Resistance to Fungal Pathogens

Plants react to attack by fungal and other pathogens by activating a series of defence mechanisms, both locally and throughout the plant. The responses may be non-specific induction of defence reactions to pathogens or specific responses based on the race of the pathogen and the genotype of the host plant. Local resistance may appear as a hypersensitive response in which a local necrotic lesion restricts the growth and spread of a pathogen. Systemic resistance, which may take several hours or days to develop, provides resistance to pathogens in parts of the plant remote form the initial site of infection and longer term resistance to secondary challenge by the initial pathogen and also to unrelated pathogens.
The hypersensitive response is characterised by rapid reactions to invasion by a potential pathogen, through recognition of pathogen or cell wall derived elicitors. It involves:
(i) opening of specific ion channels;
(ii) membrane potential changes;
(iii) oxidative burst (generation of reactive oxygen species);
(iv) synthesis of peroxidase;
(v) production of secondary metabolites (phenylpropanoids and phytoalexins);
(vi) synthesis of pathogenesis-related (PR) proteins (e.g. b-l,3-glucanases, chitinases);
(vii) cell wall changes (e.g. suberin, lignin).
It results in death of host cells, formation of a necrotic lesion and restriction or death of the pathogen. Transduction of a signal following pathogen recognition can be both local and systemic and involves a number of different pathways. The synthesis and accumulation of salicylic acid appears to be necessary for both local and systemic induction of defence responses and salicylic acid (or methyl salicylate) is a major signalling molecule. However, other compounds can activate plant defence genes(e.g. 2,6-dichloroisonicotinic acid, benzothiadiazole, ethylene, abscisic acid, jasmonic acid and systemin). Systemic signals may lead to induction of systemic acquired resistance.
For salicylic acid signalling, salicylic acid moves in the phloem and its presence may be required to establish and maintain systemic acquired resistance. Its arrival in tissues leads to expression of plant defencerelated genes in sites distant from the initial challenge, such as PR proteins and production of hydrogen peroxide and reactive oxygen species, cross-linking of cell wall proteins and lignin synthesis. There are a series of other defence systems that plants use to combat pathogens and these include natural resistance genes and antifungal proteins.

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