First report of natural infection by cowpea mild mottle virus on Sesbania cannabina and Physalis angulata in China.

Abstract

Vigna unguiculata (L.) Walp. (cowpea) is an important economic vegetable in Sanya, Hainan Province, China. But it is easily infected by various viral diseases, especially cowpea mild mottle virus (CPMMV), bean common mosaic virus (BCMV) and cowpea polevirus 2 (CPPV2), causing symptoms such as yellowing, dwarfism and mosaic of leaves, affecting photosynthesis of cowpea. Weeds are perfect intermediate hosts for crop viruses. In the cowpea fields of Sanya, there are a large number of weeds growing nearby, including Sesbania cannabina (Retz.) Poir. and Physalis angulata (L.). S. cannabina, a species of legume, has high nutritional value as the one of the most admired green-manuring plants widely used in crop field for gaining yield and decreasing the quantity of chemical fertilizer (Tiwari et al. 2004). P. angulata, commonly known as husk tomato, is an annual Solanaceaea vegetable crop grown for its fruits (Ramakrishna Pillai et al. 2022). In July 2022, S. cannabina and P. angulata surrounding one hectare of cowpea fields of Sanya with chlorotic and asymptomatic (Figure S1) were collected, and then their total RNAs were individually extracted by the TRIzol Reagent (Life, USA), and two cDNA libraries (one including the RNA samples of S. cannabina, another including the RNA sample of P. angulata) were subsequently constructed using VAHTS Universal V8 RNA-seq Library Prep Kit (Vazyme, China). High-throughput sequencing was performed on a DNBSEQ-T7 platform (BGI, China) with the 150 bp paired-end method. A total of 40,134,299 and 149,473,308 clean reads were de novo assembled by SPAdes 3.15.5 to generate contigs, followed by BLAST-x/n search in viral genomes in GenBank. From the RNA-seq library of S. cannabina, we identified one contig (6255 nt) that exhibited 96.45% nucleotide identity to the CPMMV isolate CPMMV-JS (MT366555), covering 6033 bp out of 6255 bp (6033/6255) of the contig. Additionally, we found seven contigs (ranging in length from 1870 nt to 8336 nt) in the RNA-seq library of P. angulata that showed 78.92% to 84.15% nucleotide identity to the CPMMV isolate Brunt (OK558693), with specific alignment lengths varying from 669/795 to 4244/5337. CPMMV was verified by reverse transcription - polymerase chain reaction (RT-PCR) detection using specific primer (F: 5'-CTGGCCAAGTGGTTTGTTTT-3', R: 5'- AACCGGGTTTACAATCCACA -3') (Wei et al. 2020). PCR products were extracted from an agarose gel, cloned into the pMD19-T Vector (Takara, China), and transformed into Escherichia coli DH5α cells (Sangon Biotech, Shanghai). Two clones for each PCR product were sequenced by Sanger sequencing. The amplified 1865 bp fragments of S. cannabina (PP790744) and P. angulata (PP790743) had 98.07% identity (1727 bp out to 1761 bp) and 97.92% sequence identity (1739 bp out to 1776 bp) to CPMMV-JS (MT366555). Afterwards, we collected S. cannabina and P. angulata with similar symptoms around two cowpea fields (each field covers an area of about one hectare) in three districts of Sanya, namely Yazhou, Tianya, and Haitang, and conducted CPMMV detection. We found that CPMMV was detected in all six samples. CPMMV, including genus Carlavirus and family Betaflexiviridae, was first identified on cowpea in Ghana in the 1970s. Since then, it has been shown to have a very extensive geographical distribution and a wide natural host range. CPMMV principally infects Fabaceae, it is also able to infect hosts from Solanaceaea, Nyctaginaceae, Cleomaceae, Asteraceae and Lamiaceae (Celli et al. 2016; Lamas et al. 2017; Zanardo and Carvalho 2017). To our knowledge, this is the first report of natural infection of the S. cannabina and P. angulata by CPMMV, which may represent additional reservoirs of this virus in the region. Meanwhile, investigating CPMMV in S. cannabina and P. angulata is crucial for developing integrated strategies to prevent its transmission to economically important plants such as cowpea.