Shujun Nie

The foodborne pathogen Burkholderia gladioli pv. cocovenenans ( B. cocovenenans ) produces the lethal mitochondrial toxin bongkrekic acid (BA). To more accurately assess the toxin production levels of toxigenic strains, it is necessary to further optimize the culture conditions used during laboratory detection procedures. Here, we characterized the temperature-dependent growth and BA production of two BA-producing B. cocovenenans strains isolated from commercially available dried Auricularia auricula ( A. auricula ) in China, and examined their genomic relationships to previously reported strains. Across the discrete temperatures tested, both isolates showed the highest biomass at 31 °C and the highest measured BA levels at 37 °C, respectively. These observations are specific to the two isolates and the experimental conditions used; they should not be interpreted as species-wide temperature optima. Whole-genome sequencing, phylogenetic analysis, and average nucleotide identity (ANI) analysis showed that the isolates were closely related to outbreak-associated strains, including Co14, and both carried a bon biosynthetic gene cluster. However, BA levels in the original dry products and bacterial growth or BA production after rehydration were not assessed in this study. The findings therefore support further investigation of temperature-dependent BA production and the occurrence of toxigenic B. gladioli pv. cocovenenans in dried edible fungi, but do not by themselves establish the magnitude of consumer risk or demonstrate that current surveillance procedures underestimate it.

The cellulose of the plant cell wall indirectly affects the cell shape and straw stiffness of the plant. Here, the novel brittleness mutant brittle stalk-5 (bk-5) of the maize inbred line RP125 was characterized. We found that the mutant displayed brittleness of the stalk and even the whole plant, and that the brittleness phenotype existed during the whole growth period from germination to senescence. The compressive strength was reduced, the cell wall was thinner, and the cellulose content was decreased compared to that of the wild type. Genetic analysis and map-based cloning indicated that bk-5 was controlled by a single recessive nuclear gene and that it was located in a 90.2-Kb region on chromosome 3 that covers three open reading frames (ORFs). Sequence analysis revealed a single non-synonymous missense mutation, T-to-A, in the last exon of Zm00001d043477 (B73: version 4, named BK-5) that caused the 951th amino acid to go from leucine to histidine. BK-5 encodes a cellulose synthase catalytic subunit (CesA), which is involved with cellulose synthesis. We found that BK-5 was constitutively expressed in all tissues of the germinating stage and silking stage, and highly expressed in the leaf, auricula, and root of the silking stage and the 2-cm root and bud of the germinating stage. We found that BK-5 mainly localized to the Golgi apparatus, suggesting that the protein might move to the plasma membrane with the aid of Golgi in maize. According to RNA-seq data, bk-5 had more downregulated genes than upregulated genes, and many of the downregulated genes were enzymes and transcription factors related to cellulose, hemicellulose, and lignin biosynthesis of the secondary cell wall. The other differentially expressed genes were related to metabolic and cellular processes, and were significantly enriched in hormone signal transduction, starch and sucrose metabolism, and the plant–pathogen interaction pathway. Taken together, we propose that the mutation of gene BK-5 causes the brittle stalk phenotype and provides important insights into the regulatory mechanism of cellulose biosynthesis and cell wall development in maize.