Acidovorax citrulli Type III Effector AopP Suppresses Plant Immunity by Targeting the Watermelon Transcription Factor WRKY6

Abstract

Acidovorax citrulli (Ac) is the causal agent of bacterial fruit blotch (BFB), and BFB poses a threat to global watermelon production. Despite its economic importance, the molecular mechanisms underlying Ac pathogenicity and virulence are not well understood, particularly with regard to its type III secreted effectors. We identify a new effector, AopP, in Ac and confirm its secretion and translocation. AopP suppresses reactive oxygen species burst and salicylic acid (SA) content and significantly contributes to virulence. Interestingly, AopP interacts with a watermelon transcription factor, ClWRKY6, in vivo and in vitro. ClWRKY6 shows typical nuclear localization, and AopP and ClWRKY6 co-localize in the nucleus. Ac infection, SA, and the pathogen-associated molecular pattern flg22Ac promote ClWRKY6 production, suggesting that ClWRKY6 is involved in plant immunity and SA signaling. Furthermore, ClWRKY6 positively regulates PTI and SA production when expressed in Nicotiana benthamiana. Importantly, AopP reduces ClWRKY6 mRNA and ClWRKY6 protein levels, suggesting that AopP suppresses plant immunity by targeting ClWRKY6. In summary, we identify a novel effector associated with the virulence mechanism of Ac, which interacts with the transcription factor of the natural host, watermelon. The findings of this study provide insights into the mechanisms of watermelon immune responses and may facilitate molecular breeding for bacterial fruit blotch resistance.

Autophagy Inhibits Intercellular Transport of Citrus Leaf Blotch Virus by Targeting Viral Movement Protein

“…For the transient expression assay, the full-length coding sequences of replicase, CP, MP, NbATG8c1, NbATG8d, NbATG8f, and NbATG8i were amplified by PCR, cloned into pBin41 [49], pBin61-GFP [50], pCambia1302-GFP [51] or pBI121-mCherry [52] digested with BamHI, BamHI, SpeI or BamHI, respectively, to generate CP-HA, MP-HA, GFP-ATG8c1, GFP-ATG8C1, GFP-ATG8d, GFP-ATG8f, GFP-ATG8i, replicase-GFP, CP-GFP, and MP-GFP, CP-mCherry, and MP-mCherry.…”

yggS Encoding Pyridoxal 5′-Phosphate Binding Protein Is Required for Acidovorax citrulli Virulence

“…More than 50 T3Es sharing similarity with known T3Es from other pathogenic bacteria were revealed in A. citrulli strain M6 by computational approach, and seven new putative T3Es were further validated as real effectors through T3SS-dependent translocation assay ( Jiménez-Guerrero et al, 2020 ). Two effectors, AopN and AopP, were confirmed to inhibit plant immunity by interacting with ClHIPP, CILTP and ClWRKY6 in watermelon, respectively ( Zhang et al, 2020a , b ). In addition, type II and VI secretion systems were reported to play key roles in A. citrulli virulence ( Johnson et al, 2009 ; Tian et al, 2015 ).…”

Essential Acidovorax citrulli Virulence Gene hrpE Activates Host Immune Response against Pathogen

“…Furthermore, the deletion of regulators or structural genes such as hrpG , hrpX , hrcN , and hrcQ significantly reduces or eliminates A. citrulli virulence [ 39 , 40 , 41 ]. Additionally, type three effectors (T3Es) promote A. citrulli infection by inhibiting the immune responses of host plants [ 42 , 43 , 44 , 45 ]. To date, several T3Es have been characterized [ 42 , 43 , 44 , 45 , 46 ].…”

“…Additionally, type three effectors (T3Es) promote A. citrulli infection by inhibiting the immune responses of host plants [ 42 , 43 , 44 , 45 ]. To date, several T3Es have been characterized [ 42 , 43 , 44 , 45 , 46 ]. The model plant Nicotiana benthamiana Domin can be used to study A. citrulli [ 46 ], which has furthered research on T3SS in A. citrulli .…”