Evidence that resistance in squash mosaic comovirus coat protein-transgenic plants is affected by plant developmental stage and enhanced by combination of transgenes from different lines.

Three transgenic lines of squash hemizygous for the coat protein genes of squash mosaic virus (SqMV) were shown previously to have resistant (SqMV-127), susceptible (SqMV-22) or recovery (SqMV-3) phenotypes. Post-transcriptional gene silencing (PTGS) was the underlying mechanism for resistance of SqMV-127. Here, experiments conducted to determine the mechanism of the recovery phenotype and whether enhanced resistance could be obtained by combining transgenes from susceptible and recovery plants are reported. Upper leaves of SqMV-3 plants were sampled for Northern analysis at 17, 31 and 45 days after germination (DAG) and a proportion of plants were inoculated with SqMV. SqMV-3 plants inoculated at a young stage (17 DAG) showed susceptible or recovery phenotypes. However, a number of plants inoculated at later developmental stages (31 or 45 DAG) were resistant to infection. Resistance of recovery plants was due to PTGS that was activated at a later developmental stage, independent of virus infection. Similar results were observed with plants grown under field conditions. To investigate the interactions of transgenes, progeny of crosses between SqMV-127, -3 and -22 were inoculated with SqMV. Progeny with the transgene of line 127 were resistant. However, a number of plants with transgenes from the recovery and susceptible lines or the self-pollinated recovery line were resistant even when inoculated at a young stage. Northern analysis suggested that resistance was due to PTGS. The results reveal that the timing of PTGS and consequent resistance of the transgenic plants were affected by their developmental stage and the interaction of transgene inserts.