Screening of aerobic endospore-forming bacterial isolates as candidate biocontrol agents against rhizoctonia solani.
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Date
2016
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Abstract
Bacterial-based biocontrol of soil-borne phytopathogens has gained prominence as a
promising technology for developing sustainable agricultural pest control practices. Aerobic
endospore-forming bacteria are seen as potential candidates for biocontrol applications due to
various ecological and physiological traits which have been shown to influence plant health
and disease control. Their ability to produce endospores also provides a major commercial
advantage over non spore-forming bacteria. Appropriate screening methods are central to the
discovery of successful biocontrol agents and should ideally be both ecologically relevant and
able to evaluate a large number of isolates. A study was therefore undertaken with the aim of
establishing screening methods that facilitate the selection of aerobic endospore-forming
bacteria as candidate biocontrol agents against Rhizoctonia solani, an economically important
fungal pathogen exhibiting a wide host range.
Aerobic endospore-forming bacteria were isolated from rhizosphere material of five crop
types grown in composted pine bark medium and screened for R. solani antagonism using
traditional in vitro dual-culture bioassays. Isolates exhibiting antifungal activity were then
evaluated in vivo for biocontrol activity against R. solani in cucumber seedling trials.
Selected isolates were evaluated further using several screening approaches including:
genomic fingerprinting; characterization of, and PCR-based screening for genes involved in
the biosynthesis of bioactive lipopeptide compounds; and, the use of matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) as a means of
rapidly screening bacterial isolates.
Approximately 6% of the bacterial isolates (n=400) showed antagonism towards R. solani in
vitro. Dual-culture bioassays against R. solani, Fusarium oxysporum, Botrytis cinerea and
Pythium arrhenomanes revealed that the antagonistic activity amongst isolates varied
considerably and was influenced by the duration of the assay. From these assays it was
possible to rank isolates based on the extent and stability of the inhibitory response in vitro as
well as by the spectrum of antifungal activity observed. Twenty-four isolates were selected
for in vivo screening for biocontrol activity against R. solani, using susceptible cucumber
plants grown under greenhouse conditions. In preliminary experiments the pathogen loading
rates were shown to have a marked influence on disease severity. In experiments where R.
solani was seeded in the form of colonized agar plugs, significant differences between
treatments and controls were recorded and several potential biocontrol candidates were
identified. A general observation was that isolates that achieved high rankings in vitro
performed better in the in vivo trial than those with lesser rankings; although some exceptions
were noted. These findings support the notion that fungal antagonism is an important
determinant of biocontrol potential that can be used in preliminary biocontrol screening
programmes.
Internal-transcribed spacer region (ITS) PCR and randomly amplified polymorphic DNA
(RAPD) PCR were evaluated as methods to differentiate isolates exhibiting antifungal
activity in vitro. ITS-PCR distinguished three major groupings, but proved to be limited in its
ability to detect inter- and intra-specific variation amongst closely related organisms. Based
on 16S rRNA gene sequence analysis, two of the groups were identified as members of the
“Bacillus subtilis” and “Bacillus cereus” clusters; while, the third group consisted of a single
isolate identified as a strain of Brevibacillus laterosporus. RAPD-PCR revealed further levels
of genetic diversity within each ITS grouping. The “Bacillus subtilis” cluster was
distinguished further into four distinct groups, which based on gyrA gene fragment sequence
analysis, were identified as strains of B. amyloliquefaciens subsp. plantarum and B. subtilis
respectively. Sequence matches were consistent with the RAPD-PCR groupings, indicating
that this method was appropriate for differentiating related isolates at the strain and possibly
the sub-species level. Clonal similarities were evident for a number of strains isolated from
different plant species suggesting that these may reflect populations of rhizosphere competent
strains and/or plant adapted ecotypes. Strains of B. amyloliquefaciens subsp. plantarum and
B. subtilis were amongst the best performers in the in vivo biocontrol seedling trial and
generally performed better than the “Bacillus cereus” group of isolates. RAPD-PCR of the
“Bacillus cereus” isolates showed that they exhibited greater levels of genetic heterogeneity
and that the groupings detected were not consistent when different primer sets were
evaluated. Genomic fingerprinting was found to provide an insight into the prevalence,
distribution and possible rhizosphere competency of related strains.
