OEM--a new medium for rapid isolation of onion-pathogenic and onion-associated bacteria

Isolation and Characterization of Bacteria Associated with Onion and First Report of Onion Diseases Caused by Five Bacterial Pathogens in Texas, U.S.A

Bacterial diseases pose a severe challenge to growers and cause significant loss to the billion-dollar onion industry in the United States. Texas is the sixth largest onion producing state, yet the bacterial communities associated with short-day onion crops grown in Texas have not been studied. This study was conducted to identify, characterize, and understand the diversity of bacteria associated with onion production in Texas. In 2020, 190 foliar and 210 bulb samples were collected from onion crops in the Rio Grande Valley and Winter Garden regions of Texas. Sequencing of the 16s rRNA gene was used to identify each bacterial strains to a genus. The pathogenicity to onion of each bacterial strain was tested using three assays: a red onion scale assay, a yellow onion bulb assay, and a foliar assay. Whole genome sequencing was done to identify the onion-pathogenic strains to species. Collectively, isolates of 24 genera belonging to three phyla were detected, including 19 genera from foliar samples and nine genera from bulb samples. Isolates in the Phylum Proteobacteria, including 15 genera of Gram-negative bacteria, were the most abundant of the taxa, comprising 90.0% of the strains isolated. The diversity of foliar isolates was evenly distributed between Gram-positive and Gram-negative bacteria, while Gram-negative bacteria dominated the isolates from bulb samples. In total, 83.9% of the bacterial isolates were not pathogenic on onion, with only isolates of Pantoea, Pseudomonas, Burkholderia, Erwinia, Enterobacter, and Curtobacterium proving pathogenic. Strains of Burkholderia gladioli, Pseudomonas alliivorans, Pantoea agglomerans, P. ananatis, and P. allii are the first documented cases of these pathogens of onion in Texas. Identifying and characterizing the nature of onion microflora, including pathogens of onion, is vital to developing rapid disease detection techniques via pathogenomics and minimizing losses through the application of effective disease management measures.

Rahnella spp. are commonly isolated from onion (Allium cepa) bulbs and are weakly pathogenic

AIMS

Bacterial decays of onion bulbs have serious economic consequences for growers, but the aetiologies of these diseases are often unclear. We aimed to determine the role of Rahnella, which we commonly isolated from bulbs in the United States and Norway, in onion disease.

METHODS AND RESULTS

Isolated bacteria were identified by sequencing of housekeeping genes and/or fatty acid methyl ester analysis. A subset of Rahnella spp. strains was also assessed by multilocus sequence analysis (MLSA); most onion strains belonged to two clades that appear closely related to R. aquatilis. All tested strains from both countries caused mild symptoms in onion bulbs but not leaves. Polymerase chain reaction primers were designed and tested against strains from known species of Rahnella. Amplicons were produced from strains of R. aquatilis, R. victoriana, R. variigena, R. inusitata and R. bruchi, and from one of the two strains of R. woolbedingensis.

CONCLUSIONS

Based on binational testing, strains of Rahnella are commonly associated with onions, and they are capable of causing mild symptoms in bulbs.

SIGNIFICANCE AND IMPACT OF THE STUDY

While Rahnella strains are commonly found within field-grown onions and they are able to cause mild symptoms, the economic impact of Rahnella-associated symptoms remains unclear.

Genomics-based Sensitive and Specific Novel Primers for Simultaneous Detection of Burkholderia glumae and Burkholderia gladioli in Rice Seeds

Panicle blight and seed rot disease caused mainly by Burkholderia glumae and Burkholderia gladioli is threatening rice cultivation worldwide. The bacteria have been reported as seed-borne pathogens from rice. Accurate detection of both pathogens on the seeds is very important for limiting the disease dissemination. Novel primer pairs targeting specific molecular markers were developed for the robust detection of B. glumae and B. gladioli. The designed primers were specific in detecting the target species with no apparent crossreactions with other related Burkholderia species at the expected product size. Both primer pairs displayed a high degree of sensitivity for detection of B. glumae and B. gladioli separately in monoplex PCR or simultaneously in duplex PCR from both extracted gDNA and directly preheated bacterial cell suspensions. Limit of detection was as low as 0.1 ng of gDNA of both species and 3.86 × 102 cells for B. glumae and 5.85 × 102 cells for B. gladioli. On inoculated rice seeds, the designed primers could separately or simultaneously detect B. glumae and B. gladioli with a detection limit as low as 1.86 × 103 cells per rice seed for B. glumae and 1.04 × 104 cells per rice seed of B. gladioli. The novel primers maybe valuable as a more sensitive, specific, and robust tool for the efficient simultaneous detection of B. glumae and B. gladioli on rice seeds, which is important in combating rice panicle blight and seed rot by early detection and confirmation of the dissemination of pathogen-free rice seeds.

