Plant Pathogens, Minor (Phytoplasmas)

Abstract

Phytoplasmas are economically important plant pathogens that are transmitted by phloem-feeding insects. These bacteria replicate intracellularly in their plant and insect hosts. Phytoplasmas have probably evolved from a Gram-positive Clostridium -like ancestor through genome reductions and loss of outer cell wall. The cells of these bacteria are small, averaging ∼500 nm in diameter, and are surrounded by a single membrane and no cell wall. Phytoplasma detection has been difficult, because they cannot be cultured and are frequently present in low numbers. Phytoplasma diagnostics have greatly improved with the accumulation of sequence information of primarily 16S ribosomal DNA genes. The complete genome sequences of four related phytoplasma strains provided insights into the limited metabolic capabilities of phytoplasmas. Even though phytoplasma genomes are small, they have multiple copies of units up to approximately 20 kb in length that bear similarities to replicative composite transposons. Recent efforts of scientists have focused on the functional characterization of phytoplasma virulence factors. Phytoplasmas use the Sec-dependent protein translocation system for export of virulence factors. Virulence factors characterized so far are cell surface proteins involved in insect transmission and secreted proteins of which some target plant cell nuclei and manipulate plant processes.