Abstract
Through the formation of persister cells, bacteria exhibit tolerance to multidrug and other environmental stresses without undergoing genetic changes. The toxin-antitoxin (TA) systems are involved in the formation of persister cells because they are able to induce cell dormancy. Among the TA systems, the MqsRA system has been observed to be highly induced in persister cells of Xylella fastidiosa (causal agent of citrus variegated chlorosis—CVC) activated by copper stress, and has been described in Escherichia coli as related to the formation of persister cells and biofilms. Thus, we evaluated the role of this TA system in X. fastidiosa by overexpressing the MqsR toxin, and verified that the toxin positively regulated biofilm formation and negatively cell movement, resulting in reduced pathogenicity in citrus plants. The overexpression of MqsR also increased the formation of persister cells under copper stress. Analysis of the gene and protein expression showed that this system likely has an autoregulation mechanism to express the toxin and antitoxin in the most beneficial ratio for the cell to oppose stress. Our results suggest that this TA system plays a key role in the adaptation and survival of X. fastidiosa and reveal new insights into the physiology of phytopathogen-host interactions.
Highlights
The Gram-negative bacterium Xylella fastidiosa is a phytopathogen that causes diseases in many economically important crops worldwide, including citrus, grapevine, plum, almond, peach, coffee (Hopkins and Purcell, 2002) and, more recently, olives (Saponari et al, 2013)
We determined that the MqsRA TA system from the X. fastidiosa citrus variegated chlorosis (CVC) strain is a functional bona fide TA locus because, in addition to its ability to cause growth arrest, the toxin could degrade RNA, and the antitoxin directly bound to the toxin (Figure S1)
As we found that the overexpression of MqsR induced biofilm formation and reduced cell movement, we investigated the effects of the overexpression of this toxin on X. fastidiosa pathogenicity
Summary
The Gram-negative bacterium Xylella fastidiosa is a phytopathogen that causes diseases in many economically important crops worldwide, including citrus, grapevine, plum, almond, peach, coffee (Hopkins and Purcell, 2002) and, more recently, olives (Saponari et al, 2013). X. fastidiosa lives in the xylem vessels of infected plants and in the foregut of sharpshooters insect vector, which are responsible for the transmission of the bacterium directly to the xylem of the host plant (Almeida et al, 2014). X. fastidiosa multiplies and moves systemically colonizing the plant vessels forming biofilm, which is considered the main mechanism of X. fastidiosa pathogenicity. Biofilm condition is required for X. fastidiosa insect acquisition from infected plants, characterizing the dual lifestyle of X. fastidiosa (Chatterjee et al, 2008)
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