Roles of two distinct alphasatellites modulating geminivirus pathogenesis

Geminiviridae and Alphasatellitidae Diversity Revealed by Metagenomic Analysis of Susceptible and Tolerant Tomato Cultivars across Distinct Brazilian Biomes

The diversity of Geminiviridae and Alphasatellitidae species in tomatoes was assessed via high-throughput sequencing of 154 symptomatic foliar samples collected from 2002 to 2017 across seven Brazilian biomes. The first pool (BP1) comprised 73 samples from the North (13), Northeast (36), and South (24) regions. Sixteen begomoviruses and one Topilevirus were detected in BP1. Four begomovirus-like contigs were identified as putative novel species (NS). NS#1 was reported in the semi-arid (Northeast) region and NS#2 and NS#4 in mild subtropical climates (South region), whereas NS#3 was detected in the warm and humid (North) region. The second pool (BP2) comprised 81 samples from Southeast (39) and Central-West (42) regions. Fourteen viruses and subviral agents were detected in BP2, including two topileviruses, a putative novel begomovirus (NS#5), and two alphasatellites occurring in continental highland areas. The five putative novel begomoviruses displayed strict endemic distributions. Conversely, tomato mottle leaf curl virus (a monopartite species) displayed the most widespread distribution occurring across the seven sampled biomes. The overall diversity and frequency of mixed infections were higher in susceptible (16 viruses + alphasatellites) in comparison to tolerant (carrying the Ty-1 or Ty-3 introgressions) samples, which displayed 9 viruses. This complex panorama reinforces the notion that the tomato-associated Geminiviridae diversity is yet underestimated in Neotropical regions.

Development of Stable Infectious cDNA Clones of Tomato Black Ring Virus Tagged with Green Fluorescent Protein

Tomato black ring virus (TBRV) is a member of the Nepovirus genus in the Secoviridae family, which infects a wide range of important crop species worldwide. In this work, we constructed four cDNA infectious clones of the TBRV tagged with the green fluorescent protein (TBRV-GFP), which varied in (i) the length of the sequences flanking the GFP insert, (ii) the position of the GFP insert within the RNA2 polyprotein, and (iii) the addition of a self-cutting 2A protein. The presence of the GFP coding sequence in infected plants was verified by RT-PCR, while the infectivity and stability of the constructs were verified by mechanical inoculation of the host plants. The systemic spread of TBRV-GFP within plants was observed under UV light at a macroscopic level, monitoring GFP-derived fluorescence in leaves, and at a microscopic level using confocal microscopy. The obtained clones are a valuable tool for future studies of TBRV-host interactions, virus biology, and the long-term monitoring of its distribution in infected plants.

Geminiviral betasatellites: critical viral ammunition to conquer plant immunity

Geminiviruses have mastered plant cell modulation and immune invasion to ensue prolific infection. Encoding a relatively small number of multifunctional proteins, geminiviruses rely on satellites to efficiently re-wire plant immunity, thereby fostering virulence. Among the known satellites, betasatellites have been the most extensively investigated. They contribute significantly to virulence, enhance virus accumulation, and induce disease symptoms. To date, only two betasatellite proteins, βC1, and βV1, have been shown to play a crucial role in virus infection. In this review, we offer an overview of plant responses to betasatellites and counter-defense strategies deployed by betasatellites to overcome those responses.

Patterns of Genetic Diversity among Alphasatellites Infecting Gossypium Species

Alphasatellites are small single-stranded circular DNA molecules associated with geminiviruses and nanoviruses. In this study, a meta-analysis of known alphasatellites isolated from the genus Gossypium (cotton) over the last two decades was performed. The phylogenetic and pairwise sequence identity analysis suggested that cotton-infecting begomoviruses were associated with at least 12 different alphasatellites globally. Three out of twelve alphasatellite were associated with cotton leaf curl geminiviruses but were not isolated from cotton plants. The cotton leaf curl Multan alphasatellite, which was initially isolated from cotton, has now been reported in several plant species, including monocot plants such as sugarcane. Our recombination analysis suggested that four alphasatellites, namely cotton leaf curl Lucknow alphasatellites, cotton leaf curl Multan alphasatellites, Ageratum yellow vein Indian alphasatellites and Ageratum enation alphasatellites, evolved through recombination. Additionally, high genetic variability was detected among the cotton-infecting alphasatellites at the genome level. The nucleotide substitution rate for the replication protein of alphasatellites (alpha-Rep) was estimated to be relatively high (~1.56 × 10−3). However, unlike other begomoviruses and satellites, the first codon position of alpha-Rep rapidly changed compared to the second and third codon positions. This study highlights the biodiversity and recombination of alphasatellites associated with the leaf curl diseases of cotton crops.