Phloem protein 2 (PP2) regulates phloem-based defense against apple scar skin viroid and cucumber mosaic virus

Abstract

The phloem protein 2 (PP2), one of the major phloem proteins, is known to be involved in the transport of nutrients, RNA, and defense; however, the molecular basis of its RNA-binding activity and its role in antiviral defense remain poorly understood. In the present study, we investigated the domains responsible for RNA-binding properties and defense-related functions of PP2 in interactions with apple scar skin viroid (ASSVd) and cucumber mosaic virus (CMV). The cucumber PP2 (CsPP2) was divided into four RNA-binding domains (RBD1-4), and their interactions with ASSVd RNA were analyzed. This study revealed that both the C-termini and the conventionally predicted single RNA-binding domain interact with ASSVd RNA. To investigate the role of the ASSVd-PP2 interaction function by using AtPP2-A1 (the closest homolog of CsPP2 ) T-DNA knockout mutants, we found that the PP2 protein plays an important role in phloem-based defense in contrast to the long-distance movement of RNA. The AtPP2-A1 gene knockout mutants showed high disease severity and accumulation of ASSVd and reduced expression of the phloem defense marker gene PDF1.2 compared with wild-type plants, indicating a role for PP2 in phloem-based defense. and. In the protoplast study, we found that the PP2 protein does not affect the replication of ASSVd. Like ASSVd, AtPP2-A1 mutants exhibited enhanced susceptibility to cucumber mosaic virus (CMV), symptom severity, and increased viral accumulation. Overall, these findings showed that PP2 functions as an unconventional RNA-binding protein and plays an important role in restricting virus and viroid accumulation through phloem-associated defense mechanisms.