Selective autophagy limits cauliflower mosaic virus infection by NBR1-mediated targeting of viral capsid protein and particles

Abstract

Autophagy plays a paramount role in mammalian antiviral immunity including direct targeting of viruses and their individual components, and many viruses have evolved measures to antagonize or even exploit autophagy mechanisms for the benefit of infection. In plants, however, the functions of autophagy in host immunity and viral pathogenesis are poorly understood. In this study, we have identified both anti-and proviral roles of autophagy in the compatible interaction of cauliflower mosaic virus (CaMV), a double-stranded DNA pararetrovirus, with the model plant Arabidopsis thaliana. We show that the autophagy cargo receptor NEIGHBOR OF BRCA1 (NBR1) targets nonassembled and virus particle-forming capsid proteins to mediate their autophagy-dependent degradation, thereby restricting the establishment of CaMV infection. Intriguingly, the CaMV-induced virus factory inclusions seem to protect against autophagic destruction by sequestering capsid proteins and coordinating particle assembly and storage. In addition, we found that virus-triggered autophagy prevents extensive senescence and tissue death of infected plants in a largely NBR1-independent manner. This survival function significantly extends the timespan of virus production, thereby increasing the chances for virus particle acquisition by aphid vectors and CaMV transmission. Together, our results provide evidence for the integration of selective autophagy into plant immunity against viruses and reveal potential viral strategies to evade and adapt autophagic processes for successful pathogenesis. A utophagy is a conserved intracellular pathway that engages specialized double-membrane vesicles, called "autophagosomes," to enclose and transport cytoplasmic content to lytic compartments for degradation and subsequent recycling (1). Autophagosome formation relies on extensive membrane rearrangements and is mediated by the concerted action of a core set of autophagy-related proteins (ATGs) (2, 3). At basal levels, autophagy serves mainly housekeeping functions in cellular homeostasis, whereas stimulated autophagy activity facilitates adaptation to developmental and environmental stress conditions including starvation, aging, and pathogen infection (1, 4). Ample evidence now indicates that autophagy, initially recognized as a mainly bulk catabolic process, is able specifically to target and degrade a multitude of cellular structures ranging from individual and aggregated proteins to entire organelles and invading microbes (5, 6). Selectivity is provided by a growing number of autophagic adaptor or receptor proteins identified in eukaryotic organisms that recruit the cargo to the developing autophagosome through interaction with membrane-associated ATG8/LC3 proteins (7, 8). Several mammalian autophagy receptors have been implicated in the targeting of intracellular bacterial and viral pathogens in a process called "xenophagy" (8-10). For instance, the cargo receptor p62 (SQSTM1) was shown to bind directly to and mediate autophagic clearance of different viral capsid pr...

Nuclear autophagy degrades a geminivirus nuclear protein to restrict viral infection in solanaceous plants

“…C1, C2, C3, V1 from DNA-A, and NSP from DNA-B), showing the functional importance of nuclear localization for the geminivirus infection cycle (Fondong 2013;Wang & Zhou 2016;Kushwaha et al, 2017). Recently, plant virus-infection-induced autophagy has been described in many plant-virus interactions (Nakahara et al, 2012;Hafr en et al, 2017Hafr en et al, , 2018Haxim et al, 2017;Li et al, 2017Li et al, , 2018Yang et al, 2018). Interestingly, we found that the C1 protein of TLCYnV is translocated from the nucleus to the cytoplasm after 24 hpi, with a concomitant decrease in protein accumulation (Figs 1, 2, S1).…”

Proteasome‐independent functions of lysine‐63 polyubiquitination in plants

“…Interestingly, ELC/ AtVPS23A and AtFYVE1 directly bind to the AtPYL4 abscisic acid (ABA) receptor and AtIRT1, two ubiquitinated endocytosed cargos (Barberon et al, 2014;Belda-Palazon et al, 2016;Yu et al, 2016). Consistently, genetic interference with ELC/AtVPS23A or AtFYVE1 expression yields ubiquitinated protein accumulation in (Zhou et al, 2013), sequestration of anthocyanins in the vacuole (Pourcel et al, 2010), chlorophagy of physiological and damaged chloroplasts (Izumi et al, 2017), reticulophagy (Liu et al, 2012), pexophagy (Shibata et al, 2013), mitophagy (Li et al, 2014) and xenophagy (Hafren et al, 2017). Whether K63 polyubiquitination plays a role in these various types of selective autophagy in plants has either been ruled out (Hafren et al, 2017), or still requires confirmation.…”

“…Consistently, genetic interference with ELC/AtVPS23A or AtFYVE1 expression yields ubiquitinated protein accumulation in (Zhou et al, 2013), sequestration of anthocyanins in the vacuole (Pourcel et al, 2010), chlorophagy of physiological and damaged chloroplasts (Izumi et al, 2017), reticulophagy (Liu et al, 2012), pexophagy (Shibata et al, 2013), mitophagy (Li et al, 2014) and xenophagy (Hafren et al, 2017). Whether K63 polyubiquitination plays a role in these various types of selective autophagy in plants has either been ruled out (Hafren et al, 2017), or still requires confirmation. Chloroplast damage resulting from UV-B exposure results in the vacuolar transport of damaged chloroplasts to the vacuole in an autophagy-dependent manner (Izumi et al, 2017).…”

Autophagy–virus interplay in plants: from antiviral recognition to proviral manipulation

“…More recent studies focused on unravelling the immune functions of autophagy during virulent virus infections. Autophagy appears to be activated in response to various unrelated DNA and RNA viruses with negative consequences for virus accumulation, suggesting the integration of autophagic mechanisms in basal antiviral defences (Hafren et al , , ; Haxim et al , ; Li et al , ). A seminal example in this regard is given by Cauliflower mosaic virus (CaMV), a double‐stranded DNA pararetrovirus that is targeted by selective autophagy to impair the establishment of infection in Arabidopsis (Fig.…”

“…A seminal example in this regard is given by Cauliflower mosaic virus (CaMV), a double‐stranded DNA pararetrovirus that is targeted by selective autophagy to impair the establishment of infection in Arabidopsis (Fig. ) (Hafren et al , ). Strikingly, the cargo receptor NEIGHBOR OF BRCA1 (NBR1) was found to directly bind to and mediate the vacuolar clearance of the viral capsid protein and particles, thereby establishing xenophagy as antiviral pathway in plants (Hafren and Hofius, ; Hafren et al , ).…”