Multiple mechanisms at work in TYLCSV Rep-derived transgenic plants challenged with Geminiviruses.

The replication-associated protein (Rep) of geminiviruses is involved in several biological processes brought about owing to the presence of distinct functional domains and different states of protein aggregation. In the present study, using both biochemical and transgenic approaches, we carry out an extensive investigation of the molecular resistance mechanisms operating in transgenic plants expressing the N-terminal 210 amino acids of Tomato yellow leaf curl Sardinia virus Rep (Rep-210). We show that Rep-210 confers resistance through two distinct molecular mechanisms, depending on the challenging virus. Resistance to the homologous virus is achieved by a tight inhibition of viral C1 gene transcription, while that to heterologous virus (Tomato yellow leaf curl virus, Portugal isolate) is due to the interacting property of the Rep-210 oligomerization domain. Furthermore, we show that in Rep-210 expressing plants the duration of resistance is related to the unexpected ability of the challenging virus to shut off transgene expression by a post-transcriptional homology-dependent gene silencing mechanism and to spread in silenced transgenic plants. We propose a model in which the resistant or susceptible phenotype can be regarded as the result of a battle where the transgenic plant uses the Rep-210 to repress viral transcription and to assemble dysfunctional oligomers, while the virus induces silencing of the transgene.