Cell wall defense structures

Cell wall defense structures

Induced structural defenses - Histological defense structures: Cork layers

Induced structural defenses - Histological defense structures: Cork layers

Induced structural defenses - Histological defense structures: Tyloses Deposition of gums

Induced structural defenses - Histological defense structures: Abscission layers

NECROTIC STRUCTURAL DEFENSE REACTION

NECROTIC STRUCTURAL DEFENSE REACTION

HYPERSENSITIVE REACTION

INDUCED BIOCHEMICAL DEFENSES ACTIVE OXYGEN SPECIES: SUPEROXIDE (O -2), HYDROGEN PEROXIDE (H2O2) AND HYDROXYL RADICAL (OH)

INDUCED BIOCHEMICAL DEFENSES REINFORCEMENT OF HOST CELL WALLS BY CALLOSE GLYCOPROTEINS (EXTENSIN) PHENOLIC COMPOUNDS (SUBERIN, LIGNIN) MINERAL ELEMENTS (SILICON, CALCIUM)

INDUCED BIOCHEMICAL DEFENSES PRODUCTION OF ANTIMICROBIAL SUBSTANCES IN ATTACKED CELLS: PATHOGENESIS RELATED PROTEINS

INDUCED BIOCHEMICAL DEFENSES PRODUCTION OF SECONDARY METABOLITES: PHENOLIC COMPOUNDS LIKE CHLOROGENIC ACID, CAFFEIC ACID AND FERULIC ACID PHENOL OXIDIZING ENZYMES – QUINONES (MORE TOXIC THAN ORIGINAL COMPOUND)

INDUCED BIOCHEMICAL DEFENSES PRODUCTION OF SECONDARY METABOLITES: PHYTOALEXINS – PHASEOLIN, PISATIN, GOSSYPOL, CAPSIDIOL ETC. ELICITORS OF PHA ARE RELEASED FROM PATHOGEN CELL WALL BY HOST ENZYMES OR PRODUCET BY HOST CELL ITSELF

DETOXIFICATION OF PATHOGEN TOXINS BY PLANTS E.G. HC TOXIN, PYRICULARIN

IMMUNIZATION OF PLANTS AGAINST PATHOGENS PLANTIBODIES

SYSTEMIC ACQUIRED RESISTANCE

DEFENSE THROUGH GENETICALLY ENGINEERING DISEASE-RESISTANT PLANTS PLANT-DERIVED GENES: E.G. Hml GENE FOR ENZYME INACTIVATING HC TOXIN GENES ACTIVATED BY AVR GENES OF PATHOGENS ANTIVIRAL GENES

DEFENSE THROUGH GENETICALLY ENGINEERING DISEASE-RESISTANT PLANTS PATHOGEN-DERIVED GENES SEE VIROLOGY GENE FROM ASPERGILLUS NIGER FOR H2O2 GENERATING GLUCOSE OXIDASE INCREASES RESISTANCE AGAINST P. INFESTANS, A. SOLANI

DEFENSE THROUGH GENETICALLY ENGINEERING DISEASE-RESISTANT PLANTS PATHOGEN-DERIVED GENES T4 GENE FOR LYSOZYME DEGRADING CELL WALL OF SOME BACTERIA AND FUNGI GENE FOR CHITINASE FROM T. HARZIANUM AGAINST APPLE SCAB

RNA SILENCING