Mulberry mosaic dwarf disease (MMDD) is a graft-transmissible syndrome long-known in China, where it can seriously affect silk farming industry. Infected mulberry plants showed severe dwarf symptoms on mulberry leaves such as mottling, deformation, curling and puckering showing a typical MMDD (Figure 1). Although studies on this disease have lasted for nearly a half century, the etiology is not yet clear.

Recently, the cooperation between the Institute of Plant Protection of CAAS and the Institute for Sustainable Plant Protection of Italian National Research Council has made a great progress in etiology research on MMDD. A novel geminivirus was identified from mulberry trees using next generation sequencing (NGS) technology. Experiment results revealed a close association of the geminivirus with MMDD, so we tentatively named this novel geminivirus as mulberry mosaic dwarf associated virus (MMDaV). The related research results have been published online in Journal of General Virology on May 6, 2015.

Rolling circle amplification (RCA) and polymerase chain reaction (PCR) with specific primers, followed by sequencing of eight independent full-length clones, showed that this virus has a monopartite circular single strand DNA (ssDNA) genome (~2.95 kb) containing seven Open Reading Frames (ORFs) in both polarity strands, as previously reported for geminiviruses. MMDaV genome codes for five and two putative proteins in the virion-sense and in the complementary-sense strands, respectively. As previously reported for members of the genera Mastrevirus and Becurtovirus, MMDaV replication-associated proteins are expressed through the alternative splicing of an intron, which was shown to be functional in vivo. A similar intron was found in the genome of citrus chlorotic dwarf associated virus (CCDaV) - a divergent geminivirus recently found in citrus. Based on pairwise comparisons and phylogenetic analyses, CCDaV and MMDaV appear to be closely related to each other, thus supporting their inclusion in a putative novel genus in the family Geminiviridae.