Defense Responses Of Plants

Following inoculation, plants activate a broad spectrum of responses, either general ones after several types of stress, pathogens and wounding, or specific ones that require the recognition of a particular pathogen. Both reaction types may be linked or act separately and determine whether the virus is virulent (the host susceptible) or avirulent (the host resistant). The resulting interaction is mostly the outcome of a race between virus multiplication and the velocity of defense.

A. General Responses

As with other pathogens (see Chap. 23), viruses encounter an oxidative burst early (4), followed by the transcriptional activation of pathogenesis-related (PR) proteins (5), accumulation of phytoalexins (6), and a systemic signal mediated by salicylic acid, leading to systemically acquired resistance (SAR) in noninoculated tissues or organs (7). No evidence is available that PR proteins interact directly with plant viruses, and it is believed that the broad defense reaction is rationalized by the evolutionary experience that a virus seldom comes alone in nature (8). A further general response has been attributed to ribosome-inactivating proteins (RIPs), which accumulate in certain plants to high levels in an inactive pre-form that is rendered active during wounding to destroy the translational machinery (9-12). Several components of the general response system have been manipulated in order to obtain protection against viruses but so far with only limited success (13,14).

B. Virus-Specific Responses

As Flor (15) elaborated for fungi, genetic traits of a pathogen coevolve with the respective host resistance genes in a pairwise manner. This gene-forgene concept also holds true for plant viruses. Gene products of a virus are recognized by a host and lead to a defense response. The viral genes are then said to be avirulence (avr) genes, although they fulfill different functions in the context of the viral life cycle. Almost all viral genes may function as avr genes by triggering a host response (16-37) (Table 2). Breaking resistance is most frequently caused by mutations in avr genes, and during classical breeding the main task has been to cope with this challenge by

Table 2 Examples of the Relation between Host Resistance (R Genes) and Virus (avr Genes)

Host species

R gene




Arabidopsis thaliana

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