Stem-pitting caused by Citrus tristeza virus is associated with increased phloem occlusion

Stem-pitting (SP) disease results from disruption of normal phloem and xylem development. In citrus, a characteristic manifestation of SP caused by Citrus tristeza virus (CTV) is phloem regeneration. We hypothesized that phloem regeneration occurs due to reduced functionality of CTV infected phloem cells. To examine phloem cell occlusions in CTV-SP, we analyzed callose and phloem-protein (PP) accumulation in Citrus macrophylla trees infected with CTV mutants exhibiting different SP phenotypes from very mild (CTVΔp13) to severe (CTVΔp33), in addition to full-length CTV and healthy plants. CTV infection was accompanied by callose and PP accumulation in the phloem. With the increase in the SP symptoms from very mild to severe, there was a constant increase in the levels of callose and PP, accompanied by an increase in PHLOEM-PROTEIN 2 and a decrease in BETA-1,3-GLUCANASE gene expression levels. These results indicate that SP symptom development is associated with increased phloem occlusion.