From the smallest to the largest subcellular plant pathogen: Citrus tristeza virus and its unique p23 protein

•P23 is a CTV-specific protein regulating critical functions in the life cycle.•Its RNA-binding activity depends on a zinc-finger motif and several basic amino acids.•In infected cells, p23 is targeted to the nucleolus, cajal bodies and plasmodesmata.•P23 suppresses both the RNA silencing and the basal defense system of the plant.•The 157 N-terminal amino acids of p23 are involved in symptom expression. Knowledge on diseases caused by Citrus tristeza virus (CTV) has greatly increased in last decades after their etiology was demonstrated in the past seventies. Professor Ricardo Flores substantially contributed to these advances in topics like: i) improvement of virus purification to obtain biologically active virions, ii) sequencing mild CTV isolates for genetic comparisons with sequences of moderate or severe isolates and genetic engineering, iii) analysis of genetic variation of both CTV genomic RNA ends and features of the highly variable 5′ end that allow accommodating this variation within a conserved secondary structure, iv) studies on the structure, subcellular localization and biological functions of the CTV-unique p23 protein, and v) potential use of p23 and other 3′-proximal regions of the CTV genome to develop transgenic citrus resistant to the virus. Here we review his main achievements on these topics and how they contributed to deeper understanding of CTV biology and to new potential measures for disease control.