Center Rot of Onion (Allium cepa) Caused by Pantoea ananatis Requires pepM, a Predicted Phosphonate-Related Gene

Pantoea ananatis, a cause of center rot of onion, is problematic in the United States and elsewhere. The bacterium lacks disease determinants common to most other bacterial pathogens of plants. A genomic island containing the gene pepM was detected within many onion-pathogenic strains of P. ananatis of diverse origins. The pepM gene of P. ananatis putatively encodes a protein that converts phosphoenolpyruvate to phosphonopyruvate, the first step in the biosynthesis of phosphonates and related molecules. This gene appears to be essential for center rot disease. Deletion of pepM rendered the mutant strain unable to cause lesions in leaves of growing onions and water-soaking of inoculated yellow onion bulbs. Furthermore, growth of the deletion mutant in onion leaves was significantly diminished compared with wild-type bacteria, and the mutant failed to cause cell death in tobacco. Complementation of the mutated strain with pepM restored the phenotype to wild-type capability. The pepM gene is the first pathogenicity factor identified that affects bacterial fitness as well as symptom development in both leaves and bulbs in a pathogen causing center rot of onion.

Lactic Acid Bacteria Cause a Leaf Blight and Bulb Decay of Onion (Allium cepa)

Several members of the lactic acid bacteria group were isolated from diseased onion plants and bulbs. Based on growth characteristics and sequence analysis of 16S rRNA and rpoA genes, the strains were identified as Lactococcus lactis, Lactobacillus plantarum, and three species of Leuconostoc, i.e., citreum, mesenteroides, and pseudomesenteroides. Pathogenic potential to onion leaves and mature onion bulbs was assessed. L. plantarum and all three Leuconostoc species caused symptoms in both leaves and bulbs. L. lactis caused scale discoloration in bulbs but failed to cause lesions on leaves. Leuconostoc citreum caused bulb decay in 7 days at 18°C as well as 37°C. This is the first report of a group of gram-positive bacteria able to cause disease in onion leaves.

PCR Primers for Detection of Pantoea ananatis, Burkholderia spp., and Enterobacter sp. from Onion

Bacterial decays of onion bulbs cause sporadic and sometimes serious losses to onion (Allium cepa). In New York, three groups of bacteria were identified as problematic: Burkholderia spp., Pantoea ananatis, and Enterobacter spp. To aid in efficient detection and diagnosis of these pathogens, pairs of specific polymerase chain reaction primers were designed and validated, based on a strategy that utilized various genome sequences now available in public databases. Primer pairs were tested against numerous strains of target bacteria, closely related bacteria, and other onion-pathogenic bacteria. Each primer pair yielded a single, apparently highly specific amplicon from aqueous suspensions of the target bacteria. Minimum sensitivities were approximately 10³ CFU per 25-μl reaction mixture for all three primer pairs.

Evaluation of economically feasible, natural plant extract-based microbiological media for producing biomass of the dry rot biocontrol strain Pseudomonas fluorescens P22Y05 in liquid culture

The production of microbial biomass in liquid media often represents an indispensable step in the research and development of bacterial and fungal strains. Costs of commercially prepared nutrient media or purified media components, however, can represent a significant hurdle to conducting research in locations where obtaining these products is difficult. A less expensive option for providing components essential to microbial growth in liquid culture is the use of extracts of fresh or dried plant products obtained by using hot water extraction techniques. A total of 13 plant extract-based media were prepared from a variety of plant fruits, pods or seeds of plant species including Allium cepa (red onion bulb), Phaseolus vulgaris (green bean pods), and Lens culinaris (lentil seeds). In shake flask tests, cell production by potato dry rot antagonist Pseudomonas fluorescens P22Y05 in plant extract-based media was generally statistically indistinguishable from that in commercially produced tryptic soy broth and nutrient broth as measured by optical density and colony forming units/ml produced (P ≤ 0.05, Fisher's protected LSD). The efficacy of biomass produced in the best plant extract-based media or commercial media was equivalent in reducing Fusarium dry rot by 50-96% compared to controls. In studies using a high-throughput microbioreactor, logarithmic growth of P22Y05 in plant extract-based media initiated in 3-5 h in most cases but specific growth rate and the time of maximum OD varied as did the maximum pH obtained in media. Nutrient analysis of selected media before and after cell growth indicated that nitrogen in the form of NH4 accumulated in culture supernatants, possibly due to unbalanced growth conditions brought on by a scarcity of simple sugars in the media tested. The potential of plant extract-based media to economically produce biomass of microbes active in reducing plant disease is considerable and deserves further research.