Polyamine signaling communications play a key role in regulating the pathogenicity of Dickeya fangzhongdai.

IMPORTANCE: Dickeya fangzhongdai is a newly identified plant bacterial pathogen with a wide host range. A clear understanding of the cell-to-cell communication systems that modulate the bacterial virulence is of key importance for elucidating its pathogenic mechanisms and for disease control. In this study, we present evidence that putrescine molecules from the pathogen and host plants play an essential role in regulating the bacterial virulence. The significance of this study is in (i) demonstrating that putrescine signaling system regulates D. fangzhongdai virulence mainly through modulating the bacterial motility and production of PCWD enzymes, (ii) outlining the signaling and regulatory mechanisms with which putrescine signaling system modulates the above virulence traits, and (iii) validating that D. fangzhongdai could use both arginine and ornithine pathways to synthesize putrescine signals. To our knowledge, this is the first report to show that putrescine signaling system plays a key role in modulating the pathogenicity of D. fangzhongdai.

The GacA-GacS Type Two-Component System Modulates the Pathogenicity of Dickeya oryzae EC1 Mainly by Regulating the Production of Zeamines.

The GacS-GacA type two-component system (TCS) positively regulates pathogenicity-related phenotypes in many plant pathogens. In addition, Dickeya oryzae EC1, the causative agent of soft rot disease, produces antibiotic-like toxins called zeamines as one of the major virulence factors that inhibit the germination of rice seeds. The present study identified a GacS-GacA type TCS, named TzpS-TzpA, that positively controls the virulence of EC1, mainly by regulating production of the toxin zeamines. RNA-seq analysis of strain EC1 and its tzpA mutant showed that the TCS regulated a wide range of virulence genes, especially those encoding zeamines. Protein-protein interaction was detected between TzpS and TzpA through the bacterial two-hybrid system and pull-down assay. In trans expression of tzpA failed to rescue the defective phenotypes in both the ΔtzpS and ΔtzpSΔtzpA mutants. Furthermore, TzpA controls target gene expression by direct binding to DNA promoters that contain a Gac-box motif, including a regulatory RNA rsmB and the vfm quorum-sensing system regulator vfmE. These findings therefore suggested that the EC1 TzpS-TzpA TCS system mediates the pathogenicity of Dickeya oryzae EC1 mainly by regulating the production of zeamines.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

First Report of Bacterial Soft Rot Disease on Taro Caused by Dickeya fangzhongdai in China.

Taro [Colocasia esculenta (L.) Schott.] is an important root crop in the world with great economic value. In recent years, outbreaks of soft rot were observed on taro plants in several plantation areas located in Shaoguan, Guangdong Province, China (25°7'57" N, 113°19'5" E). Root tubers of taro (Paodan variety) infected by soft rot had water-soaked lesions with a dark brown-black margin including a rotten smell, they also had internal rot that was also found in root tubers with no external symptoms. In some areas, the incidence of soft rot can reach up to 30%. To isolate the causal agent, ten pieces of taro root tubers with typical symptoms were surface-sterilized with 75% ethanol and 0.1% HgCl2 solution and then washed thrice with sterile water. The tuber slices were soaked in 50 ml sterile water and shaken at 28°C, 200 rpm for 2 h, and 100 µl was streaked onto the modified Yeast Extract Beef (YEB) agar medium (1% peptone, 0.5% yeast extract, 0.5% sucrose, 0.5% NaCl, 1 Mmol/L MgSO4•7H2O, 1.5% agar, pH 7.0) plates (Zhou et al. 2011) and incubated at 28°C for 24 h. Single colonies grown on YEB were selected for preliminary inoculation onto healthy taro (Paodan variety) slices. Two of the Gram-negative bacteria, named as ZXC1 and MPC2, developed symptoms consistent in rotted decay inside the root tubers after incubation for 24h at 30°C. ZXC1 and MPC2 were biochemically profiled using a Biolog Gen III MicroPlate (Microlog 3, 5.2) (Shen et al. 2019) and resulted Dickeya sp. (SIM 0.856 and 0.704). To determine the species of the Dickeya isolates, 16S rRNA sequences were amplified by primers 27F and 1492R (Hauben et al. 1998). Housekeeping genes including gyrB, atpD, rpoB, and infB were also amplified using degenerate primers (Brady et al. 2008). Results from the BLASTn analysis of the 16S rRNA (GenBank accession numbers MN853405, MN853406), gyrB (GenBank accession numbers MN866299, MN866303), atpD (GenBank accession numbers MN866298, MN866302), rpoB (GenBank accession numbers MN866301, MN866305), and infB (GenBank accession numbers MN866300, MN866304) genes in the isolates ZXC1 and MPC2 showed 99% identities to those of the previously reported D. fangzhongdai isolates from Phalaenopsis (Zhang et al. 2018). Multilocus sequence analysis (MLSA) by MEGA 7.0 performed with four housekeeping genes (gyrB, atpD, rpoB, infB) showed that they clustered with D. fangzhongdai isolates. Analyses using scanning and transmission electron microscopy showed that ZXC1 and MPC2 bacteria were rod-shaped, 0.5-1.0 µm × 1.0-3.0 µm, with peritrichous flagella. Pathogenicity tests were performed thrice using surface-sterilized 2-month-old taro seedlings (Paodan variety). Six individual seedlings were inoculated using a sterile syringe with ten microliters of bacterial suspension (108 CFU/ml) in Tris buffer (0.1 mol/L Tris and 0.1 mol/L HCl, pH 7.4). Taro seedlings injected with sterile Tris buffer were used as the negative control. These taro seedlings were grown in the greenhouse (30 ± 2°C, 90 ± 5% relative humidity). At the 25th day post inoculation, soft rot symptoms were observed in inoculated taro, while all control taro plants remained symptom-free. Small and pale yellow with irregular margins colonies consistent with morphological characteristics of those of D. fangzhongdai were re-isolated from symptomatic taro tubers and the housekeeping genes presence was verified by sequencing as described above, fulfilling Koch's postulates. D. fangzhongdai is a newly emerging bacterial pathogen, which causes bleeding cankers in pear trees (Tian et al. 2016), and soft rot of Phalaenopsis (Zhang et al. 2018). This is the first report of D. fangzhongdai causing soft rot disease in taro. Considering the high incidence of soft rot, this pathogen might pose a significant threat to taro and other economically important crops. Therefore, further researches are needed to investigate host range of the pathogen and develop appropriate integrated management to contain this disease spreading.