Bacterial Brown Leaf Spot of Citrus, a New Disease Caused by Burkholderia andropogonis

Draft Genome Sequences of Robbsia andropogonis Isolated from Sorghum bicolor Exhibiting Bacterial Leaf Stripe in Australia

Robbsia andropogonis causes leaf spots, streaks, or stripes on a wide range of commercially important crops. Here, we present the draft genome sequences of two isolates of R. andropogonis sourced from Sorghum bicolor displaying symptoms of bacterial leaf stripe disease in Australia.

First report of bacterial leaf spot disease on Pueraria montana var. thomsonii caused by Robbsia andropogonis in China

Pueraria montana var. thomsonii (Hereinafter referred to as Pmt) belongs to the Leguminosae and is widely distributed in China, Laos, Thailand, Myanmar, Bhutan and other Asian countries. The plant is called "Fenge" in China, and its root is widely used in medicine and food. In recent years, an unknown leaf spot disease of Pmt has occurred in Gaoming, Zhaoqing and Yunfu districts of Guangdong Province in China, where 1,600 hectares of Pmt plants were affected. The incidence rate of plants were more than 80% and led to 10-15% death of Pmt plants in Gaoming district. . In the early stage of the disease, radiating and water-soaking lesions appeared between the main veins and side veins of Pmt leaves. After the spread of the lesions, they formed brown and short strips with yellow haloes around them, which led to leaf shedding, plant death and decline of production. To isolate bacteria, diseased leaves were surface sterilized with 0.6% sodium hypochlorite solution for 30 s, followed by three consecutive rinses in distilled water. The leaves were aseptically macerated, and the macerate streaked on PDA medium. Whitish to dull white, mucoid, raised, round, and translucent colonies were obtained. All isolates were gram-negative and had a single, polar, sheathed flagellum. Sequences (approx. 1,458 bp each) of the 16S rRNA gene amplified from five isolates (FG2, FG3, FG9, FG12 and FG17) using primer pair 27F/1492R (Lane et al,1991) (GenBank Accession Nos. OL677034, OL677351, OL677352, OL677353 and OL677354 respectively) shared 99.93% sequence identity with that of Robbsia andropogonis (Synonyms: Burkholderia andropogonis) (Lopes-Santos et al,2017) type strain LMG2129 (NR104960.1). The specific 410-bp and 704-bp target fragments were also amplified from isolates using R. andropogonis-specific primers Pf/Pr (Bagsic et al,1995) and LJ23f/LJ24R (Duan et al,2009). The four housekeeping genes atpD, lepA, gyrB and rpoD were partially sequenced for FG9 isolates using primers atpD-F3/atpD-R3, lepA-F2/lepA-R, LJ23f/LJ24R and LJ25f/LJ26r (Duan et al,2009; Estrada-De et al,2013) respectively. Multilocus sequence analyses confirmed the isolates from Pmt as R. andropogonis. Physiological and biochemical tests revealed the isolates are negative for oxidase, arginine dihydrolase, saccharose and betaine, and positive for sorbitol, lactose and galactose (Gillis et al,1995; Lopes-Santos et al,2017). In addition, all isolates caused a hypersensitive reaction on leaves of Nicotiana benthamiana and were pathogenic to some crops, including maize (Zea mays), sorghum (Sorghum bicolor), carnation (Dianthus caryophilus), common bean (Phaseolus vulgaris), tomato. Five isolates (FG2, FG3, FG9, FG12 and FG17) pathogenicity were tested twice with a total of three replications per isolate. Two young leaves each of 3-month-old Pmt plants grow in greenhouse were sprayed a bacterial suspension at 108 CFU/ml, then covered the inoculated leaves individually with plastic bags for 24 h, and incubated at 100% relative humidity with 16 h of daylight at 30°C and 8 h of darkness at 22°C in a greenhouse. Radiating and water-soaked lesions with yellow haloes were observed between the main veins and side veins of Pmt leaves 5 days after inoculation and were similar to those caused by R. andropogonis in the field. Koch's postulates were fulfilled by reisolating bacteria from typical lesions on inoculated plants. And the reisolated bacteria were identical to the inoculated ones. To our knowledge, this is the first report of R. andropogonis on Pueraria montana var. thomsonii in China.

