Historical Review on Sugarcane Streak Mosaic Virus That Has Recently Emerged in Africa.

Sugarcane streak mosaic virus (SCSMV), now assigned to the genus Poacevirus of the family Potyviridae, was reported for the first time in 1932 in Louisiana and was believed to be strain F of sugarcane mosaic virus (SCMV) for more than six decades. SCMV-F was renamed SCSMV in 1998 after partial sequencing of its genome and phylogenetic investigations. Following the development of specific molecular diagnostic methods in the 2000s, SCSMV was recurrently found in sugarcane exhibiting streak mosaic symptoms in numerous Asian countries, but not in the Western hemisphere or in Africa. In this review, we give an overview of the current knowledge on this disease and the progression in research on SCSMV. This includes symptoms, geographical distribution and incidence, diagnosis and genetic diversity of the virus, epidemiology, as well as control. Finally, we highlight future challenges as sugarcane streak mosaic has recently been found in Africa where this disease represents a new threat to sugarcane production.

Virion-Associated Nucleic Acid-Based Metagenomics: A Decade of Advances in Molecular Characterization of Plant Viruses.

Over the last decade, viral metagenomic studies have resulted in the discovery of thousands of previously unknown viruses. These studies are likely to play a pivotal role in obtaining an accurate and robust understanding of how viruses affect the stability and productivity of ecosystems. Among the metagenomics-based approaches that have been developed since the beginning of the 21st century, shotgun metagenomics applied specifically to virion-associated nucleic acids (VANA) has been used to disentangle the diversity of the viral world. We summarize herein the results of 24 VANA-based studies, focusing on plant and insect samples conducted over the last decade (2010 to 2020). Collectively, viruses from 85 different families were reliably detected in these studies, including capsidless RNA viruses that replicate in fungi, oomycetes, and plants. Finally, strengths and weaknesses of the VANA approach are summarized and perspectives of applications in detection, epidemiological surveillance, environmental monitoring, and ecology of plant viruses are provided. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Occurrence of a novel mastrevirus in sugarcane germplasm collections in Florida, Guadeloupe and Réunion.

BACKGROUND: In Africa and Asia, sugarcane is the host of at least seven different virus species in the genus Mastrevirus of the family Geminiviridae. However, with the exception of Sugarcane white streak virus in Barbados, no other sugarcane-infecting mastrevirus has been reported in the New World. Conservation and exchange of sugarcane germplasm using stalk cuttings facilitates the spread of sugarcane-infecting viruses. METHODS: A virion-associated nucleic acids (VANA)-based metagenomics approach was used to detect mastrevirus sequences in 717 sugarcane samples from Florida (USA), Guadeloupe (French West Indies), and Réunion (Mascarene Islands). Contig assembly was performed using CAP3 and sequence searches using BLASTn and BLASTx. Mastrevirus full genomes were enriched from total DNA by rolling circle amplification, cloned and sequenced. Nucleotide and amino acid sequence identities were determined using SDT v1.2. Phylogenetic analyses were conducted using MEGA6 and PHYML3. RESULTS: We identified a new sugarcane-infecting mastrevirus in six plants sampled from germplasm collections in Florida and Guadeloupe. Full genome sequences were determined and analyzed for three virus isolates from Florida, and three from Guadeloupe. These six genomes share >88% genome-wide pairwise identity with one another and between 89 and 97% identity with a recently identified mastrevirus (KR150789) from a sugarcane plant sampled in China. Sequences similar to these were also identified in sugarcane plants in Réunion. CONCLUSIONS: As these virus isolates share <64% genome-wide identity with all other known mastreviruses, we propose classifying them within a new mastrevirus species named Sugarcane striate virus. This is the first report of sugarcane striate virus (SCStV) in the Western Hemisphere, a virus that most likely originated in Asia. The distribution, vector, and impact of SCStV on sugarcane production remains to be determined.

Bioassay to Study the Attachment of Xanthomonas albilineans on Sugarcane Leaves.

