One DNA A (KA30) and five different DNA B components (KA21, KA22, KA27, KA28 and KA34) of a geminivirus, Mungbean yellow mosaic virus-Vigna (MYMV-Vig) were cloned from a pooled sample of field-infected Vigna mungo plants from Vamban, South India. MYMV-Vig DNA A (KA30) and one of the DNA B components (KA27) exhibited 97% and 95% sequence identities, respectively, to those of MYMV reported from Thailand. However, the DNA B components KA21, KA22, KA28 and KA34 exhibited only 71 to 72% sequence identity to MYMV DNA B. Co-existence of multiple DNA B components in field-infected V. mungo was proved by Southern and PCR analyses. Each of the five DNA B components was infective together with the DNA A upon agroinoculation. Agroinoculation with mixed cultures of Agrobacterium with partial dimers of DNA A and all five DNA Bs proved that all five DNA B components can co-infect a single V. mungo plant.

Mungbean yellow mosaic virus-Vigna (MYMV-Vig) is a bipartite geminivirus that causes a severe yellow mosaic disease in blackgram. An assay was developed to study MYMV-Vig replication by agroinoculation of tobacco leaf discs with partial dimers of the virus. This assay, in a non-host model plant, was used to evaluate pathogen-derived resistance contributed by MYMV-Vig genes in transgenic plants. Viral DNA accumulation was optimum in tobacco leaf discs cultured for 10 days after infection with Agrobacterium tumefaciens strain Ach5 containing partial dimers of both DNA A and DNA B of MYMV-Vig. Transgenic tobacco plants with MYMV-Vig genes for coat protein (CP), replication-associated protein (Rep)-sense, Rep-antisense, truncated Rep (T-Rep), nuclear shuttle protein (NSP) and movement protein (MP) were generated. Leaf discs from transgenic tobacco plants, harbouring MYMV-Vig genes, were agroinoculated with partial dimers of MYMV-Vig and analyzed for viral DNA accumulation. The leaf discs from transgenic tobacco plants harbouring CP and MP genes supported the accumulation of higher levels of MYMV-Vig DNA. However, MYMV-Vig accumulation was inhibited in one transgenic plant harbouring the Rep-sense gene and in two plants harbouring the T-Rep gene. Northern analysis of these plants revealed a good correlation between expression of Rep or T-Rep genes and inhibition of MYMV-Vig accumulation.

Mungbean yellow mosaic virus-Vigna (MYMV-Vig), a Begomovirus that causes yellow mosaic disease, was cloned from field-infected blackgram (Vigna mungo). One DNA A clone (KA30) and five different DNA B clones (KA21, KA22, KA27, KA28 and KA34) were obtained. The sequence identity in the 150-nt common region (CR) between DNA A and DNA B was highest (95%) for KA22 DNA B and lowest (85.6%) for KA27 DNA B. The Rep-binding domain had three complete 11-nt (5'-TGTATCGGTGT-3') iterons in KA22 DNA B (and KA21, KA28 and KA34), while the first iteron in KA27 DNA B (5'-ATCGGTGT-3') had a 3-nt deletion. KA27 DNA B, which exhibited 93.9% CR sequence identity to the mungbean-infecting MYMV, also shared the 3-nt deletion in the first iteron besides having an 18-nt insertion between the third iteron and the conserved nonanucleotide. MYMV was found to be closely related to KA27 DNA B in amino acid sequence identity of BV1 (94.1%) and BC1 (97.6%) proteins and in the organization of nuclear localization signal (NLS), nuclear export signal (NES) and phosphorylation sites. Agroinoculation of blackgram (V. mungo) and mungbean (V. radiata) with partial dimers of KA27 and KA22 DNA Bs along with DNA A caused distinctly different symptoms. KA22 DNA B caused more intense yellow mosaic symptoms with high viral DNA titre in blackgram. In contrast, KA27 DNA B caused more intense yellow mosaic symptoms with high viral DNA titre in mungbean. Thus, DNA B of MYMVVig is an important determinant of host-range between V. mungo and V. radiata.

The vir genes of octopine, nopaline, and L,L-succinamopine Ti plasmids exhibit structural and functional similarities. However, we observed differences in the interactions between octopine and nopaline vir components. The induction of an octopine virE A6 ::lacZ fusion (pSM358cd) was 2.3-fold higher in an octopine strain (A348) than in a nopaline strain (C58). Supplementation of the octopine virG A6 in a nopaline strain with pSM358 did not completely restore virE A6 induction. However, addition of the octopine virA A6 to the above strain increased virE A6 induction to a level almost comparable to that in octopine strains. In a reciprocal analysis, the induction of a nopaline virE C58 ::cat fusion (pUCD1553) was two-to threefold higher in nopaline (C58 and T37) strains than in octopine (A348 and Ach5) and L,L-succinamopine (A281) strains. Supplementation of nopaline virA C58 and virG C58 in an octopine strain (A348) harboring pUCD1553 increased induction levels of virE C58 ::cat fusion to a level comparable to that in a nopaline strain (C58). Our results suggest that octopine and L,L-succinamopine VirG proteins induce the octopine virE A6 more efficiently than they do the nopaline virE C58 . Conversely, the nopaline VirG protein induces the nopaline virE C58 more efficiently than it does the octopine virE A6 . The ability of Bo542 virG to bring about supervirulence in tobacco is observed for an octopine vir helper (LBA4404) but not for a nopaline vir helper (PMP90). Our analyses reveal that quantitative differences exist in the interactions between VirG and vir boxes of different Ti plasmids. Efficient vir gene induction in octopine and nopaline strains requires virA, virG, and vir boxes from the respective Ti plasmids.Agrobacterium tumefaciens has the unique capability of transferring the T-DNA portion of its Ti plasmid into plant cells at infected wound sites, which results in the formation of crown gall tumors (8, 9, 73). The infection process involves a set of chromosome-encoded genes (chv) involved in attachment of bacteria to plant cells and Ti plasmid-encoded vir genes that function in trans, helping in the generation, transfer, and integration of T strands into the plant genome (reviewed in references 19, 31, and 32).Sensing of signal molecules released by wounded plant cells is the first step of signal transduction leading to vir gene induction in Agrobacterium (68, 70). VirA, an inner membrane protein, senses the signal molecules (45, 48) and gets autophosphorylated in the His-474 residue (33, 36). The phosphorylated VirA in turn activates the cytoplasmic protein VirG by phosphorylating it at Asp-52 (35). VirG, a DNA binding protein (54, 56), acts as a transcriptional activator of vir genes by binding to vir boxes present upstream of all vir operons (11, 37). Based on protein sequence similarities, VirA and VirG have been assigned to a large group of His-Asp two-component regulatory systems, involving a sensor and a response regulator (45, 76).The T-DNA portion of the Ti plasmid carries genes that specif...