Phytoplasma

 ( Mollicutes ) 

In 2009, 56 genes for secreted proteins were identified in the genome of Aster yellows phytoplasma strain (AY-WB); these were named secreted AY-WB proteins (SAPs) and considered effectors. Also in 2009, effector SAP11 was shown to target plant cell nuclei and unload from phloem cells in AY-WB-infected plants. SAP11 was later found to induce changes in leaf shapes of plants and stem proliferations which resembled the witches' broom symptoms of AY-WB-infected plants. In addition, it was demonstrated that SAP11 interacts with and destabilizes plant class II TCP protein domain transcription factors that lead to shoot proliferation and leaf shape changes. TCPs also control the expression of lipoxygenase genes required for jasmonate biosynthesis. Jasmonate levels are decreased in phytoplasma-infected Arabidopsis plants and plants that transgenically express the AY-WB SAP11 effector. The downregulation of jasmonate production is beneficial to phytoplasmas because jasmonate is involved in plant defenses against herbivorous insects such as . Leafhoppers lay increased numbers of eggs on AY-WB-infected plants, at least in part because of SAP11 production. For example, the leafhopper Macrosteles quadrilineatus laid 30% more eggs on plants expressing SAP11 transgenically than control plants 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 compromising jasmonate production, SAP11 encourages leafhoppers to lay more eggs on phytoplasma-infected plants, thereby ensuring that newly hatched leafhopper nymphs feed upon infected plants to become phytoplasma vectors. SAP11 effectors are identified in a number of divergent phytoplasmas and these effectors also interact with TCPs and modulate plant defenses. SAP11 is the first phytoplasma virulence protein for which plant targets and effector functions were identified. TCPs were found to be targeted by a number of other pathogen effectors.

The AY-WB phytoplasma effector SAP54 was shown to induce virescence and phyllody when expressed in plants, and homologs of this effector were found in at least three other phytoplasmas. Two SAP54 homologs, PHYL1 of the onion yellows phytoplasma and PHYL1_PnWB of the peanut witches' broom phytoplasma, also induce phyllody-like floral abnormalities. These results suggest that PHYL1, SAP54, and their homologs form a phyllody-inducing gene family, the members of which are termed phyllogens. MADS-box transcription factors (MTFs) of the ABCE model play critical roles in floral organ development in Arabidopsis. Phyllogens interact directly with class A and class E MTFs, inducing protein degradation in a ubiquitin/proteasome-dependent manner that, at least for SAP54, is dependent on interactions with the proteasome shuttle factor RAD23. Interestingly, RAD23 mutants do not show phyllody when infected with phytoplasma indicating that RAD23 proteins are susceptibility factors; i.e. phytoplasmas and SAP54 require these plant proteins to induce phyllody symptoms. The accumulation of mRNAs encoding class B MTFs, the transcription of which is positively regulated by class A and class E MTFs, is drastically decreased in Arabidopsis constitutively expressing PHYL1. Phyllogens induce abnormal floral organ development by inhibiting the functions of these MTFs. RAD23 proteins are also required for promoting leafhopper vector egg laying on plants that express SAP54 and are infected with AY-WB phytoplasma.

Phytoplasmas enter the insect body through the stylet, pass through the intestine, and then move to the hemolymph and colonize the . The entire process can take up to 3 weeks. Once established in an insect host, phytoplasmas are found in most major organs. The time between ingestion by the insect and attainment of an infectious titer in the salivary glands is termed the latency period.

Phytoplasmas can also be spread via dodders ( Cuscuta) or by vegetative propagation such as the grafting of infected plant tissue onto a healthy plant.