Sugarcane (interspecific hybrids of Saccharum species) is an economically important crop that provides 70% of raw table sugar production worldwide and contributes, in some countries, to bioethanol and electricity production. Leaf scald, caused by the bacterial plant pathogen Xanthomonas albilineans, is one of the major diseases of sugarcane. Dissemination of X. albilineans is mainly ensured by contaminated harvesting tools and infected stalk cuttings. However, some strains of this pathogen are transmitted by aerial means and are able to survive as epiphytes on the sugarcane phyllosphere before entering the leaves and causing disease. Here we present a protocol to estimate the capacity of attachment of X. albilineans to sugarcane leaves. Tissue-cultured sugarcane plantlets were immersed in a bacterial suspension of X. albilineans and leaf attachment of X. albilineans was determined by two methods: leaf imprinting (semi-quantitative method) and leaf washing/homogenization (quantitative method). These methods are important tools for evaluating pathogenicity of strains/mutants of the sugarcane leaf scald pathogen.

Surface polysaccharides and quorum sensing are involved in the attachment and survival of Xanthomonas albilineans on sugarcane leaves.

Xanthomonas albilineans, the causal agent of sugarcane leaf scald, is a bacterial plant pathogen that is mainly spread by infected cuttings and contaminated harvesting tools. However, some strains of this pathogen are known to be spread by aerial means and are able to colonize the phyllosphere of sugarcane before entering the host plant and causing disease. The objective of this study was to identify the molecular factors involved in the survival or growth of X. albilineans on sugarcane leaves. We developed a bioassay to test for the attachment of X. albilineans on sugarcane leaves using tissue-cultured plantlets grown in vitro. Six mutants of strain XaFL07-1 affected in surface polysaccharide production completely lost their capacity to survive on the sugarcane leaf surface. These mutants produced more biofilm in vitro and accumulated more cellular poly-ß-hydroxybutyrate than the wild-type strain. A mutant affected in the production of small molecules (including potential biosurfactants) synthesized by non-ribosomal peptide synthetases (NRPSs) attached to the sugarcane leaves as well as the wild-type strain. Surprisingly, the attachment of bacteria on sugarcane leaves varied among mutants of the rpf gene cluster involved in bacterial quorum sensing. Therefore, quorum sensing may affect polysaccharide production, or both polysaccharides and quorum sensing may be involved in the survival or growth of X. albilineans on sugarcane leaves.

Leaf Surface Colonization of Sugarcane by Xanthomonas albilineans and Subsequent Disease Progress Vary According to the Host Cultivar.

Spread of leaf scald in modern sugarcane cultivars in Guadeloupe occurs through aerial dissemination of Xanthomonas albilineans. However, the importance of host genotype on the foliar spread of leaf scald has never been investigated. To explore this, we followed two trials used to screen sugarcane cultivars for resistance to leaf scald under natural inoculum pressure. Leaf scald epidemic characteristics were studied by measuring epiphytic populations of X. albilineans, leaf symptom incidence and severity, and the number of infected stalks. In both trials, epiphytic X. albilineans populations and incidence of foliar symptoms varied between sugarcane cultivars (P < 0.001 in each trial for both traits) and differences in stalk infection between cultivars was also observed (P < 0.002 and P < 0.07 for trials A and B, respectively). Part of the cultivar resistance that minimizes epiphytic bacterial populations is correlated to resistance to internal leaf tissue infection as expressed by leaf symptoms. No correlation was found between epiphytic X. albilineans populations of cultivar and the incidence of stalk infection. However, foliar symptom incidence was inconsistently correlated with stalk infection. Resistance of sugarcane to leaf scald appears to involve several traits, including limiting size of epiphytic X. albilineans populations and limiting the capacity of the pathogen to produce leaf necrotic symptoms by invading the leaf vascular system or to move from the leaf into the stalk.

The RpfCG two-component system negatively regulates the colonization of sugar cane stalks by Xanthomonas albilineans.

