A schematic model for plant microbe interactions.
see monochrome version of this figure.
This model illustrates the main features of host-pathogen interactions.
- The pathogen produces various tools to establish and maintain infection. These are
called pathogenicity factors and include toxins, hydrolytic enzymes and various substances
which otherwise interfere with the host.
- The host recognises substances produced by the pathogen (including pathogenicity
factors) using receptors which may be membrane-bound or cytoplasmatic
- The recognition event to activate a defence response which can be both transcriptionally
activated and post-translationally activated. The signalling processes by which the
defences are activated is is termed signal transduction and can use substances called
secondary messengers, e.g.,
calcium(which we won't go into here - see the
Drastic site at
SCRI, especially the response
flash animation).
- Transcriptional activation entails:
- The de novo transcription of genes to produce mRNA.
- The translation of mRNA to produce a polypeptide (protein) product.
- The protein can act:
- directly on the pathogen (e.g. chitinase to break down the cell wall)
- indirectly by contributing to a biosynthetic pathway for the production of a
phytoalexin.
- Post-translation activation is the activation of constitutively produced defences. The
mechanisms involved can include:
- Activation of enzymes by phosphorylation (e.g. production of H2O2/superoxide
- Release of substances sequestered in vacuoles (a chemical bomb)
- The substances used by the plant to detect the pathogen are termed
elicitors.
- The defence response includes the production of antimicrobial substances, enzymes,
strengthening of the cell wall (e.g.
lignification) and the hypersensitive response. The defences are activated
both in compatible and incompatible interactions. It is rarely clear why an interaction
has a particular outcome.
Other links:
created for the web 8th March 1999,
revised 28. marts 2004