Molecular, Phenotypical, and Host-Range Characterization of Robbsia andropogonis Strains Isolated from Bougainvillea spp. in Mexico

Characteristic leaf spot and blight symptoms caused by Robbsia andropogonis on bougainvillea plants were found in three locations in different provinces of Mexico from 2019 to 2020. Eleven bacterial isolates with morphology similar to R. andropogonis were obtained from the diseased bougainvillea leaves. The isolates were confirmed as R. andropogonis by phenotypic tests and 16S rRNA, rpoD, and gyrB gene sequencing. In addition to bougainvillea, the strains were pathogenic to 10 agriculturally significant crops, including maize (Zea mays), sorghum (Sorghum bicolor), barley (Hordeum vulgare), coffee (Coffea arabiga), carnation (Dianthus caryophilus), Mexican lime (Citrus × aurantifolia), common bean (Phaseolus vulgaris), broadbeans (Vicia faba), and pea (Pisum sativum), but not runner bean (Phaseolus coccineus). The haplotypes network reveals the genetic variability among Mexican strains and its phylogeographic relationship with Japan, the U.S.A., and China. The presence of this pathogen represents a challenge for plant protection strategies in Mexico.

Reassessment of the taxonomic position of Burkholderia andropogonis and description of Robbsia andropogonis gen. nov., comb. nov

The phylogenetic classification of the species Burkholderia andropogonis within the Burkholderia genus was reassessed using 16S rRNA gene phylogenetic analysis and multilocus sequence analysis (MLSA). Both phylogenetic trees revealed two main groups, named A and B, strongly supported by high bootstrap values (100%). Group A encompassed all of the Burkholderia species complex, whi.le Group B only comprised B. andropogonis species, with low percentage similarities with other species of the genus, from 92 to 95% for 16S rRNA gene sequences and 83% for conserved gene sequences. Average nucleotide identity (ANI), tetranucleotide signature frequency, and percentage of conserved proteins POCP analyses were also carried out, and in the three analyses B. andropogonis showed lower values when compared to the other Burkholderia species complex, near 71% for ANI, from 0.484 to 0.724 for tetranucleotide signature frequency, and around 50% for POCP, reinforcing the distance observed in the phylogenetic analyses. Our findings provide an important insight into the taxonomy of B. andropogonis. It is clear from the results that this bacterial species exhibits genotypic differences and represents a new genus described herein as Robbsia andropogonis gen. nov., comb. nov.

Draft Genome Sequence of Burkholderia andropogonis Type Strain ICMP2807, Isolated from Sorghum bicolor

Here, we report the draft genome sequence of Burkholderia andropogonis ICMP2807, a phytopathogenic bacterium isolated from Sorghum bicolor plants in the United States.

Pathogenicity and biotechnological applications of the genus Burkholderia

Bacteria belonging to the genus Burkholderia are well known for their adaptability to habitats as diverse as freshwater sediments, lungs of cystic fibrosis patients and plant tissues. This genus includes also plant, animal and human pathogenic species, such as Burkholderia glumae, Burkholderia pseudomallei and the Burkholderia cepacia complex. Over the past few years, several newly discovered non-pathogenic plant associated Burkholderia species have raised particular interest for their potential use in plant growth promotion, biocontrol of plant pathogens, phytoremediation and xenobiotics degradation. Highlights from recent studies on the taxonomy, ecology and pathogenicity of different species of the Burkholderia genus are presented with the aim to evaluate their potential use in biotechnology.

Genomic and resistance gene homolog diversity of the dominant tallgrass prairie species across the U.S. Great Plains precipitation gradient

BACKGROUND

Environmental variables such as moisture availability are often important in determining species prevalence and intraspecific diversity. The population genetic structure of dominant plant species in response to a cline of these variables has rarely been addressed. We evaluated the spatial genetic structure and diversity of Andropogon gerardii populations across the U.S. Great Plains precipitation gradient, ranging from approximately 48 cm/year to 105 cm/year.

METHODOLOGY/PRINCIPAL FINDINGS

Genomic diversity was evaluated with AFLP markers and diversity of a disease resistance gene homolog was evaluated by PCR-amplification and digestion with restriction enzymes. We determined the degree of spatial genetic structure using Mantel tests. Genomic and resistance gene homolog diversity were evaluated across prairies using Shannon's index and by averaging haplotype dissimilarity. Trends in diversity across prairies were determined using linear regression of diversity on average precipitation for each prairie. We identified significant spatial genetic structure, with genomic similarity decreasing as a function of distance between samples. However, our data indicated that genome-wide diversity did not vary consistently across the precipitation gradient. In contrast, we found that disease resistance gene homolog diversity was positively correlated with precipitation.

SIGNIFICANCE

Prairie remnants differ in the genetic resources they maintain. Selection and evolution in this disease resistance homolog is environmentally dependent. Overall, we found that, though this environmental gradient may not predict genomic diversity, individual traits such as disease resistance genes may vary significantly.