The genome of Xanthomonas albilineans, the causal agent of sugar cane leaf scald, carries a gene cluster encoding a predicted quorum sensing system that is highly related to the diffusible signalling factor (DSF) systems of the plant pathogens Xylella fastidiosa and Xanthomonas campestris. In these latter pathogens, a cluster of regulation of pathogenicity factors (rpf) genes encodes the DSF system and is involved in control of various cellular processes. Mutation of Xanthomonas albilineans rpfF, encoding a predicted DSF synthase, in Florida strain XaFL07-1 resulted in a small reduction of disease severity (DS). Single-knockout mutations of rpfC and rpfG (encoding a predicted DSF sensor and regulator, respectively) had no effect on DS or swimming motility of the pathogen. However, capacity of the pathogen to cause disease was slightly reduced and swimming motility was severely affected when rpfG and rpfC were both deleted. Similar results were obtained when the entire rpfGCF region was deleted. Surprisingly, when the pathogen was mutated in rpfG or rpfC (single or double mutations) it was able to colonize sugar cane spatially more efficiently than the wild-type. Mutation in rpfF alone did not affect the degree of spatial invasion. We conclude that the DSF signal contributes to symptom expression but not to invasion of sugar cane stalks by Xanthomonas albilineans strain XaFL07-1, which is mainly controlled by the RpfCG two-component system.

High molecular variability of sugarcane bacilliform viruses in Guadeloupe implying the existence of at least three new species.

Thirty-five unique partial sugarcane bacilliform virus (SCBV) sequences extending over 529 bp were identified in sugarcane samples from Guadeloupe diagnosed by Immunocapture-PCR (IC-PCR) using specific badnavirus primers. Phylogenetic analysis of these sequences along with the two known genome sequences of Sugarcane bacilliform Mor virus (SCBMV) and Sugarcane bacilliform IM virus (SCBIMV) revealed high molecular variability in the SCBV genome. Seven phylogenetic groups, named A to G, were characterized: virus isolates from groups A-B, C and D are proposed to be members of three additional SCBV species. The two (7446 and 7444 bp) and one (7317 bp) complete sequences of SCBV isolates from groups A and D, respectively, likely represented the genome of two new species. Phylogenetic analysis of the complete genome and RT/RNase H sequences confirmed the polyphyletic structure of SCBV isolates and the absence of a clear separation between SCBV and Banana streak virus (BSV) isolates within badnavirus group 1. These results showed that reconsideration of taxonomy and classification of SCBV and BSV are necessary.

Variation in Infection Capacity and in Virulence Exists Between Genotypes of Sugarcane yellow leaf virus.

Two experiments, one in Guadeloupe and one in Réunion Island, were performed to transmit different genotypes of Sugarcane yellow leaf virus (SCYLV) to eight sugarcane cultivars differing in resistance to infection by the virus and to yellow leaf. Transmission was attempted from SCYLV-infected sugarcane plants or leaves to healthy tissue-cultured plantlets grown in vitro and with the aphid vector Melanaphis sacchari. After inoculation and elimination of insects with an insecticide, plantlets were transferred to Montpellier, France and grown in a greenhouse. Plants were tested for presence of SCYLV by tissue-blot immunoassay and reverse-transcription polymerase chain reaction after 5 to 6 months of growth. SCYLV genotypes BRA-PER, CUB, and REU were detected in 47, 62, and 39% of plants inoculated with these genotypes in Guadeloupe, respectively. SCYLV genotypes BRA-PER and REU and a mixed infection of genotypes BRA-PER and REU were detected in 56, 33, and 42% of plants inoculated with these genotypes in Réunion Island, respectively. Genotypes BRA-PER and CUB could be transmitted to all eight sugarcane cultivars, but genotype REU could never be transmitted to resistant sugarcane cvs. H78-4153 and H78-3567. SCYLV genotype REU was transmitted successfully to sugarcane cv. R570 in Guadeloupe, but not in Réunion Island. Genotypes BRA-PER and CUB induced yellow leaf symptoms in susceptible or highly susceptible sugarcane cultivars, whereas genotype REU induced very few symptoms. SCYLV was not found in several symptomatic plants, suggesting an association of disease with undetectable populations of the virus or a nonviral cause. This is the first report of variation in infection capacity and in virulence of SCYLV.