Final Activity Report Summary - PHYTOPLASMA GENOMICS (Functional characterization of phytoplasma candidate virulence proteins that target plant cell nuclei) Phytoplasmas are insect-transmitted plant pathogens that can manipulate their plant and insect hosts for their own benefit. Indeed, phytoplasmas interfere with plant development often leading to the production of more young leafy tissues and inhibition of flowering and plant senescence. This provides more leaf tissue for phytoplasma replication and better opportunities for feeding and egg laying of the phytoplasma insect vectors, which spread phytoplasmas to new plant hosts. We found that phytoplasmas interfere with plant development through the secretion of virulence factors (called effectors) into plant cells where they perturb a variety of cell processes. In this project, we have further functionally characterized one such effector from Aster Yellows phytoplasma strain Witches' Broom (AY-WB), named secreted AY-WB protein (SAP) 11. SAP11 localizes to plant cell nuclei where it inactivates transcription factors that counteract leaf proliferation and induce leaf senescence. Production of SAP11 in Arabidopsis thaliana leads to the differential regulation of plant developmental genes and downregulation of genes involved in leaf senescence and secondary metabolism. These plants have large kale-like crinkled leaves, delayed flowering and produce more young shoots. The gene transcription profiles and phenotype of Arabidopsis plants that manufacture SAP11 overlap with those of AY-WB-infected plants and plant lines in which the production of the transcription factors is inhibited. We envision that the SAP11-mediated delay of leaf senescence and increase in shoots and leaf surface enhances and prolong opportunities for AY-WB replication and feeding and egg laying of AY-WB insect vectors. Thus, SAP11 has a true effector function. This research has led to a better understanding of phytoplasma biology that will be shared with the larger scientific community and industry. Research on phytoplasmas is important, because these pathogens are becoming increasingly a problem in (durable) crops, flower bulbs and ornamentals worldwide, including third world countries. This is because of intensified agriculture, stricter regulations of the use of insecticides and climate change that together lead to better survival rates of the insect vectors that transmit phytoplasmas.