Figure 1. Bacterial Vector-Borne Diseases That Persist in Plant Vascular Tissues.

Pathogenic bacteria are delivered directly into the phloem (liberibacter, spiroplasma, and phytoplasma) or xylem (Xylella) during vector feeding. Upon delivery into the phloem, liberibacters and phytoplasmas are confined to sieve elements, move intracellularly through sieve pores, and tend to accumulate in sink tissues. Spiroplasmas preferentially localize near nucleated cells or in phloem parenchyma cells. Phytoplasmas attach to the sieve element membrane. Phloem-limited bacteria are pleomorphic. Vector feeding and bacterial proliferation induce Ca²⁺ production, accumulation of Sieve Element Occluding Relatives (SEOR) proteins, and callose deposition through callose synthases (CalS). Xylella cells move between xylem vessels through the pit membrane and exhibit differential distribution in resistant hosts.