ORF 2a (replicase)
introducing new resistance genes into crop plants, essentially by crossing wild predecessors and cultivated plants (3).
The avr gene products elicit host responses on different levels. One example, extreme resistance (38), inhibits viral multiplication in the inoculated plant cells and in protoplasts. The fact that this genetic trait is termi-nologically considered resistance and not immunity is merely based on the observation that other strains of the same virus multiply readily in single cells.
A second and most common example, the hypersensitive reaction (HR) (38), allows multiplication in the inoculated cells and limited spread in the tissues, but then infected parts die and further infection is impaired. It has been questioned whether resistance is solely due to this "suicide for survival" (8,36,39), and at least in some cases resistance can be uncoupled from cell death (26,33).
Extreme resistance and hypersensitive reactions are currently the best known barriers for viral infections; others are being investigated to elucidate additional mechanisms in the near future (40,41). In particular, apoptosis, the programmed cell death of single cells thereby preventing necrosis of tissues, might be an interesting candidate. This mechanism has been intensively studied in animals (42) and might also be responsible for resistance traits in plants (43-45). Most virus-specific host responses rely on the general signal transduction cascades of eukaryotic cells, but for plants signal perception between virus and host takes place mostly inside the cell (in the symplast).
The word "symptom" originates from the Greek cru/A-nTco/xa, which means "chance, transient peculiarity" (46). Correspondingly, symptoms may disappear during the infection process. Some shoots may escape from infection just by chance; others may develop virus-specific resistance that does not allow reinfection by the same virus, but unrelated viruses are propagated without problems. This recovery phenomenon gave rise to intensive research and led to the discovery of virus-induced gene silencing, discussed later (47,48).
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