Plant Antiviral Immunity Against Geminiviruses and Viral Counter-Defense for Survival

Abstract

The family Geminiviridae includes plant-infecting viruses whose genomes are composed of one or two circular non-enveloped ssDNAs(+) of about 2.5–5.2 kb each in size. These insect-transmissible geminiviruses cause significant crop losses across continents and pose a serious threat to food security. Under the control of promoters generally located within the intergenic region, their genomes encode five to eight ORFs from overlapping viral transcripts. Most proteins encoded by geminiviruses perform multiple functions, such as suppressing defense responses, hijacking ubiquitin-proteasomal pathways, altering hormonal responses, manipulating cell cycle regulation, and exploiting protein-signaling cascades. Geminiviruses establish complex but coordinated interactions with several host elements to spread and facilitate successful infection cycles. Consequently, plants have evolved several multilayered defense strategies against geminivirus infection and distribution. Recent studies on the evasion of host-mediated resistance factors by various geminivirus proteins through novel mechanisms have provided new insights into the development of antiviral strategies against geminiviruses. This review summarizes the current knowledge concerning virus movement within and between cells, as well as the recent advances in our understanding of the biological roles of virus-encoded proteins in manipulating host-mediated responses and insect transmission. This review also highlights unexplored areas that may increase our understanding of the biology of geminiviruses and how to combat these important plant pathogens.

Molecular Insights into Host and Vector Manipulation by Plant Viruses

“…Despite encoding only a few proteins, most begomoviruses are able to manipulate various host components and pathways. In particular, they have evolved diverse RSS targeting plant defence mechanisms and capable of reprogramming multiple pathways such as transcriptional and post-transcriptional gene silencing, hormonal responses, ubiquitin-dependant degradation, autophagy and antiviral chloroplast defence [51][52][53]. Most monopartite begomoviruses associate with betasatellites that enhance accumulation of the helper virus and are essential for induction of typical disease symptoms.…”

Susceptibility to South African cassava mosaic virus is Associated with a RING Finger E3 Ubiquitin Ligase

“…Some E3 ligases and viral proteins can localise to the nucleus, such as the tobacco E3 ligase, NtHUB1 which has a nuclear localisation sequence, is recruited by geminiviral Rep protein, and co-localises and interacts with the Rep protein to monoubiquitinate cellular chromatin and thus enable infection (Kushwaha et al, 2017). The viral coat protein, CP, also has a nuclear localisation signal, can localise in the nucleolus and nucleoplasm, and facilitates entry of ssDNA into the nucleus (Wang et al, 2017;Kumar, 2019). However, the mechanisms for these processes are yet to be investigated.…”

“…Begomoviruses signi cantly contribute to reduced production of economically and nutritionally important crops by causing diseases such as Pumpkin yellow mosaic disease, Tomato yellow vein streak disease, Bean dwarf mosaic disease, Sweet potato leaf curl disease, Squash mild leaf curl disease, and Cassava mosaic disease (Ramesh et al, 2017;Rey and Vanderschuren, 2017;Yang et al, 2019). Geminiviruses have the ability to reprogram host cellular processes for their own bene t (Voinnet et al, 1999;Hanley-Bowdoin et al, 2004;Hanley-Bowdoin et al, 2013;Wu et al, 2019), and one such process is ubiquitination (Kumar, 2019). They interfere with host UPSs to redirect protein metabolism, alter defense processes (Ramesh et al, 2017) and even suppress the plant's response to insect vectors (Li et al, 2019).…”

A Begomovirus Nuclear Shuttle Protein-Interacting Immune Hub: Hijacking Host Transport Activities and Suppressing Incompatible Functions

“…Begomovirus represents the largest genus of the Geminiviridae family and consists of whiteflytransmitted single-stranded DNA viruses, which severely inflict several important crops and vegetables in tropical and subtropical regions (Rojas et al, 2018). Species of the Begomovirus genus can be either monopartite (with one single genomic component) or bipartite (two genomic components, referred to as DNA-A and DNA-B) (Zerbini et al, 2017;Kumar, 2019). The nuclear shuttle protein (NSP) is encoded by the DNA-B of bipartite begomoviruses, which also encodes the movement protein (MP).…”

“…The host range of begomovirus species depends on multiple molecular interactions involved in both co-opting the cellular machinery and evading the innate immune recognition (for reviews see Hanley-Bowdoin et al, 2013;Li et al, 2018;Kumar, 2019). Failure of any one of these steps can decrease or impair the virus proliferation on a given host.…”