Expression of Xylella fastidiosa RpfF in Citrus Disrupts Signaling in Xanthomonas citri subsp. citri and Thereby Its Virulence

Abstract

Xylella fastidiosa and Xanthomonas citri subsp. citri, that cause citrus variegated chlorosis (CVC) and citrus canker diseases, respectively, utilize diffusible signal factor (DSF) for quorum sensing. DSF, produced by RpfF, are similar fatty acids in both organisms, although a different set of genes is regulated by DSF in each species. Because of this similarity, Xylella fastidiosa DSF might be recognized and affect the biology of Xanthomonas citri. Therefore, transgenic Citrus sinensis and Carrizo citrange plants overexpressing the Xylella fastidiosa rpfF were inoculated with Xanthomonas citri and changes in symptoms of citrus canker were observed. X. citri biofilms formed only at wound sites on transgenic leaves and were thicker; however, bacteria were unable to break through the tissue and form pustules elsewhere. Although abundant growth of X. citri occurred at wound sites on inoculated transgenic leaves, little growth was observed on unwounded tissue. Genes in the DFS-responsive core in X. citri were downregulated in bacteria isolated from transgenic leaves. DSF-dependent expression of engA was suppressed in cells exposed to xylem sap from transgenic plants. Thus, altered symptom development appears to be due to reduced expression of virulence genes because of the presence of antagonists of DSF signaling in X. citri in rpfF-expressing plants.

Mechanistic insights into host adaptation, virulence and epidemiology of the phytopathogenXanthomonas

“…Few studies have introduced bacterial genes into crops to combat Xanthomonas diseases in greenhouse conditions. For example, Caserta and colleagues have introduced DSF producing gene rpfF from Xylella into citrus and demonstrated enhanced resistance to X. citri, possibly through misregulation of virulence-associated genes in an improper bacterial quorum (Caserta et al 2014). In addition, Zhang and colleagues expressed Pantoea albD, which was shown to detoxify the X. albilineans toxin albicidin in sugarcane.…”

The DSF Family of Cell–Cell Signals: An Expanding Class of Bacterial Virulence Regulators

“…fastidiosa and Xcc has indicated that DSF signaling is normally finely balanced during the disease process and that such a fine balance might therefore be readily disrupted by either degradation or over-production of the signal, with consequences for disease control [ 31 ]. One approach has been to engineer plants to express RpfF and hence elevate DSF levels [ 47 , 48 ]. Plant expression of RpfF from X .…”

Diffusible signal factor-dependent quorum sensing in pathogenic bacteria and its exploitation for disease control

“…fastidiosa and X. citri , respectively, (Caserta et al . ; Lindow et al . ) although the underlying mechanisms are not fully understood.…”

“…fastidiosa in grape and citrus reduces the virulence of Xy. fastidiosa and X. citri, respectively, (Caserta et al 2014;Lindow et al 2014) although the underlying mechanisms are not fully understood. RpfF expression in grape directs the production of XfDSF as well as XfDSF2 .…”