Volatile organic compounds (VOCs) play important roles in the regulation of plant growth and pathogen resistance. However, little is known about the influence of VOCs released from endophytic strains (Burkholderia pyrrocinia strain JK-SH007) on controlling pathogens or inducing systemic resistance in poplar. In this study, we found that VOCs produced by strain JK-SH007 inhibit three poplar canker pathogens (Cytospora chrysosperma, Phomopsis macrospora, and Fusicoccum aesculi) and promote defense enzyme activity and malondialdehyde (MDA) and total phenol (TP) accumulation. Thirteen kinds of VOC components were identified using the solid-phase microextraction combined with gas chromatography-mass spectrometry method. Dimethyl disulfide (DMDS) accounted for the largest proportion of these VOCs. Treatments of poplar seedlings with different volumes of VOC standards (DMDS, benzothiazole, dimethylthiomethane, and phenylacetone) showed that DMDS had the greatest effects on various defense enzyme activities and MDA and TP accumulation. We also found that the inhibitory effect of the VOCs on the three pathogens was gradually enhanced with increasing standard volume. Moreover, the treatment of samples with DMDS significantly reduced the severity and development of the disease caused by three poplar canker pathogens. Comparative transcriptomics analysis of poplar seedlings treated with DMDS showed that there were 1,586 differentially expressed genes in the leaves and stems, and quantitative PCR showed that the gene expression trends were highly consistent with the transcriptome sequencing results. The most significant transcriptomic changes induced by VOCs were related to hormone signal transduction, transcriptional regulation of plant–pathogen interactions, and energy metabolism. Moreover, 137 transcription factors, including members of the ethylene response factor, NAC, WRKY, G2-like, and basic helix-loop-helix protein families, were identified to be involved in the VOC-induced process. This study elucidates the resistance induced by Burkholderia pyrrocinia strain JK-SH007 to poplar canker at the molecular level and can make possible a new method for the comprehensive prevention and control of poplar disease.

Pseudomonas brassicacearum GS20 is an antagonistic strain of bacteria recently isolated from the rhizosphere of Codonopsis pilosula. No validated reference gene has been indentified from P. brassicacearum to use in the normalization of real-time quantitative reverse transcription-PCR data. Therefore, in this study, nine candidate reference genes (recA, gyrA, rpoD, proC, gmk, rho, 16S, ftsz, and secA) were assessed at different growth phases and under various growth conditions. The expression stability of these candidate genes was evaluated using BestKeeper, NormFinder and GeNorm. In general, the results showed rho, rpoD and gyrA were the most suitable reference genes for P. brassicacearum GS20. The relative expression of iron-regulated gene (fhu) was normalized to verify the reliability of the proposed reference genes under iron-replete and iron-limited conditions. The trend in relative expression was consistent with the change in siderophore production under different iron conditions. This study presents reliable reference genes for transcriptional studies in P. brassicacearum GS20 under the chosen experimental conditions.

The efficient industrial conversion of plant-derived cellulose to simple sugars and other value-added chemicals requires various highly stable and reactive enzymes. Industrial processes especially synchronous saccharification and fermentation (SSF)-based production of cellulosic bio-ethanol require enzymes that are active at lower temperatures. In this study, we have identified, characterized, and expressed the cold-adaptive endo-1,4-β-glucanase (BpEG) isolated from the Burkholderia pyrrocinia JK-SH007. The analysis of the predicted amino acid sequence indicated that BpEG belongs to GH family 8. The BpEG without the signal peptide was cloned into the expression vector pET32a and significantly expressed in Escherichia coli BL21 (DE3) competent cells. The SDS-PAGE and Western blot analysis of BpEG revealed that the recombinant BpEG was approximately 60 kDa. Purified recombinant BpEG exhibited hydrolytic activity against carboxymethyl cellulose (CMC) and phosphoric acid swollen cellulose (PASC), but not crystalline cellulose and xylan substrates. High performance, anion exchange, chromatography-pulsed amperometric detector (HPAEC-PAD) analysis of the enzymatic products obtained from depolymerization of 1,4-β-linked biopolymers of different lengths revealed an interesting cutting mechanism employed by endoglucanases. The recombinant BpEG exhibited 6.0 of optimum pH and 35°C of optimum temperature, when cultured with CMC substrate. The BpEG enzyme exhibited stable activity between pH 5.0 and 9.0 at 35°C. Interestingly, BpEG retained about 42% of its enzymatic activity at 10°C compared to its optimal temperature. This new cold-adaptive cellulase could potentially achieve synchronous saccharification and fermentation (SSF) making BpEG a promising candidate in the fields of biofuel, biorefining, food and pharmaceutical industries.