Genomic diversity of Areca Palm Velarivirus 1 (APV1) in Areca palm (Areca catechu) plantations in Hainan, China

Genomic diversity of Capillovirus uniheveae (Betaflexiviridae) infecting Hevea brasiliensis Muell. Arg. in Hainan, China

BACKGROUND

Rubber tree (Hevea brasiliensis Muell. Arg.) is a significant commercial crop in tropical areas worldwide, with rubber production threatened by Tapping Panel Dryness (TPD). Rubber tree virus 1 (Capillovirus uniheveae; RTV1) was identified in rubber tree samples with TPD symptoms through RNA-seq. However, its genetic diversity may have hindered the detection of RTV1 via RT-PCR, complicating the further identification of RTV1 as the causative agent of TPD. To assess RTV1 prevalence and genomic diversity, rubber tree bark samples with TPD syndrome were collected from various sites in Hainan, China, for RNA-seq and RTV1 genome determination.

RESULTS

Twenty complete RTV1 genomes were determined from 22 samples with TPD syndrome via RNA-seq and RT-PCR. Using degenerate primers based on conserved sequences in the 3'- and 5'-UTR, 20 complete RTV1 genomes were identified directly from 48 trees affected by TPD via RT-PCR. The 40 RTV1 genome sequences showed significant variations, particularly in the RdRp domain. Phylogenetic analysis of full-genome nucleotide sequences divided RTV1 isolates into three phylogroups (A, B, and C), with phylogroup A being the most prevalent (67.5%). Similar results were observed based on RdRp and CP phylogenetic analysis. Additionally, mixed infections with different genotypes were identified in the same tree. Notably, no genetic recombination was observed among different phylogroups, while ten recombination events were identified within phylogroup A.

CONCLUSIONS

RTV1 was identified in approximately 50% of samples with TPD syndrome collected in Hainan, China, with phylogroup A being the most prevalent. Considerable variations were observed in RTV1 nucleotide sequences among different phylogroups. These findings lay a foundation for accurate diagnostics, etiological characterization, and elucidation of the evolutionary relationships of RTV1 populations, providing a strong guarantee for obtaining virus-free rubber tree seedlings, and promoting the healthy and sustainable development of rubber tree plantations.

Areca palm velarivirus 1 infection caused disassembly of chloroplast and reduction of photosynthesis in areca palm

The expansion of betel palm cultivation is driven by rising demand for betel nut, yet this growth is accompanied by challenges such as decreased agricultural biodiversity and the spread of infectious pathogens. Among these, Yellow Leaf Disease (YLD) emerges as a prominent threat to betel palm plantation. Areca Palm Velarivirus 1 (APV1) has been identified as a primary causative agent of YLD, precipitating leaf yellowing, stunted growth, and diminished yield. However, the precise mechanisms underlying APV1-induced damage remain elusive. Our study elucidates that APV1 infiltrates chloroplasts, instigating severe damage and consequential reductions in chlorophyll a/b and carotene levels, alongside notable declines in photosynthetic efficiency. Moreover, APV1 infection exerts broad regulatory effects on gene expression, particularly suppressing key genes implicated in chloroplast function and photosynthesis. These disruptions correlate with growth retardation, yield diminishment, and compromised nut quality. Intriguingly, the paradoxical destruction of the host's photosynthetic machinery by APV1 prompts inquiry into its evolutionary rationale, given the virus's dependence on host resources for replication and proliferation. Our findings reveal that APV1-induced leaf yellowing acts as a beacon for transmission vectors, hinting at a nuanced "host-pathogen-vector co-evolutionary" dynamic.

Evaluation of aerial spraying application of multi-rotor unmanned aerial vehicle for Areca catechu protection

Multi-rotor unmanned aerial vehicle (UAV) is a new chemical application tool for tall stalk tropical crop Areca catechu, which could improve deposit performance, reduce operator healthy risk, and increase spraying efficiency. In this work, a spraying experiment was carried out in two A. catechu fields with two leaf area index (LAI) values, and different operational parameters were set. Spray deposit quality, spray drift, and ground loss were studied and evaluated. The results showed that the larger the LAI of A. catechu, the lesser the coverage of the chemical deposition. The maximum coverage could reach 4.28% and the minimum 0.33%. At a flight speed of 1.5 m/s, sprayed droplets had the best penetration and worst ground loss. The overall deposition effect was poor when the flight altitudes were greater than 11.09 m and the flight speed was over 2.5 m/s. Comparing flight speed of 2.5 to 1.5 m/s, the overall distance of 90% of the total drift increased to double under the same operating parameters. This study presents reference data for UAV chemical application in A. catechu protection.

Effect of temperature on yellow leaf disease symptoms and its associated areca palm velarivirus 1 titer in areca palm (Areca catechu L.)

Yellow leaf disease (YLD) has been a major limiting factor threatening areca palm commonly known as betel palm (Areca catechu L.) plantations in Hainan, China. The YLD disease is closely associated with areca palm velarivirus 1 (APV1), which belongs to the family Closteroviridae. YLD-affected betel palms show more serious yellowing symptoms in winter than in summer based on anecdotal observations. In the present work, the underlying mechanism was investigated. We first observed that the severity of YLD symptoms was closely related with the APV1 viral titer determined by qRT-PCR and ELISA under natural conditions. To further investigate whether temperature plays a key role in APV1 accumulation, the areca palm seedlings were artificially inoculated with APV1-positive mealybugs (Ferrisia virgata) and then cultivated under controlled conditions. According to our results, the YLD symptoms severity in inoculated seedlings were closely associated with temperature, e.g., severest symptoms at low temperature (16/22 ± 2°C, night/day), severer symptoms at room temperature (24/26 ± 2°C, night/day), while moderate symptoms at high temperature (27/34 ± 2°C, night/day). The qRT-PCR and ELISA results showed that APV1 titer accumulates significantly abundant at low temperature as compared to high and room temperatures. In conclusion, this is the first report about the temperature effects on the symptoms severity of YLD and APV1 titer, which may have important implications for the epidemiology of YLD.