RESUMO
The major threat for cassava cultivation on the Indian subcontinent is cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses which are bipartite begomoviruses with DNA A and DNA B components. Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosaic virus (SLCMV) cause CMD in India. Two isolates of SLCMV infected the cassava cultivar Sengutchi in the fields near Malappuram and Thiruvananthapuram cities of Kerala State, India. The Malappuram isolate was persistent when maintained in the Madurai Kamaraj University (MKU, Madurai, Tamil Nadu, India) greenhouse, whereas the Thiruvananthapuram isolate did not persist. The recovered cassava plants with the non-persistent SLCMV, which were maintained vegetative in quarantine in the University of Basel (Basel, Switzerland) greenhouse, displayed re-emergence of CMD after a six-month period. Interestingly, these plants did not carry SLCMV but carried ICMV. It is interpreted that the field-collected, SLCMV-infected cassava plants were co-infected with low levels of ICMV. The loss of SLCMV in recovered cassava plants, under greenhouse conditions, then facilitated the re-emergence of ICMV. The partial dimer clones of the persistent and non-persistent isolates of SLCMV and the re-emerged isolate of ICMV were infective in Nicotiana benthamiana upon agroinoculation. Studies on pseudo-recombination between SLCMV and ICMV in N. benthamiana provided evidence for trans-replication of ICMV DNA B by SLCMV DNA A.
RESUMO
We report a high rate of IS426 transposition in Agrobacterium tumefaciens in the presence of the Sri Lankan cassava mosaic virus (SLCMV) replication associated protein gene (Rep). Upon conjugal transfer of the binary plasmid pCam-SLCMV-Rep with the SLCMV Rep gene in the sense orientation under the transcriptional control of the Cauliflower mosaic virus (CaMV) 35S promoter into the A. tumefaciens vir helper strain EHA105, the binary plasmid size increased in all 15 transconjugants studied. Southern blot analysis of the transconjugants with the binary plasmid probe revealed that the 35S promoter and its proximal sequences in the T-DNA were rearranged. The rearranged sequences harboured the 1.3-kb IS426 element of A. tumefaciens. Conjugal mobilisation of the binary plasmid pCam-SLCMV-asRep, with the SLCMV Rep gene in antisense orientation, did not cause DNA rearrangement in EHA105. A mutated SLCMV Rep, in which a frame shift mutation caused retention of only 27 of the 351 amino acids, did not cause IS426 transposition in A. tumefaciens. These findings show that the multifunctional begomoviral Rep protein of SLCMV triggers transposition of IS426 in Agrobacterium.
