Witches' Broom Disease of Lime Contributes to Phytoplasma Epidemics and Attracts Insect Vectors

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 (SAP11(WBDL)) is present in all WBDLp isolates collected from diseased trees. SAP11(WBDL) 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 SAP11(WBDL) expression levels were higher in symptomatic leaves compared with 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 with 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.

Automated summary

The Witches' Broom Disease of Lime (WBDL) caused by an insect-transmitted phytoplasma has led to a significant decrease in lime production in multiple countries. Research has shown that the gene encoding SAP11 of WBDL phytoplasma interacts with lime TCP transcription factors, inducing witches' broom symptoms and attracting insect vectors. High phytoplasma titers and expression levels of SAP11 were found in symptomatic leaves, suggesting that affected parts of trees contribute to WBDL epidemics by supporting higher phytoplasma levels and attracting vectors.