Jujube witches’ broom phytoplasmas inhibit ZjBRC1-mediated abscisic acid metabolism to induce shoot proliferation

Abstract

Jujube witches' broom (JWB) phytoplasmas parasitize the sieve tubes of diseased phloem and cause an excessive proliferation of axillary shoots from dormant lateral buds to favour their transmission. In previous research, two JWB effectors, SJP1 and SJP2, were identified to induce lateral bud outgrowth by disrupting ZjBRC1-mediated auxin flux. However, the pathogenesis of JWB disease remains largely unknown. Here, tissue-specific transcriptional reprogramming was examined to gain insight into the genetic mechanisms acting inside jujube lateral buds under JWB phytoplasma infection. JWB phytoplasmas modulated a series of plant signalling networks involved in lateral bud development and defence, including auxin, abscisic acid, ethylene, jasmonic acid, and salicylic acid. JWB-induced bud outgrowth was accompanied by downregulation of ABA synthesis within lateral buds. ABA application rescued the bushy appearances of the transgenic Arabidopsis overexpressing SJP1 and SJP2 in Col-0 and ZjBRC1 in brc1-2 mutant. Furthermore, the expression of ZjBRC1 and ABA-related genes ZjHB40 and ZjNCED3 was negatively correlated with lateral main bud outgrowth in decapitated healthy jujube. Molecular evidence showed that ZjBRC1 interacted with ZjBRC2 via its N-terminus to activate ZjHB40 and ZjNCED3 expression and ABA accumulation in transgenic jujube calli. In addition, ZjBRC1 widely regulated differentially expressed genes related to ABA homeostasis and ABA signalling, especially by binding to and suppressing PYL receptors. Therefore, these results suggest that JWB phytoplasmas hijack the ZjBRC1-mediated ABA pathways to stimulate lateral bud outgrowth and expansion, providing a strategy to engineer plants resistant to JWB phytoplasma disease and regulate woody plant architecture to promote crop yield and quality.

The ‘Candidatus phytoplasma ziziphi’ effectors SJP1 and SJP2 destabilise the bifunctional regulator ZjTCP7 to modulate floral transition and shoot branching

“…After the initiation of the floral meristem, the middle flower buds of the bearing shoots from stage 1 (the formation of flower buds), stage 2 (2 weeks after flower bud formation) and stage 3 (5 weeks after flower bud formation) were collected (Ma et al, 2020) and used for gene expression. 35S::SJP1‐GFP, 35S::SJP2‐GFP and 35S::GFP transgenic Nicotiana benthamiana and Arabidopsis lines were produced in previous research (Ma et al, 2023; Zhou et al, 2021) and used to investigate flowering phenotypes. Seeds of the ap1 mutant (SALK_151561C) were obtained from AraShare (https://www.arashare.cn/index/).…”

“…Before sowing on Murashige and Skoog media, all the seeds were cold‐treated at 4°C for 3 days and then surface‐sterilised. Twelve‐day‐old plants were transplanted into nutrient soil and grown in an artificial climate chamber at 23°C ± 1°C under a 16 h/8 h (light/dark) cycle (Ma et al, 2023). Four‐week‐old Arabidopsis thaliana (Col‐0) plants were used for the bimolecular fluorescence complementation (BiFC) assays, and N. benthamiana was used for the coexpression and dual luciferase (LUC) assays.…”

“…To produce a gain of repression activity, ZjTCP7 coding sequences were fused to the N‐terminus of the codon‐optimised repressor domain SRDX (Ma et al, 2023) under the control of the CaMV35S promoter. The coding sequence of ZjAP1 (LOC107429076) was fused to the N‐terminus of the GFP fluorescent protein under the control of the CaMV35S promoter.…”

“…The diseased jujube plants exhibit excessive proliferation of lateral shoot branches and phyllodes and repress the conversion of vegetative state to the initiation of flowering in the following years. Notably, two secreted JWB phytoplasma proteins, SJP1 and SJP2, have been identified as homologues of SAP11 (the secreted aster yellows phytoplasma strain witches' broom protein 11), serving as core effectors responsible for the manifestation of witches’ broom symptoms (Ma et al, 2023; Ma et al, 2024; Zhou et al, 2021). However, the mechanisms by which the effectors SJP1 and SJP2 disrupt the well‐regulated process of flowering have yet to be fully understood.…”

“…Among these functions, the role of SAP11 in promoting the proliferation of axillary meristems was found to be conserved across different phytoplasma‐host plant interaction systems, thereby stimulating vegetative growth. This mechanism was elucidated through the destabilization of CYC/TB1‐TCP transcription factors and disruption of their downstream hormonal homoeostasis (Chang et al, 2018; Correa Marrero et al, 2024; Ma et al, 2023; Pecher et al, 2019; Sugio et al, 2011; Wang et al, 2018; Zhou et al, 2021). Furthermore, the SAP11 effector proteins exhibited varying capacities to destabilise CIN‐TCP transcription factors (Chang et al, 2018; Pecher et al, 2019; Sugio et al, 2011).…”