Liquid chromatography was used in conjunction with electrospray-ionization time-of-flight
(ESI-TOF) mass spectrometry (MS) to characterize bioactive lipopeptides purified from
culture supernatants of selected strains that ranked highly in the in vitro/in vivo assays.
Phylogenetically related strains produced very similar lipopeptide profiles. Bacillus subtilis
strains were found to produce isoforms of surfactin and fengycin. In addition to these
lipopeptides, B. amyloliquefaciens subsp. plantarum strains were also found to produce
isoforms of bacillomycin D or iturin A. Bacillomycin/iturin and fengycin fractions exhibited
antifungal activity in vitro, whereas surfactin fractions did not. Isolates that ranked the
highest in the R. solani dual-culture bioassays all produced either isoforms of bacillomycin D
or iturin A. Bacillomycin D producing isolates were amongst the best performers in the in
vivo biocontrol trials.
Gene markers targeting the biosynthetic apparatus of the detected lipopeptide classes were
then assessed for screening purposes using PCR. BACC1F/1R primers targeting the
bacillomycin D synthetase C (bmyC) gene correlated well with the ESI-TOF MS findings,
whereas ITUD1F/1R primers targeting the malonyl-CoA-transacylase (ituD) gene linked to
iturin A biosynthesis were unable to distinguish between isolates that produced iturin or
bacillomycin in culture. Disparities between some of the PCR and ESI-TOF MS results
suggested that primers targeting srfA (surfactin) and fenD (fengycin) biosynthetic genes
showed limited specificity amongst the strains screened. Phylogenetic comparisons of srfD
and fenD gene sequences from selected strains of B. amyloliquefaciens subsp. plantarum and
B. subtilis revealed that these genes clustered according to species with marked heterogeneity
between clusters being evident. Using fenD gene sequence data from B. amyloliquefaciens
subsp. plantarum FZB42, primers (FENG1F/1R) targeting fengycin synthetase genes of
strains of B. amyloliquefaciens subsp. plantarum isolated in this study were successfully
established.
MALDI-TOF MS was assessed as a means of identifying isolates antagonistic to R. solani in
vitro and determining their associated lipopeptide profiles. Mass spectra were obtained in the
m/z range 2000 to 20000 for identification and grouping purposes and in the m/z 750 to 2500
range in order to profile lipopeptide production. The available Bruker BDal spectral library
allowed for the identification of isolates to the genus level but proved to be limited for
identifying environmental isolates to the species level. Extension of the library using “inhouse”
mass spectra generated from isolates identified in this study significantly improved
the level of isolate identification in subsequent identification runs. Cluster analysis of mass
spectra allowed for the relationships between isolates to be established and provided a means
of grouping closely related isolates. Strains of B. subtilis and B. amyloliquefaciens were
clearly distinguished from one another and the potential for differentiating strains at the subspecies
level was also shown. MALDI-TOF MS also provided a convenient means of
detecting bioactive lipopeptides directly from whole cell preparations, cell extracts and crude
culture filtrates. Lipopeptide profiles varied depending on taxonomic groupings. Results for
isolates within the “Bacillus subtilis” group supported the earlier ESI-TOF MS findings and
were found to be more reliable than PCR screening for lipopeptide synthesis genes. “Bacillus
cereus” group isolates produced distinct spectral profiles with peaks that were consistent with
biomarkers previously described in the literature as isoforms of the kurstakin class of
lipoheptapeptides. Brevibacillus laterosporus CC-R4 yielded a unique spectral profile in the
m/z 750-2000 range with mass fragments which were similar to antimicrobial compounds
recently reported in the literature. Overall, MALDI-TOF MS was found to fulfil the
requirement for a practical yet robust technique suitable for processing large numbers of
aerobic endospore-forming bacteria for biocontrol screening. This study has shown that genomic fingerprinting and MALDI-TOF MS characterization of
bacterial isolates are worthwhile additions to preliminary in vitro screening practices. They
provide a level of isolate differentiation and characterization that is beneficial for selecting
candidate biocontrol agents, which is not possible with traditional screening practices.
Effectively, they allow traditional biocontrol screening to move away from empirically based
approaches to ones which are “knowledge” based, allowing for representative groups of
bacteria with specific traits to be selected for further evaluation.
Description
Doctor of Philosophy in Microbiology. University of KwaZulu-Natal, Pietermaritzburg 2016.
Keywords
Rhizoctonia solani., Aerobic bacteria., Sporeforming bacteria., Biological pest control agents., Theses -- Microbiology.