Phytoplasma - Effector (virulence) Proteins

Effector (virulence) Proteins

Many plant pathogens produce virulence factors (or effectors) that modulate or interfere with normal host processes in a way that is beneficial to the pathogen. Aster Yellows phytoplasma strain Witch's Broom (often referred to as 'AY-WB') produces two effector proteins that have been characterized experimentally. SAP11 is a small protein (~10 kDa) that is released into plants upon infection with AY-WB. SAP11 selectively interacts with and destabilizes CINCINNATA (CIN)-related TEOSINTE BRANCHED1, CYCLOIDEA, PROLIFERATING CELL FACTORS 1 and 2 (TCP) transcription factors in the model plant Arabidopsis. TCP transcription factors normally regulate plant development and control the expression of lipoxygenase (LOX) genes that are required for the biosynthesis of jasmonate. In AY-WB-infected Arabidopsis plants (and plants that express SAP11 transgenically), jasmonate levels are decreased due to the SAP11-mediated degradation of CIN-TCPs. The downregulation of jasmonate production is beneficial to the phytoplasma because jasmonate is involved in plant defence against herbivorous insects such as leafhoppers, and leafhoppers have been shown to lay more eggs on AY-WB-infected plants atleast in part because of SAP11. For example, the leafhopper Macrosteles quadrilineatus lay 30% more eggs on plants that express SAP11 transgenically, and 60% more eggs on plants infected with AY-WB. Phytoplasmas cannot survive in the external environment and are dependent upon insects such as leafhoppers for transmission to new (healthy) plants. Thus, by interfering with jasmonate production, SAP11 'encourages' leafhoppers to lay more eggs on phytoplasma-infected plants, thereby ensuring that newly hatching leafhopper nymphs feed upon infected plants and become vectors for the bacteria.

AY-WB produces a second effector (called SAP54) that induces symptoms of virescence and phyllody in infected plants. The mechanism by which SAP54 converts flowers into leaf-like organs is presently being investigated by researchers at the John Innes Centre in Norwich, United Kingdom.