Alterations of plant architecture and phase transition by the phytoplasma virulence factor SAP11.

As key mediators linking developmental processes with plant immunity, TCP (TEOSINTE-BRANCHED, CYCLOIDEA, PROLIFERATION FACTOR 1 and 2) transcription factors have been increasingly shown to be targets of pathogenic effectors. We report here that TB/CYC (TEOSINTE-BRANCHED/CYCLOIDEA)-TCPs are destabilized by phytoplasma SAP11 effectors, leading to the proliferation of axillary meristems. Although a high degree of sequence diversity was observed among putative SAP11 effectors identified from evolutionarily distinct clusters of phytoplasmas, these effectors acquired fundamental activity in destabilizing TB/CYC-TCPs. In addition, we demonstrate that miR156/SPLs and miR172/AP2 modules, which represent key regulatory hubs involved in plant phase transition, were modulated by Aster Yellows phytoplasma strain Witches' Broom (AY-WB) protein SAP11. A late-flowering phenotype with significant changes in the expression of flowering-related genes was observed in transgenic Arabidopsis plants expressing SAP11AYWB. These morphological and molecular alterations were correlated with the ability of SAP11 effectors to destabilize CIN (CINCINNATA)-TCPs. Although not all putative SAP11 effectors display broad-spectrum activities in modulating morphological and physiological changes in host plants, they serve as core virulence factors responsible for the witches' broom symptom caused by phytoplasmas.

Post-translational cleavage and self-interaction of the phytoplasma effector SAP11.

Phytoplasmas are insect-transmitted intracellular plant bacterial pathogens that secrete effector molecules into host cells that interfere with the host's developmental or metabolic processes. Recently, the secreted Aster Yellows phytoplasma strain Witches' Broom protein11 (SAP11) has been shown to act as a virulence factor that alters the development, hormone biosynthesis, phosphate (Pi) homeostasis, and defense responses in the affected plants. We found that SAP11 undergoes proteolytic processing in planta and self-interaction in vitro. These biochemical studies provide foundational insights necessary for the functional characterization of SAP11; however, the biological relevance of post-translational cleavage and self-interaction of SAP11 to its role as a virulence factor warrants further investigation.