Relationship between viral distribution in the leaf primordia/young developing leaves and symptom severity in the fully expanded leaves of tobacco plants infected with Cucumber mosaic virus

Tobacco plants inoculated with the Cucumber mosaic virus pepo strain (CMV) induced cyclic mosaic symptom expression; mosaic and mottled leaves appeared first among the newly developing leaves (L7-9) above inoculated leaf (L0). Next, symptomless leaves developed (L10-11), and then mosaic leaves appeared again (L12-14). To clarify the formation of the cyclic symptoms by CMV, the relationships between viral amounts and distributions in the leaf primordia (2 to 3 mm) or young developing leaves (1 to 2 cm) and symptom severity in the fully expanded leaves were analyzed. Large amounts of CMV were detected in most cells of the young developing leaves in the early infection periods, which later led to the development of mosaic and mottled leaves. Virtually no signals were detected in the leaf primordia/young developing leaves for L11, which developed into symptomless leaves. However, CMV was already distributed in the young developing leaves at L13 in the later mosaic leaves. The relationship between the viral distributions in the leaf primordia/young developing leaves and the symptoms in the fully developed leaves in CMV-infected tobacco is discussed.