When considering substances produced by plants that act as agents that protect the plant from pathogens and insect pests, we must first consider whether the compounds are present prior to the time of infection or whether they are synthesized in response to infection. When compounds are present prior to attempted infection they are known as preformed antimicrobial metabolites. Such preformed compounds are part of a passive resistance mechanism. In general, such preformed metabolites are toxic to a broad spectrum of fungi and bacteria, but the compounds have a relatively low level of toxicity. Thus, preformed compounds are present in all plant species and help plants to ward off pathogens that are not considered as highly aggressive organisms. They are also referred to as phytoanticipins (Van Etten et al., 1994).
When one considers resistance expression in plants, it is necessary to consider whether resistance expression is part of a passive or active response system. There are several situations that could arise:
1. Compound "A" is present in the plant and is toxic to the potential pathogen. The compound is present in cells or tissues that the pathogen must come into contact with, at some time during the attempted infection. The compound is not further metabolized, but may or may not be changed by the pathogen, and the compound as such then accounts for toxicity to the pathogen. This example represents a form of passive resistance.
2. A preformed substance is degraded or metabolized to a different compound by the host in direct response to the pathogen and it is this compound that accounts in part for toxicity to the pathogen. Because the host changes the compound, this would be considered a mechanism of active resistance.
There are several criteria that must be satisfied before it can be decided whether a particular compound plays a significant role in the resistance to a pathogen. These are as follows:
1. The compound must be present in those parts of the plant where the pathogen will come in contact with it. For example, apple leaves contain phloridzin (6.1) and its aglycone phloretin (6.2), but these compounds are not present in the fruit.
2. The compound must be present at concentrations high enough to affect the pathogen. For example, maize contains the preformed cyclic hydroxamic acid DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one; 6.3). The concentration of DIMBOA, however, decreases over time and is initially not high enough for it to serve as a fungi-toxic agent.
3. Directly related to the criterion above is the requirement that the compound be "available" in the host in a form or place where it can express its toxicity. This may ultimately be a question of whether the compound is changed when it is extracted. Is the compound compartmentalized in a cell and possibly inaccessible to the pathogen? Is it in only one tissue?
4. Another criterion that must be met deals with the time of appearance of the compound. Thus, even if preformed, the compound must be at a sufficient in vivo concentration by the time resistance is being expressed.
Phenolic substances are often thought of when referring to preformed rsistance compounds. The following classes of metabolites, however, should also be included: alkaloids, carbohydrates that can bind bacteria, proteins that act as lectins, amino acids, terpenoids, and polyacetylenes. Some of these compounds will be included in the discussion below.
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