A newly isolated cotton-infecting Polerovirus with cryptic pathogenicity encodes a weak suppressor of RNA silencing

Abstract

Cotton is a multipurpose crop grown globally, including the United States. Cotton leafroll dwarf virus (CLRDV), a phloem-limited virus (Solemoviridae) transmitted by aphids, causes significant economic losses to cotton cultivation. CLRDV strains (CLRDV-typical and atypical) that were previously prevalent in other countries cause severe symptoms leading to high yield loss. Recently, a new isolate of CLRDV (CLRDV-AL) has been characterized from infected cotton plants in Alabama that are often asymptomatic and difficult to detect, implying a low titer and pathogenicity within the host. Different pathogenicity among certain strains within the same species often correlates with both environmental and molecular factors. Thus, better management and control of the vector-borne disease can be achieved by elucidating host-pathogen interaction, such as host immune response and pathogen counter-response. In this study, we demonstrate the ability of CLRDV-AL to suppress a major host defense response known as RNA silencing and compare the potency of silencing suppression to other strains of the same virus. Also, we discuss the difference in pathogenicity among them by evaluating the observations based on the amino acid variation within the functional domain. Our study provides and suggests a future direction for specifying the strategy to mitigate potential cotton disease severity.

Functional Characterization of RNA Silencing Suppressor Encoded by Cotton Leafroll Dwarf Virus

“…Similar changes in symptom development, VSR potency, intracellular localization, and viral pathogenicity resulting from aa substitution within the F-box-like motif were also reported in other members of the genus Polerovirus [15][16][17]29,31,42]. Interestingly, our recent study reported that cryptic pathogenicity displayed by a new strain of CLRDV, recently isolated from the U.S. and designated as CLRDV-AL [2,7], could be due to relatively weaker P0 protein activity as a VSR compared to other strains [24]. In that study, we discussed the three different aa residues within CLRDV-AL's F-box-like motif compared to CLRDV-ty and CLRDV-at.…”

“…The cDNA constructs of the ORF 0 from three different CLRDV strains (-ty, -at, and -AL) placed under the CaMV 35S promoter sequence in the binary plasmid pAIDEE (pAI-P0 ty , pAI-P0 at , and pAI-P0 AL , respectively) were previously described [24]. The cDNA constructs expressing a series of mutant P0 AL , which carry a single aa substitution mutation within the F-box-like motif, were constructed by Q5 site-directed mutagenesis kit (New England Biolabs, Ipswich, MA, USA).…”

“…The relative expression levels of GFP mRNA were evaluated as previously described [24]. Briefly, 20 ng total RNA extracted using the RNeasy Plant Mini Kit (Qiagen, Hilden, Germany) was used as a template with specific pairs of oligomers as primers for RT-qPCR reactions in a total volume of 10 µL using the Luna ® universal RT-qPCR system (New England Biolabs, Ipswich, MA, USA).…”

“…In our previous study [24], we reported that the P0 protein encoded by CLRDV-AL (P0 AL ) acts as a weak VSR like the P0 protein of CLRDV-at (P0 at ) and hypothesized its potential correlation with the cryptic traits of the virus during host infection. While P0 AL shares a conserved F-box-like motif with other CLRDV strains (Figure 1B), there are notable differences in specific aa residues when compared to CLRDV-ty and CLRDV-at [24]. We postulated that these residues might contribute to the distinctive characteristics of P0 AL and the cryptic traits of CLDRV-AL.…”

“…However, plant viruses have evolved their proteins to interfere with this process. In the case of the members of the genus Polerovirus, this interference is accomplished through the P0 proteins [12,14,15,17,[24][25][26][27][28][29]. Consequently, the P0 protein is often associated with virus pathogenicity and symptom development [12,16,17,[30][31][32].…”