First Report of Pectobacterium aroidearum Causing Soft Rot of Pinellia ternata in China

Pinelliae rhizoma is the dried tuber of Pinellia ternata (Thunb.) Breit., and has been used for thousand of years in traditional Chinese medicine as an antivomit, anticough, and analgesic (Ying et al. 2007). In September 2022, P. ternata planted in Bijie, Guizhou Province, showed severe soft rot symptoms with incidence of about 50%. The diseased plants showed water-soaked symptoms and produced a foul soft rot smell, and finally the whole plant collapsed. Lesions were first observed at the tip of a leaf or wound, and symptoms of the disease spread rapidly, with the entire plant collapsing and dying within a week. The tissue sections of six plants with typical symptoms from the diseased field were disinfected with 75% ethanol for 30 seconds and 0.3% NaClO for 3 minutes. The tissue sections were then washed with sterile water for three times. A small piece of tissue (5x5mm) was removed from the edge of the lesion and mashed in a 1.5 ml centrifuge tube containing 20 μl of sterile water. The tissue liquid was then diluted 100 times with prepared sterile water. The bacteria were streaked on LB (tryptone/yeast extract/NaCl) AGAR medium and cultured at 37°C for 48 h (Kravitz, 1962). Isolated colonies were streaked on Luria-Bertani (LB) AGAR medium to obtain single colonies for further identification. A total of 13 representative isolates were selected for PCR amplification using primers targeting the conserved region of the 16S rDNA gene, which were in turn analyzed via the BLASTn search engine on the NCBI website. The results of the analysis revealed that seven of the isolates were similar to P. aroidearum strain SCRI 109 (GenBank accession no. NR_159926), with strain BX13 exhibiting the highest similarity to P. aroidearum (99.93% similarity), and therefore, this strain was selected for further investigation. The strain BX13 was incubated on LB solid medium for 24 h at 37 °C, and the single colonies were creamy white, translucent and round, slightly elevated in the center, with smooth surfaces and neat edges (Figure S1 B1). Then,the Scanning Electron Microscope revealed that the thalli of strain BX13 were short rod-shaped and somewhat blunt round at both ends (Figure S1 B2). The steward genes (icdA, gapA, proA) of BX13 were amplified and sequenced for further identification. The sequences of the amplified fragments were all deposited in GenBank 16S rDNA (OQ874505,) icdA (OQ954122)，gapA (OQ954123), proA (OQ954124). Sequence analysis using the BLASTn program at the NCBI revealed gene icdA, gapA, and proA had 100% identity to P. aroidearum strain QJ002 (GenBank accession no. CP090597).. Meanwhile, a maximum likelihood phylogenetic tree was constructed based on multigene sequence analysis of BX13 16S rDNA and steward genes (gapA, icdA, proA) by MEGA X (Liang et al. 2022). Phylogenetic results also showed that BX13 and P. aroidearum strain QJ002 gathered in the same clade(Figure S2). Accordingly, the morphological and molecular characteristics of strain BX13 indicate that it is P. aroidearum. (Nabhan S., et al.2013,Xu et al. 2020). In order to confirm the pathogenicity of strain BX13, a bacterial suspension containing 107 CFU/ml (10 ml/ inoculation point) was injected into the base of a one-week-old P. ternata stems, control seedlings were inoculated with sterile water, inoculated and control seedlings (each of six plants) were kept in a growth chamber maintained at 26°C with a relative humidity range of 70% to 80%. Plants were watered as needed. After 3 days, the stem base of the plants inoculated with bacteria solution showed water-soaked necrosis and stems began to rot, while the plants inoculated with water did not show this symptom. The strains were then successfully re-isolated from the symptomatic P. ternata. Then the strain re-isolated was identified using the BLASTn program at the NCBI and found that it has the same 16S rDNA, icdA, gapA, and proA sequences as strain BX13, thus completing the Koch's postulates. To our knowledge, this is the first report of P. aroidearum causing P. ternata soft rot in China, which expands its known host range. Accordingly, this study provides essential information for the breeding of P. ternata resistant to bacterial soft rot and the development of control measures in China.