Inhibition of Bemisia tabaci vectored, GroEL mediated transmission of tomato leaf curl New Delhi virus by garlic leaf lectin (Allium sativum leaf agglutinin)

GroEL or symbionin synthesized by the endosymbionts of whitefly (Bemisia tabaci)/ aphids play a cardinal role in the persistent, circulative transmission of plant viruses by binding to viral coat protein/ read-through protein. Allium sativum leaf agglutinin (ASAL), a Galanthus nivalis agglutinin (GNA)- related mannose-binding lectin from garlic leaf has been reported as a potent controlling agent against hemipteran insects including whitefly and aphids. GroEL related chaperonin- symbionin was previously identified as a receptor of ASAL by the present group in the brush border membrane vesicle (BBMV) of mustard aphid. In the present study similar GroEL receptor of ASAL has been identified through LC-MS/MS in the BBMV of B. tabaci which serves as a vector for several plant viruses including tomato leaf curl New Delhi virus (ToLCNDV). Ligand blot analysis of ASAL-fed B. tabaci showed that when GroEL is pre-occupied by ASAL, it completely blocks its further binding to ToLCNDV coat protein (ToLCNDV-CP). Prior feeding of ASAL hindered the co-localization of ToLCNDV-CP and GroEL in the midgut of B. tabaci. Immunoprecipitation followed by western blot with ASAL-fed B. tabaci yielded similar result. Moreover, ASAL feeding inhibited viral transmission by B. tabaci. Together, these results confirmed that the interaction of ASAL with GroEL interferes with the binding of ToLCNDV-CP and inhibits further B. tabaci mediated viral transmission.

Tomato geminivirus encoded RNAi suppressor protein, AC4 interacts with host AGO4 and precludes viral DNA methylation

Plant RNA silencing systems are organized as a network, regulating plant developmental pathways and restraining invading viruses, by sharing cellular components with overlapping functions. Host regulatory networks operate either at the transcriptional level via RNA-directed DNA methylation, or at the post-transcriptional stage interfering with mRNA to restrict viral infection. However, viral-derived proteins, including suppressors of RNA silencing, favour virus establishment, and also affect plant developmental processes. In this investigation, we report that Tomato leaf curl New Delhi virus-derived AC4 protein suppresses RNA silencing activity and mutational analysis of AC4 showed that Asn-50 in the SKNT-51 motif, in the C-terminal region, is a critical determinant of its RNA silencing suppressor activity. AC4 showed interaction with host AGO4 but not with AGO1, aggregated around the nucleus, and influenced cytosine methylation of the viral genome. The possible molecular mechanism by which AC4 interferes in the RNA silencing network, helps virus establishment, and affects plant development is discussed.

Identification, characterization, expression profiling, and virus-induced gene silencing of armadillo repeat-containing proteins in tomato suggest their involvement in tomato leaf curl New Delhi virus resistance

Armadillo repeat family is well-characterized in several plant species for their involvement in multiple regulatory processes including growth, development, and stress response. We have previously shown a three-fold higher expression of ARM protein-encoding in tomato cultivar tolerant to tomato leaf curl New Delhi virus (ToLCNDV) compared to susceptible cultivar upon virus infection. This suggests the putative involvement of ARM proteins in defense response against virus infection; however, no comprehensive investigation has been performed to address this inference. In the present study, we have identified a total of 46 ARM-repeat proteins (SlARMs), and 41 U-box-containing proteins (SlPUBs) in tomato. These proteins and their corresponding genes were studied for their physicochemical properties, gene structure, domain architecture, chromosomal localization, phylogeny, and cis-regulatory elements in the upstream promoter region. Expression profiling of candidate genes in response to ToLCNDV infection in contrasting tomato cultivars showed significant upregulation of SlARM18 in the tolerant cultivar. Virus-induced gene silencing of SlARM18 in the tolerant tomato cultivar conferred susceptibility, which suggests the involvement of this gene in resistance mechanism. Further studies are underway to functionally characterize SlARM18 to delineate its precise role in defense mechanism.