Aixin Liu

Lysobacter capsici X2-3, a plant growth-promoting rhizobacteria (PGPR), was isolated from wheat rhizosphere and has inhibitory effects against a wide range of pathogens. One important characteristic of L. capsici is its ability to produce diverse antibiotics and lytic enzymes. The GntR family of transcription factors is a common transcription factor superfamily in bacteria that has fundamental roles in bacterial metabolism regulation. However, the GntR family transcription factor in Lysobacter has not been identified. In this study, to obtain an understanding of the GntR/HutC gene function in L. capsici X2-3, a random Tn5-insertion mutant library of X2-3 was constructed to select genes showing pleiotropic effects on phenotype. We identified a Tn5 mutant with an insertion in LC4356 that showed reduced biofilm levels, and sequence analysis indicated that the inserted gene encodes a GntR/HutC family transcription regulator. Furthermore, the LC4356 mutant showed reduced extracellular polysaccharide (EPS) production, diminished twitching motility and decreased survival under UV radiation and high-temperature. The RT‒qPCR results indicated that the pentose phosphate pathway-related genes G6PDH, 6PGL and PGDH were upregulated in the LC4356 mutant. Thus, since L. capsici is an efficient biocontrol agent for crop protection, our findings provide fundamental insights into GntR/HutC and will be worthwhile to improve PGPR biocontrol efficacy.

Southern blight, stem rot, and root rot are serious soil-borne fungal diseases of peanut, which are caused by Sclerotium rolfsii, Lasiodiplodia theobromae, and Fusarium oxysporum, respectively. These diseases are difficult to be diagnosed in early stage of infection, causing the optimal treatment period was often missed. Therefore, establishing a rapid detection system is of great significance for early prevention of peanut soil-borne fungal diseases. Here, we have invented a multiplex PCR detection system to detect fungal pathogens of peanut southern blight, stem rot, and root rot at the same time. The quarantine fungal pathogen primer pairs were amplified to the specific number of base pairs in each of the following fungal pathogens: 1005-bp (F. oxysporum), 238-bp (L. theobromae), and 638-bp (S. rolfsii). The detection limit for the single and multiplex PCR primer sets was 1 ng of template DNA under in vitro conditions. Amplification of fungi of non-target species yielded no non-specific products. The validation showed that the multiplex PCR could effectively detect single and mixed infections in field samples. Overview, this study proved that this mPCR assay was a rapid, reliable, and simple tool for the simultaneous detection of three important peanut soil-borne diseases, which facilitated prompt treatment and prevention of peanut root diseases.

Glucose-inhibited division protein (GidA) plays a critical role in the growth, stress response, and virulence of bacteria. However, how gidA may affect plant growth-promoting bacteria (PGPB) is still not clear. Our study aimed to describe the regulatory function of the gidA gene in Lysobacter capsici, which produces a variety of lytic enzymes and novel antibiotics. Here, we generated an LC_GidA mutant, MT16, and an LC_GidA complemented strain, Com-16, by plasmid integration. The deletion of LC_GidA resulted in an attenuation of the bacterial growth rate, motility, and biofilm formation of L. capsici. Root colonization assays demonstrated that the LC_GidA mutant showed reduced colonization of wheat roots. In addition, disruption of LC_GidA showed a clear diminution of survival in the presence of high temperature, high salt, and different pH conditions. The downregulated expression of genes related to DNA replication, cell division, motility, and biofilm formation was further validated by real-time quantitative PCR (RT–qPCR). Together, understanding the regulatory function of GidA is helpful for improving the biocontrol of crop diseases and has strong potential for biological applications.

Bacterial strain 71-2 with phosphate-solubilizing activity was isolated from tobacco rhizosphere and classified as Burkholderia cenocepacia based on sequence analyses of 16S rRNA and recA genes. To learn phosphate-solubilizing mechanisms of 71-2, mutants showing reduced solubilizing phosphate activity were obtained using the EZ-Tn5 transposon. Mutant 71-2-MT51 was reduced in the solubilizing phosphate content to 34.36% as compared with the wild-type strain 71-2. The disrupted gene in 71-2-MT51 was cloned and sequenced, and the putative protein from the gene shared 65.26% identity to protein sequences of enolase from Escherichia coli, which suggests the gene encodes an enzyme of enolase. Complementation analyzing showed that Eno was responsible for phosphate solubilizing for B. cenocepacia strain 71-2. To our knowledge, this is the first report of Eno involved in phosphate solubilizing in B. cenocepacia as well as in other bacteria.

Fifty-four single protoplast isolates (SPIs) were regenerated from three Rhizoctonia cerealis strains. A total of 169 rDNA-ITS regions were cloned and sequenced from these 54 SPIs. Variations in the ITS1 and ITS2 regions that flank the 5.8S gene were found within clones from the same strain, as well as within clones from the same SPI. These include variations in GC content and ITS length, and single-nucleotide polymorphisms (SNPs). The different strains and SPIs GC contents range from 40.25 to 41.74% and from 42.40 to 45.02%, in the ITS1 and ITS2 regions, respectively. All SNPs occur in the ITS1 and ITS2 regions, with 3–6 and 4–6 polymorphic sites in each region, respectively, in the different strains. SNP variation is relatively stable within the same strain. For example, the 89 ITS sequences generated from isolate WK-207, regardless of SPI or clone, predominantly cluster into two separate clades on a phylogenetic tree, suggesting that nuclei genetic heterogeneity is related to ITS variation in R. cerealis. Although rDNA-ITS sequences from the three strains and different SPIs are somewhat variable, all of our ITS sequences cluster together in anastomosis subgroup AG-DI during phylogenetic analysis. The ITS variation we observed does not negatively influence R. cerealis anastomosis group or subgroup classification.

Salvia miltiorrhiza Bge. is an important perennial herbaceous plant. The radix of S. miltiorrhiza is an important traditional Chinese medicine and has been widely used in the treatment of cardiovascular and cerebrovascular diseases. The seedling blight caused by Rhizoctonia solani AG1-IA on S. miltiorrhiza Bge. is a severe disease that can bring serious consequences to the areas where this plant was cultivated. In this study, we identified this pathogen as R. solani AG1-IA based on cultural and molecular characteristics and anastomosis tests. To our knowledge, this is the first report of R. solani AG1-IA causing seedling blight on S. miltiorrhiza Bge. in China or anywhere else in the world.

Lysobacter capsici strain X2-3 was isolated from the wheat rhizosphere in China and exhibits a remarkable capacity to inhibit the growth of multiple pathogens. Here, we report the draft genome sequence of L. capsici strain X2-3 in China.