Plants are resistant to most pathogens, therefore

1
Plants are resistant to most pathogens,
therefore most pathogens are specialized for
particular hosts. Example APS compendium lists
approximately 100 pathogens of soybean, of which
only 35 are economically important.
2
Resistance mechanisms A succesful pathogen must
overcome, evade, or otherwise tolerate resistance
mechanisms the plant deploys.
3

• Resistance Mechanisms
• Preformed v. induced
• Active v. passive
• Specific v. general

4

• Preformed resistance mechanisms
• Physical barriers (cutin, cell wall.)
• Preformed antimicrobial compounds

5
How do pathogens overcome these physical
barriers? enzymes (cutinases, pectinases,
cellulases, etc.) Mechanical force
(appresoria) Alternate routes of entry (wounds,
stomates, etc.)
6
Preformed antimicrobial compounds are
called phytoanticipins plants produce a
diverse array of secondary metabolites, some with
antimicrobial activity Phytoanticipins are
unique as they are preformed, rather than being
synthesized from remote precursors after pathogen
infection (phytoalexins)
7
Phytoanticipins 2 classes constitutive
saponins (glycosylated antimicrobials) inactive
precursors processed after tissue damage or
pathogen attack (cyanogenic glycosides
and glucosinolates)
8
Induced resistance mechanisms physical barriers
(cell wall thickening, callose deposition,
lignification.) oxidative burst, resulting in
thickening of cell wall induced antimicrobials
(phytoalexins) antimicrobial proteins (PR
proteins, defensins) Hypersensitive response
(PCD)
9
Lignification during the invasion of Phytophthora
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
Induced antimicrobials - Phytoalexins low
molecular weight, accumulate after pathogen
infection chemically diverse (ie. not all the
same!) legume isoflavanoids are best understood
14
Selected phytoalexin structures