The p33 protein of Citrus tristeza virus affects viral pathogenicity by modulating a host immune response

Abstract

Accumulation of reactive oxygen species (ROS) is a general plant basal defense strategy against viruses. In this study, we show that infection by Citrus tristeza virus (CTV) triggered ROS burst in Nicotiana benthamiana and in the natural citrus host, the extent of which was virus-dose dependent. Using Agrobacterium-mediated expression of CTV-encoded proteins in N. benthamiana, we found that p33, a unique viral protein, contributed to the induction of ROS accumulation and programmed cell death. The role of p33 in CTV pathogenicity was assessed based on gene knockout and complementation in N. benthamiana. In the citrus-CTV pathosystem, deletion of the p33 open reading frame in a CTV variant resulted in a significant decrease in ROS production, compared to that of the wild type CTV, which correlated with invasion of the mutant virus into the immature xylem tracheid cells and abnormal differentiation of the vascular system. By contrast, the wild type CTV exhibited phloem-limited distribution with a minor effect on the vasculature. We conclude that the p33 protein is a CTV effector that negatively affects virus pathogenicity and suggest that N. benthamiana recognizes p33 to activate the host immune response to restrict CTV into the phloem tissue and minimize the disease syndrome.

A Long Non-Coding RNA of Citrus tristeza virus: Role in the Virus Interplay with the Host Immunity

“…As we demonstrated recently, infection by CTV triggers a signaling cascade resulting in ROS accumulation, and three viral proteins—p33, p23, and p20—contribute to this reaction [47]. Here, we show that prevention of the LMT1 production escalates the host immune response beyond that upon infection with the wild-type virus, and that LMT1 functions as an antagonist of the SA-mediated signaling.…”

“…3,3′-Diaminobenzidine (DAB) staining was performed as previously described [47]. Briefly, N. benthamiana leaves were washed with double distilled H 2 O three times and stained with 1 mg/mL DAB-HCl solution (Sigma-Aldrich, St. Louis, MO, USA) overnight.…”

Citrus tristeza virus P33 Protein Is Required for Efficient Transmission by the Aphid Aphis (Toxoptera) citricidus (Kirkaldy)

“…Recently, Sun and Folimonova [ 39 ] showed that the expression of p33 gene of CTV enhanced the accumulation of reactive oxygen species (ROS) in N. benthamiana . On the other hand, the deletion of the p33 gene from the CTV variant significantly decreased the ROS and programmed cell death (PCD) in N. benthamiana [ 39 ].…”

“…Recently, Sun and Folimonova [ 39 ] showed that the expression of p33 gene of CTV enhanced the accumulation of reactive oxygen species (ROS) in N. benthamiana . On the other hand, the deletion of the p33 gene from the CTV variant significantly decreased the ROS and programmed cell death (PCD) in N. benthamiana [ 39 ]. It is suggested that the P33 protein is an effector, and its presence in CTV allows N. benthamiana to recognize the virus and activate its immune response, which restricts CTV into the phloem tissues and decreases disease symptoms [ 39 ].…”

Citrus tristeza virus: A large RNA virus with complex biology turned into a valuable tool for crop protection

“…The p33 protein possesses certain characteristics found in a number of movement proteins of other plant viruses: It localizes at plasmodesmata (the channels within the cell walls of plant cells connecting the cytoplasm of adjacent cells and enabling transport of materials) and has the ability to polymerize forming extracellular tubules [9] (Fig 1E). Furthermore, p33 is a key factor in CTV superinfection exclusion (SIE; this phenomenon is discussed below in more detail) and also is a viral effector that influences virus pathogenicity by modulating a host immune response [10][11][12].…”