Browsing by Subject "Plant-microbial interactions"

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Hidden miners – the roles of cover crops and soil microorganisms in phosphorus cycling through agroecosystems
(2019) Hallama, Moritz; Pekrun, Carola; Lambers, Hans; Kandeler, Ellen
Background Phosphorus (P) is a limiting nutrient in many agroecosystems and costly fertilizer inputs can cause negative environmental impacts. Cover crops constitute a promising management option for sustainable intensification of agriculture. However, their interactions with the soil microbial community, which is a key driver of P cycling, and their effects on the following crop, have not yet been systematically assessed. Scope We conducted a meta-analysis of published field studies on cover crops and P cycling, focusing on plant-microbe interactions. Conclusions We describe several distinct, simultaneous mechanisms of P benefits for the main crop. Decomposition dynamics, governed by P concentration, are critical for the transfer of P from cover crop residues to the main crop. Cover crops may enhance the soil microbial community by providing a legacy of increased mycorrhizal abundance, microbial biomass P, and phosphatase activity. Cover crops are generally most effective in systems low in available P, and may access ‘unavailable’ P pools. However, their effects on P availability are difficult to detect by standard soil P tests, except for increases after the use of Lupinus sp. Agricultural management (i.e. cover crop species selection, tillage, fertilization) can improve cover crop effects. In summary, cover cropping has the potential to tighten nutrient cycling in agricultural systems under different conditions, increasing crop P nutrition and yield.
Microbial consortia as inoculants for improvedcrop performance
(2020) Bradácová, Klára; Neumann, Günter
The use of microbial consortia products (MCP) based on combinations of different strains of plant growth-promoting microorganisms (PGPM) and frequently also on non-microbial bio-stimulants (BS) with complementary beneficial properties, is discussed as a strategy to increase the efficiency and the flexibility of BS-based crop production strategies under variable environmental conditions. Moreover, MCP application aims at the restoration of plant-beneficial, soil biological processes disturbed by soil degradation and intensive use of agro-chemicals. This PhD thesis was initiated to characterize the modes of action and the potential advantages of a representative commercial MCP formulation over selected single strain PGPM inoculants, with documented effects on plant growth promotion and pathogen suppression. In total, nine pot and field experiments were conducted with three crops (maize, spring wheat, tomato) on seven different soils with three organic and inorganic fertilization regimes. Only in one out of nine experiments conducted in this thesis, clear evidence for superior MCP performance was detectable in a drip-irrigated tomato field experiment conducted under the challenging environmental conditions of the Negev desert in Israel (Bradáčová et al., 2019c). This finding demonstrates that MCP inoculants can exhibit an advantage over single strain inoculants but not as a general feature. Selective interactions with the type and dosage of the selected fertilizers, as well as avoidance of inhibitory effects on root growth during MCP rhizosphere establishment, have been identified as critical factors. A further characterization of the conditions, promoting beneficial plant-MCP interactions is mandatory for a more targeted and reproducible MCP application.
Molecular and phenotypic diversity in populations of Fusarium culmorum on cereal hosts
(2022) Castiblanco Vargas, Eveline Valheria; Miedaner, Thomas
Fusarium head blight is one of the most devastating diseases of cereals globally and responsible for large harvest losses, not only due to the reduction in productivity but also due to the contamination of the grain with mycotoxins. The major causal agent worldwide is Fusarium graminearum; in Europe also other Fusarium species, among them Fusarium culmorum (FC) play an important role. The interaction between Fusarium species and cereals has been categorized as quantitative according to previous phenotypic and genetic observations. We studied the molecular and phenotypic diversity of natural populations of FC and how they interact with four cereals (bread wheat, durum wheat, triticale, rye) as host. Specifically, we sought (i) to understand the interaction between host and isolate, and between isolate and environment using the variance partition approach offered by mixed models applied to analyze multi-environmental studies; (ii) to identify or validate the association of Fusarium genes previously assigned as candidates using field aggressiveness and deoxynivalenol (DON) production; and (iii) to compare the application and results of the candidate gene association mapping approach applied to the same population of FC isolates but with different phenotypic data obtained from inoculation in different hosts-bread wheat and rye. Phenotyping was based on multi-environmental field experiments where each plot of the host plant was artificially inoculated with spores of the respective isolate in accordance with the experimental design. Aggressiveness was visually quantified as the percentage of spikelets with symptoms per plot and was repeatedly evaluated over time. The content of the mycotoxin deoxynivalenol (DON) in the harvested grain was evaluated by double enzyme linked immunosorbent assays (ELISA). Genes previously reported in the literature as related to aggressiveness were selected for sequencing. Using the available F. graminearum genome sequence, specific primers were constructed to amplify and sequence the most variable regions of the respective genes. The partitioning of the phenotypic variance using mixed models, for a subpopulation of 38 FC isolates in four cereal hosts, allowed to disaggregate the magnitude of the genotypic and environmental variance, and the environmental variance in turn into its different components. The genotypic variance was significant, but was exceeded by the magnitude of the environmental variance and its interactions with genotype, showing that the role of plasticity in the pathosystem of Fusarium culmorum and its cereal hosts is highly important. In contrast, the variance associated with the host