evaluation of sinorhizobium meliloti retreat garcia.pdf · •castleton –chris villa, molly...
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Evaluation of Sinorhizobium meliloti
Signaling, Metabolism & Genetics
with their Symbiotic Plant Host,
Alfalfa
Preston P. Garcia, Ph.D.
Sinorhizobium meliloti
□ Exists in free living state or symbiont inside plant
roots
□ Ability to fix atmospheric nitrogen
Sinorhizobium + Alfalfa N2-fixing nodule
Medical significance
□ Sinorhizobium is classified within the α2
subdivision of Proteobacteria
□ Mammalian pathogens Rickettsia &
Brucella
□ Sinorhizobium & Brucella are members
of the Rhizobiales order, both forming
chronic infections of eukaryotic cells
□ Exchange chemical signals with their
eukaryotic host to gain entry & prevent
their own destruction once inside.
Using catabolite repression to
study S. meliloti physiology and
genetics
Bacterial catabolite repression
□ When bacteria use one carbon source preferentially over another □ Exhibits diauxic growth.
□ First described by Jacques Monod as the “glucose effect” in E. coli in the 1940’s. □ Model system for understanding carbon
usage, gene regulation, and global gene expression.
Identification of mutants with
altered Succinate-Mediated
Catabolite Repression (SMCR)
phenotype
SMCR screen results
30,000 colonies
52 strains showed SMCR relief on plates
(blue)
Further tested in liquid with succinate and 2° carbon
sources: α-galactoside (raffinose)
β-galactoside (lactose)
α-glucoside (maltose)
One strain showed relieved SMCR with these
secondary carbon sources
Mutation of sma0113 affects SMCR on multiple 2° C-sources
Garcia, P. P. Bringhurst, R.M. and D. J. Gage (2010). J Bacteriol. 192: 5725
Two component systems
Pi Pi
Physiological signal
PAS
H670 D57
□ Regulatory circuits that mediate responses to
diverse environmental signals & play a central
role in bacterial physiology.
□ Excellent context in which to study cell signaling and biochemical circuits
Utilization of fluorescent reporter
plasmids to monitor S. meliloti
competition and succinate sensing in
vivo
Succinate Biosensor Plasmid □ Used to visually monitor specific carbon
sources in the rhizosphere.
□ GFP (green fluorescent protein) is
expressed by the dctA promoter of the
DiCarboxylate Transport system
□ Facilitates movement of succinate, fumarate,
malate and aspartate across cell membrane
□ Cells fluoresce when actively utilizing C4
dicarboxylic acids
Glass slide
Pour Nod3 Agar
Media Place sterile
germinated
seeds
Overlay with moistened
dialysis tubing S. meliloti strain
Visualization of S. meliloti on
Medicago sativa (alfalfa) root
5ml Nod3 Media
Growth chamber cycle:
16 hour light
8 hour dark
Prepare slide for growth
chamber
Glass Slide
Dialysis tubing
Larger Glass
Slide
Inverted
microscope
objective
Orientation on Microscope
S. meliloti growth in the
rhizosphere □ Actively grows on C4-dicarboxylic
acid root exudate
Rm1021/pPG12 (dctA::gfp)
S. meliloti growth in infection
thread □ Actively grows on C4-dicarboxylic acids
within root hairs
Rm1021/pPG12 (dctA::gfp)
Competitive nodulation assay
□ All S. meliloti deletion strains
are Fix+/Nod+
□ 10 distinct S. meliloti strains
(mutation in single gene of
TCS) transformed with
constitutive rfp or gfp plasmid □ Co-inoculated on a single sterile
germinated alfalfa seed.
Growth chamber
cycle:
16 hour light
8 hour dark
Constitutive gfp and rfp
plasmids
*Thank you
to Norwich U.
Future directions
□ Visually document interactions in the
rhizosphere with the biosensor strains
□ Monitor relationship of the environmental
sensing capabilities for motility / chemotaxis
□ Analyze competition data to assess
infection and nodulation ability.
□ Complete knockout mutations of
genetically similar two component
systems to counter redundancy
• VGN
– Jeanne Harris, Ph.D.
• Castleton – Chris Villa, Molly Leach, Kelsey McKay, Katelynn
Leavey
– American Society for Microbiology
– Faculty-Student Research Grants
– Advance Study grants
• UConn
– Daniel J. Gage, PhD.
– Charles Bridges
• St. Josephs University
– Dr. Catalina Arango
S. meliloti growth at root cap
□ Growing on sloughed off root cap cells
Rm1021/pPG12 (dctA::gfp) @ root cap
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