Genome-wide identification of jujube witches’ broom phytoplasma effectors revealed the role of SJP3 in inducing phyllody

Abstract

Jujube witches’ broom (JWB), associated with ‘Candidatus Phytoplasma ziziphi’, is a devastating disease in Chinese jujube (Ziziphus jujuba Mill.). One of the symptoms in plants infected with phytoplasmas is the change of floral organs into leaf-like structures, known as phyllody. Effector proteins secreted by phytoplasmas are key to inducing this and other witches’ broom symptoms. However, the phytoplasma effector proteins that induce phyllody in jujube remain unknown. We identified 43 candidate secreted JWB phytoplasma proteins (SJPs) in the JWB phytoplasma genome. Eight of the predicted candidate effector genes exhibited high transcript levels in infected flower buds. Among the eight candidate effectors, SJP3 experimentally showed the potential to induce phyllody. SJP3 expression in infected jujube was at a high level in young leaves and flowers. Ectopic expression of SJP3 in Arabidopsis induced leaf-like flowers and extra secondary flowers and significantly changed the transcript levels of PISTILLATA (PI), SHORT VEGETATIVE PHASE (SVP), AGAMOUS-LIKE 24 (AGL24) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1). The expression levels of these MADS-box homologous genes were also changed in infected jujube flower buds and all four whorls of the floral organs. The data showed that the effector SJP3 disrupts expression of several MADS-box genes associated with floral organ identity and flowering time and is likely key to inducing phyllody in phytoplasma-infected jujube. This study provides new insight into understanding the pathogenesis of JWB disease induced by phytoplasma.