Resistance to cassava mosaic disease in transgenic cassava expressing antisense RNAs targeting virus replication genes.

African cassava mosaic virus (ACMV) is a major contributor to cassava mosaic disease (CMD), the economically most important and devastating disease of cassava in Africa. We have developed transgenic cassava plants with increased ACMV resistance using improved antisense RNA technology by targeting the viral mRNAs of Rep (AC1), TrAP (AC2) and REn (AC3). Viral DNA replication assays in detached leaves demonstrated that replication of two ACMV isolates was strongly reduced or inhibited in most transgenic lines. After ACMV infection of plants using biolistic inoculation, several lines remained symptomless at lower infection pressure (100 ng viral DNA/plant). Symptom development was reduced and attenuated even at higher DNA doses. Transgenic ACMV-resistant plants had significantly reduced viral DNA accumulation in their infected leaves. Short sense and antisense RNAs specific to AC1 were identified in transgenic lines expressing AC1 antisense RNA, suggesting that the short RNAs mediate interference by post-transcriptional gene silencing. Our results demonstrate that resistance to ACMV infection of cassava can be achieved with high efficacy by expressing antisense RNAs against viral mRNAs encoding essential non-structural proteins, providing a new tool to combat CMD in Africa.