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Pathogenicity of Xanthomonas axonopodis pv. glycines, the
Causative Agent of Bacterial Pustule in Soybeans.
L. Goradia1, G. Hartman2, and S. L. Daniel1
1Department of Biological Sciences,
Eastern Illinois University, Charleston, IL
2Department of Crop Sciences, University of
Illinois, Urbana, IL
Introduction
Xanthomonas axonopodis pv. glycines (Xag)
Gram-negative aerobic, rod-shaped bacterium.
Member of family Pseudomonadaceae.
Produces extra-cellular polysaccharides and xanthum gum.
Causes bacterial pustules in soybeans.
Bacterial Pustule - Disease cycle
Spread by splashing water, wind-blown rain and cultivation when
foliage is wet.
Enters plant through stomata and wounds.
Intracellular growth (25-30oC).
Disease symptoms appear after 7-10 days.
Bacterial Pustule - Disease Symptoms
Small yellow-green spots appear initially.
Reddish brown spots appear later.
Pustules rupture and dry out.
Bacterial Pustule - Incidence
Pustule is the most prevalent bacterial of soybeans in Brazil,
china, India, Korea, USA and Taiwan.
Pustule reduces crop yield by 40% (reduces yield in USA by
4-11%).
Bacterial Pustule - Control
Crop rotation.
Burial of crop residues.
Planting resistant varieties.
Resistance developed by rxp gene
Resistance maintained through traditional plant breeding.
Reasons to study Xag
Little is known about Xag.
World wide pathogen causes significant reduction in soybean
yield.
Occasional outbreaks are observed in the USA.
Symptoms are similar to those of soybean rust.
Objectives
Isolation and comparison of Xag isolates through biochemical and
molecular techniques
Development of greenhouse assays for evaluation of factors that
impact pathogenicity of Xag.
Screen a wide variety of commercial soybean cultivars for
resistance to Xag.
Methods
Isolation of Xag
Leaf area with pustule was cut and homogenized in sterile
de-ionized water.
Streaking for isolation on potato dextrose agar (PDA) and
incubation for 48 h at 25-30oC.
Isolates were verified by Gram stain and their ability to cause
pustule on susceptible soybean cultivars.
Comparison of isolates: Growth profiles, metabolic fingerprinting
(Biolog® Plates), SDS-PAGE profiles and pathogenicity
testing using a greenhouse assay.
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Table 1. Bacteria used in this study. |
Greenhouse assay for pathogenicity testing
Isolates were grown in PDA and incubated for 8 h at 25oC
(mid log) on a shaker. Cells were harvested and washed with sterile de-ionized water. Optical Density was adjusted to 0.3 (108 cells/ml)
before spraying on two-week old plants (PI 520.733). Sixteen plants were used per experiment. Plants were damaged with a perforation needle prior to
inoculation (20 ml of inoculum per sixteen plants). Pustule severity was recorded after 7-10 days.
Results
Xag-1 and Xag-3 were isolated from susceptible soybeans PI
520.733 and Spencer, respectively. Tests confirmed Xag-1 and Xag-3 produced pustule on PI 520.733. Gram staining showed the bacteria were Gram-negative rods. The bacteria produced yellow, mucoid colonies on PDA.
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Figure 4. Growth profiles of
Xag isolates. |
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Figure 5. Metabolic
fingerprinting of Xag isolates. |
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Figure 6. SDS-PAGE profiles of
Xag isolates. |
Screening for resistance to Bacterial Pustule: Soybean Variety
Testing
Initial screening showed 146 out of 525 Roundup®
ready cultivars were susceptible to Xag-1
Out of 146 cultivars, 66 were more susceptible than PI 520.733.
Summary
Xag-1 and Xag-3 produced pustule on susceptible soybean cultivars
Xag-1 was more aggressive and slightly different than other Xag
isolates.
Factors that impact pathogenicity include cell age and growth
conditions.
Some Roundup® ready soybean
cultivars were susceptible to Xag-1
Resistance was not maintained
Potential to cause loss in yield.
Future Studies
Isolation of Xag.
Techniques for comparison.
Variety testing with different Xag isolates.
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