Phytoplasmas are insect-transmitted bacterial pathogens that colonize a wide range of plant species, including vegetable and cereal crops, and herbaceous and woody ornamentals. Phytoplasma-infected plants often show dramatic symptoms, including proliferation of shoots (witch’s brooms), changes in leaf shapes and production of green sterile flowers (phyllody). Aster Yellows phytoplasma Witches’ Broom (AY-WB) infects dicots and its effector, secreted AYWB protein 11 (SAP11), was shown to be responsible for the induction of shoot proliferation and leaf shape changes of plants. SAP11 acts by destabilizing TEOSINTE BRANCHED 1-CYCLOIDEA-PROLIFERATING CELL FACTOR (TCP) transcription factors, particularly the class II TCPs of the CYCLOIDEA/TEOSINTE BRANCHED 1 (CYC/TB1) and CINCINNATA (CIN)-TCP clades. SAP11 homologs are also present in phytoplasmas that cause economic yield losses in monocot crops, such as maize, wheat and coconut. Here we show that a SAP11 homolog of Maize Bushy Stunt Phytoplasma (MBSP), which has a range primarily restricted to maize, destabilizes specifically TB1/CYC TCPs. SAP11MBSP and SAP11AYWB both induce axillary branching and SAP11AYWB also alters leaf development of Arabidopsis thaliana and maize. However, only in maize, SAP11MBSP prevents female inflorescence development, phenocopying maize tb1 lines, whereas SAP11AYWB prevents male inflorescence development and induces feminization of tassels. SAP11AYWB promotes fecundity of the AY-WB leafhopper vector on A. thaliana and modulates the expression of A. thaliana leaf defence response genes that are induced by this leafhopper, in contrast to SAP11MBSP. Neither of the SAP11 effectors promote fecundity of AY-WB and MBSP leafhopper vectors on maize. These data provide evidence that class II TCPs have overlapping but also distinct roles in regulating development and defence in a dicot and a monocot plant species that is likely to shape SAP11 effector evolution depending on the phytoplasma host range.

28 Phytoplasmas are insect-transmitted bacterial pathogens that colonize a wide range of plant 29 species, including vegetable and cereal crops, and herbaceous and woody ornamentals.30 Phytoplasma-infected plants often show dramatic symptoms, including proliferation of shoots 31 (witch's brooms), changes in leaf shapes and production of green sterile flowers (phyllody). 32 Aster Yellows phytoplasma Witches' Broom (AY-WB) infects dicots and its effector, 33 secreted AYWB protein 11 (SAP11), was shown to be responsible for the induction of shoot 34 proliferation and leaf shape changes of plants. SAP11 acts by destabilizing TEOSINTE 35 BRANCHED 1-CYCLOIDEA-PROLIFERATING CELL FACTOR (TCP) transcription 36 factors, particularly the class II TCPs of the CYCLOIDEA/TEOSINTE BRANCHED 1 37 (CYC/TB1) and CINCINNATA (CIN)-TCP clades. SAP11 homologs are also present in 38 phytoplasmas that cause economic yield losses in monocot crops, such as maize, wheat and 39 coconut. Here we show that a SAP11 homolog of Maize Bushy Stunt Phytoplasma (MBSP), 40 which has a range primarily restricted to maize, destabilizes only TB1/CYC TCPs. 41 SAP11 MBSP and SAP11 AYWB both induce axillary branching and SAP11 AYWB also alters leaf 42 development of Arabidopsis thaliana and maize. However, only in maize, SAP11 MBSP 43 prevents female inflorescence development, phenocopying maize tb1 lines, whereas 44 SAP11 AYWB prevents male inflorescence development and induces feminization of tassels. 45 SAP11 AYWB promotes fecundity of the AY-WB leafhopper vector on A. thaliana and 46 modulates the expression of A. thaliana leaf defence response genes that are induced by this 47 leafhopper, in contrast to SAP11 MBSP . Neither of the SAP11 effectors promote fecundity of 48 AY-WB and MBSP leafhopper vectors on maize. These data provide evidence that class II 49 TCPs have overlapping but also distinct roles in regulating development and defence in a 50 dicot and a monocot plant species that is likely to shape SAP11 effector evolution depending 51 on the phytoplasma host range. 3 52 53 Author summary 57 Phytoplasmas are parasites of a wide range of plant species and are transmitted by sap-58 feeding insects, such as leafhoppers. Phytoplasma-infected plants are often easily recognized 59 because of their dramatic symptoms, including shoot proliferations (witch's brooms) and60 altered leaf shapes, leading to severe economic losses of crops, ornamentals and trees 61 worldwide. We previously found that the virulence protein SAP11 of aster yellows witches' 62 broom phytoplasma (AY-WB) interferes with a specific group of plant transcription factors, 63 named TCPs, leading to witches' brooms and leaf shape changes of the model plant 64 Arabidopsis thaliana. SAP11 has been characterized in a number of other phytoplasmas. 65 However, it is not known how phytoplasmas and their SAP11 proteins modulate processes in 66 crops, including cereals such as maize. We identified a SAP11 homolog in Maize bushy stunt 67 phytoplasma (MBSP), a pathogen that can cause severe yield l...

An insect-transmitted phytoplasma causing Witches’ Broom Disease of Lime (WBDL) is responsible for the drastic decline in lime production in several countries. However, it is unclear how WBDL phytoplasma (WBDLp) induces witches’ broom symptoms and if these symptoms contribute to the spread of phytoplasma. Here we show that the gene encoding SAP11 of WBDLp (SAP11WBDL) is present in all WBDLp isolates collected from diseased trees. SAP11WBDL interacts with acid lime (Citrus aurantifolia) TCP transcription factors, specifically members of the TB1/CYC class that have a role in suppressing axillary branching in plants. Sampling of WBDLp-infected lime trees revealed that WBDLp titers and SAP11WBDL expression levels were higher in symptomatic leaves compared to asymptomatic sections of the same trees. Moreover, the witches’ brooms were found to attract the vector leafhopper. Defense genes that have a role in plant defense responses to bacteria and insects are more downregulated in witches’ brooms compared to asymptomatic sections of trees. These findings suggest that witches’ broom-affected parts of the trees contribute to WBDL epidemics by supporting higher phytoplasma titers and attracting insect vectors.