At 23 °C, infections with alfalfa mosaic virus can be initiated with the three genomic RNA species in combination either with coat protein or with the messenger for coat protein (RNA 4). At 30 °C the combination of the three genomic RNA species with coat protein is still infectious to tobacco, whereas the infectivity of the combination with RNA 4 is almost nil. The combination of the four RNA species is infectious to bean at both temperatures. Tobacco leaf discs inoculated with the four AMV RNA species were incubated for short periods at 30 °C followed by 4 days at 23 °C. Infectivity could only be recovered when the shift-down occurred within 10 min after inoculation. Longer exposure to 30 °C probably leads to the degradation of one or more of the AMV RNA species. Translation of RNA 4 into coat protein at 23 °C in the inoculated cells presumably will render the RNA infection resistant to 30 °C. To measure the minimum time required for the penetration and translation of RNA 4, tobacco leaf discs inoculated with the four AMV RNA species were incubated for different periods at 23 °C and then for 4 days at 30 °C. From the results it was estimated that 15 to 30 min is sufficient for the penetration and translation of RNA 4.
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