Abstract
Induction of a symptom-delaying component (SDC) in tobacco plants infected with cucumber mosaic virus (CMV) was investigated. Two largest leaves of Nicotiana tabacum cv. Xanthi seedlings at the 6-8-leaf stage were inoculated with a non-satellite-RNA-containing isolate of CMV-pepo at 100μg/ml and incubated for seven days at 25°C with a 16-hr photoperiod. With a sap-preparation from each leaf directly above the inoculated leaves (LAL) tobacco seedlings were treated and subsequently inoculated with CMV at 10μg/ml four days after the LAL-sap treatment. CMV symptoms were delayed in as many as 40% of the plants which had been treated with non-infectious LAL-sap, indicating that the LAL contained SDC. SDC was detected in the treated leaves at two to eight days after SDC-treatment and appeared to be translocated to the upper leaves at four to eight days in tobacco plants. SDC appeared to delay development of CMV symptoms in cucumber and in tomato. SDC also delayed symptom development of aphid-transmitted CMV in tobacco. In contrast, SDC did not affect local lesion formation on Chenopodium quinoa or Vigna unguiculata subsp. sesquipedalis by CMV nor symptom development in tobacco by tobacco mosaic virus or potato virus Y. In most of the tobacco plants repeatedly treated with SDC, symptom development of CMV was delayed for 4-20 days under field conditions. Antigenicity was detected intermittently in sap from some LAL by immunodiffusion tests using CMV antiserum. No CMV particles were observed in SDC-containing leaf sap. SDC was inactivated with a RNase A treatment. Gel electrophoresis showed that the corresponding bands with double-stranded RNAs 3 and 4 of CMV were detectable in. SDC-containing sap. However, treatment with single-stranded RNA 3 or RNA 4 failed to delay the development of CMV symptoms in tobacco plants.
