Huan Liu

Kiwifruit (Actinidia spp.) has long been appreciated for its desirable flavor and high nutritional value. In this study, leaf samples with chlorotic mottling symptoms were collected from kiwifruit plants in Hanzhong, Shaanxi Province of China. Using high-throughput sequencing, a novel totivirus tentatively designated Actinidia totivirus 1 (AcToV1) was identified. Its complete genome sequence was determined using RT-PCR and RACE techniques. The genome of AcToV1 is 5,060 nucleotides in length and contains two open reading frames (ORFs), encoding a coat protein (CP) of 793 amino acids and an RNA-dependent RNA polymerase (RdRp) of 841 amino acids. It also features a typical slippery heptanucleotide motif, a conserved element characteristic of the genus Totivirus. AcToV1 shares the highest genome-wide nucleotide sequence identity of 57.6% with Panax notoginseng virus A (PnVA; GenBank accession NO. KT388111). Its CP and RdRp also exhibit the highest amino acid sequence identities to PnVA, at 44.0% and 56.7%, respectively. Phylogenetic analysis further supported that AcToV1 clusters closely with PnVA and Hubei toti-like virus 2 (HTLV2). Therefore, AcToV1 be considered a new species of the genus Totivirus.

Autophagy is an evolutionarily conserved cellular-degradation mechanism implicated in antiviral defense in plants. Studies have shown that autophagy suppresses virus accumulation in cells; however, it has not been reported to specifically inhibit viral spread in plants. This study demonstrated that infection with citrus leaf blotch virus (CLBV; genus Citrivirus, family Betaflexiviridae) activated autophagy in Nicotiana benthamiana plants as indicated by the increase of autophagosome formation. Impairment of autophagy through silencing of N. benthamiana autophagy-related gene 5 (NbATG5) and NbATG7 enhanced cell-to-cell and systemic movement of CLBV; however, it did not affect CLBV accumulation when the systemic infection had been fully established. Treatment using an autophagy inhibitor or silencing of NbATG5 and NbATG7 revealed that transiently expressed movement protein (MP), but not coat protein, of CLBV was targeted by selective autophagy for degradation. Moreover, we identified that CLBV MP directly interacted with NbATG8C1 and NbATG8i, the isoforms of autophagy-related protein 8 (ATG8), which are key factors that usually bind cargo receptors for selective autophagy. Our results present a novel example in which autophagy specifically targets a viral MP to limit the intercellular spread of the virus in plants.