Role of the Type VI Secretion System in the Pathogenicity of Pseudomonas syringae pv. actinidiae, the Causative Agent of Kiwifruit Bacterial Canker
Authors: Nana Wang
Yuqi Liu
Jiliang Chen
Ning Han
Xiaoning Gao
Lili Huang
Zhiran Wu
Runze Tian
Journal: Frontiers in Microbiology (2021)
Discipline: MICROBIOLOGY
Keywords: Kiwifruit bacterial canker; biofilm formation; competition; environmental adaptability; pathogenicity
Affiliations: State Key Laboratory of Crop Stress Biology for Arid Areas, Northwest A&F University, Yangling, China., College of Life Science, Northwest A&F University, Yangling, China., College of Plant Protection, Northwest A&F University, Yangling, China., Institute of Bioengineering, Guangdong Academy of Sciences, Guangzhou, China.

Abstract:  The type VI secretion system (T6SS), a macromolecular machine, plays an important role in the pathogenicity of many Gram-negative bacteria. However, the role of T6SS in the pathogenicity of Pseudomonas syringae pv. actinidiae (Psa), the pathogen of kiwifruit bacterial canker, is yet to be studied. Here, we found a T6SS gene cluster consisting of 13 core genes (A-J) in the genome of Psa M228 based on a genome-wide analysis. To determine whether the T6SS gene cluster affects the pathogenicity of Psa M228, T6SS and its 13 core gene deletion mutants were constructed and their pathogenicity was determined. The deletion mutants showed different degrees of reduction in pathogenicity compared with the wild-type strain M228; in tssM and tssJ mutants, pathogenicity was significantly reduced by 78.7 and 71.3%, respectively. The pathogenicity results were also confirmed by electron microscopy. To further confirm that the reduction in pathogenicity is related to the function of T6SS, we selected the T6SS gene cluster, comprising tssM and tssJ, for further analyses. Western blot results revealed that tssM and tssJ were necessary for hemolytic co-regulatory protein secretion, indicating that they encode a functional T6SS. Further, we explored the mechanism by which T6SS affects the pathogenicity of Psa M228. The ability of bacterial competition, biofilm formation, hydrogen peroxide tolerance, and proteolytic activity were all weakened in the deletion mutants M228ŒîT6SS, M228ŒîtssM, and M228ŒîtssJ. All these properties of the two gene complementation mutants were restored to the same levels as those of the wild-type strain, M228. Quantitative real-time results showed that during the interaction between the deletion mutant M228ŒîT6SS and the host, expression levels of T3SS transcriptional regulatory gene hrpR, structural genes hrpZ, hrcC, hopP1, and effector genes hopH1 and hopM1 were down-regulated at different levels. Taken together, our data provide evidence for the first time that the T6SS plays an important role in the pathogenicity of Psa, probably via effects on bacterial competition, biofilm formation, and environmental adaptability. Moreover, a complicated relationship exists between T6SS and T3SS. 

Retracted: No

PubPeer comments:
 Figure 6 and 7. * Red boxes: the right panel in Figure 6A (epithelial cells attached on controls) looks similar to the bottom left panel in Figure 7 (epithelial cells attached on 14% PVH hydrogels). Could the authors clarify if these are panels are displaying the same experiments or not? The legend of Figure 6A did not clearly identify the surface characteristics.  , 
 Thanks for pointing out the error in one of the figures of the manuscript ( similarity of a frame in fig 6a and fig 7). Frame in fig 7 is correct, however, mistakenly same figure got posted in the second frame of fig 6a. It happened by mistake some time during review or proof correction. Nature of surface is well defined in the paper ( cover slips). However, one can make out that it has no bearing on the results and the conclusion as well. First and communicating author ( Siddhi Gupta) has already brought it to the attention of the editor of the journal and have submitted an erratum ( with correct image). She has also communicated the same to this portal also. , 
 Thank you so much, Dr. Sinha and Dr. Gupta for correcting this error. Much appreciated. , 
 July 2020 correction: https: link.springer.com article 10.1007 s10856-020-06399-1  Following the publication of the original article [1], the authors reported an error in Fig. 6a. The image from Fig. 7 depicting the control sample was unintentionally used in place of the image depicting the 14% hydrogel sample. As such, the image has been replaced as shown below. In addition, the authors would like to update the 2nd sentence of section 3.1.4, which read u201cThese are representative of what is seen for all regions on the gelsu201d. To u201cThese are representative of what is seen for all regions on the coverslipsu201d. The purpose of this update is to better clarify what is being depicted. The authors would like to emphasize that these two corrections do not in any way alter the results of the article. I was not notified by the journal. ,