Battle of Three: The Curious Case of Papaya Sticky Disease

Abstract

Among the most serious problems in papaya production are the viruses associated with papaya ringspot and papaya sticky disease (PSD). PSD concerns producers worldwide because its symptoms are extremely aggressive and appear only after flowering. As no resistant cultivar is available, several disease management strategies have been used in affected countries, such as the use of healthy seeds, exclusion of the pathogen, and roguing. In the 1990s, a dsRNA virus, papaya meleira virus (PMeV), was identified in Brazil as the causal agent of PSD. However, in 2016 a second virus, papaya meleira virus 2 (PMeV2), with an ssRNA genome, was also identified in PSD plants. Only PMeV is detected in asymptomatic plants, whereas all symptomatic plants contain both viral RNAs separately packaged in particles formed by the PMeV capsid protein. PSD also affects papaya plants in Mexico, Ecuador, and Australia. PMeV2-like viruses have been identified in the affected plants, but the partner virus(es) in these countries are still unknown. In Brazil, PMeV and PMeV2 reside in laticifers that promote spontaneous latex exudation, resulting in the affected papaya fruit’s sticky appearance. Genes modulated in plants affected by PSD include those involved in reactive oxygen species and salicylic acid signaling, proteasomal degradation, and photosynthesis, which are key plant defenses against PMeV complex infection. However, the complete activation of the defense response is impaired by the expression of negative effectors modulated by the virus. This review presents a summary of the current knowledge of the Carica papaya-PMeV complex interaction and management strategies.

A Capsid Protein Fragment of a Fusagra-like Virus Found in Carica papaya Latex Interacts with the 50S Ribosomal Protein L17

“…Laticifers are the only cells where the PMeV complex is found [ 1 ] and it is during the laticifer differentiation that papaya proteases are accumulated. Early laticifer cells undergo autophagy of their well-developed organelles, but later in differentiation, their endoplasmic reticulum splits into fragments and initiates the production and accumulation of proteases which are stored within vesicles of the mature laticifer [ 4 , 45 ]. Several viruses encode proteases which are necessary for post-translational modifications of their proteins to ensure that proteins can travel together to the viral assembly site, to ensure the proper timing for the initiation of folding and assembly, and to control the concentration of key viral proteins [ 44 ].…”

“…Its causal agent was initially identified as a double-stranded (ds) RNA virus with an isometric particle of approximately 50 nm in diameter, named papaya meleira virus (PMeV) [ 1 , 2 ]. A second positive-sense single-stranded ((+) ssRNA) virus, designated as papaya meleira virus 2 (PMeV2), was also proved to be an important agent in the onset of symptoms associated with PSD [ 3 ] (reviewed in [ 4 ]). These two viruses make up the PMeV complex, an association of a virus tentatively classified in the proposed Fusagraviridae family [ 5 , 6 ] (PMeV) and an umbra-like virus (PMeV2) [ 3 , 7 ].…”

The mysterious non‐arbuscular mycorrhizal status of Brassicaceae species

“…However, trading‐off this loss of NBS‐LRR for AM symbiosis might have been too costly for the plant defence and thus was stabilized or is still being stabilized by the retention of glucosinolates. Obstinate viral diseases are reported to cause crop losses of up to 85% in papaya (Sa Antunes et al, 2020). Since NBS‐LRR are also known to confer resistance against the viruses, these overall compromised defences recently being experienced by papaya further predicts that this plant species is probably struggling to balance its defence trade‐off for maintaining AM symbiosis.…”

The Role of Plant Latex in Virus Biology

“…PMeV is an 8.7 kbp double-stranded RNA (dsRNA) virus ( Fusagraviridae ) [ 81 ] that, on co-infection with PMeV2, a 4.5 kbp single-stranded RNA (ssRNA) (umbra-like virus) [ 36 ], causes papaya sticky disease (PSD) [ 36 , 82 ]. Sequence similarity and phylogenetic analysis that included PMeV2, PMeV-Mx, and PpVQ suggest that these viruses may be different isolates from the same umbravirus [ 36 ].…”