factor and the interactions with host were not significant, confirmed by high values of genetic correlation amogn host. This result supports the categorization of the cereal/Fusarium culmorum interaction as unspecific and quantitatively inherited also from the view of the pathogen. For the present study, plasticity was understood as the changes in the phenotype of the pathogen that could be attributed to changes induced by the environment. Our data revealed the year as factor with the highest influence on plasticity, meaning that the isolates with high performance values under humid conditions did not exhibit the same high values under dry conditions. Because the environmental conditions are erratic between the years, the lack of a constant selection pressure in the same direction reduces the probability of achieving a speciation event per environment. The phenotypic data of the DON content in harvested grain showed a high correlation with the aggressiveness data. An association mapping study with 17 candidate genes for aggressiveness using a population of 100 isolates of FC inoculated on bread wheat revealed the significant association of the HOG1 gene, explaining 10.29% of the genetic variance of aggressiveness and 6.05% of the genetic variance corresponding to the accumulation of DON in mature grain. HOG1 is a kinase-like protein involved in the communication within the oxidative metabolism of the fungus. In a similar study using the same population of FC isolates and the same candidate genes but rye as host, the gene CUT showed a significant association with aggressiveness, explaining 16.05% of the genetic variance. The CUT gene encodes a cutinase protein, belonging to the secretome and involved in the process of unleashing the membranes and cuticles of the host plant. Taken together, our results suggest that i) field trials of breeding for resistance to FC in cereals should be carried out in several years to properly account for the genotype-by-year interaction; ii) despite the fact that molecular communication may present some type of host specificity the high plasticity guarantees that the effects on the phenotype are very similar among the cereal hosts; and iii) the high genetic correlation of aggressiveness for different cereals invites to involve non-cereal crops in the rotation plans focused on Fusarium disease management.
Molecular perspectives on the ecologically inconsistent effectiveness of the mycoherbicide Fusarium oxysporum f. sp. strigae against Striga hermonthica
(2022) Anteyi, Williams Oyifioda; Rasche, Frank
Cereals are a major staple that is crucial for food security in sub-Saharan Africa (SSA). Sadly, the obligate hemiparasitic witchweed, Striga spp., especially Striga hermonthica (Delile) Benth., is a major biotic constraint to cereal production in SSA, causing enormous crop yield losses estimated at US$10 billion annually. Fusarium oxysporum f. sp. strigae (Fos) is the most renowned fungal biological control agent (BCA) for specifically and significantly tackling S. hermonthica under agricultural systems. Field surveys, however, have revealed the inconsistent effectiveness of Fos isolates against S. hermonthica in differing zones of SSA (i.e., West Africa, East Africa). This daunting phenomenon is a critical challenge that affects Fos reliability and deters its use for S. hermonthica management. The inconsistent effectiveness of Fos against S. hermonthica was presumably ascribed to the interactions that occur between the differing location-specific ecological factors of the pathosystem i.e., abiotic (climate, moisture, or soil physico-chemistry) or biotic (S. hermonthica, Fos isolate, or the plant microbiome). Without doubt, the diversity of a host or pathogen is a primary determinant of the innate susceptibility or virulence of the host or pathogen, respectively. In terms of S. hermonthica diversity, genomic variation of individuals, or regional genetic variation of the sampling zone, were the two major forces suspected. However, the important determiner out of the two forces was unknown. Besides, despite the suppression/death that Fos causes to S. hermonthica, the physiological damage S. hermonthica initiates to an infested cereal crop is mostly irreversible. Hence, in examining strategies for circumventing the main problem of Fos inconsistent effectiveness against S. hermonthica, and the physiological consequences of S. hermonthica on the host cereal crop, the integration of other (non-Fos inoculum) BCA were suggested as possible means for improving the efficiency of S. hermonthica biocontrol. For example, by utilizing a bioherbicide cocktail of Fos and plant growth promoting rhizobacteria (PGPR), or Striga seed germination-inhibiting fungal toxins. Apart from the popular reputation of PGPR in enhancing crop health and growth, certain PGPR strains (especially Bacillus subtilis isolate GB03) have been earlier reported for their highly-promising potential of antagonizing S. hermonthica development. Similarly, certain fungal extracellular metabolites (exometabolites), especially of Fusarium origin, were reported to completely inhibit S. hermonthica seed germination in vitro at very low concentrations (≤ 1 mM). Unfortunately, knowledge of the microbe (Fos)–microbe (PGPR) interaction, their localization and ecological niche, for enabling their expected synergistic impact of simultaneously suppressing S. hermonthica and enhancing the Striga-infected cereal crop biomass, was unknown. Also, it was unknown if highly potent/efficient Striga seed germination-inhibiting fungal exometabolites will consistently suppress S. hermonthica in planta. Thus, in the context of genetic diversity in S. hermonthica, the PhD study focused on gaining (molecular) insights into the inconsistent effectiveness of Fos against S. hermonthica; including the examination of some strategies for improving S. hermonthica biocontrol efficiency, precisely by integrating PGPR, or Striga seed germination-inhibiting Fusarium exometabolites, into a S. hermonthica biocontrol system. The first research examined the molecular genetic basis, underlying the variable susceptibility of S. hermonthica populations sampled from differing zones of SSA (West Africa, East Africa) to contrasting Fos isolates (Foxy-2, FK3). Regardless of sampling zone, the S. hermonthica populations displayed divergent susceptibility patterns to the Fos isolates i.e., a S. hermonthica class was susceptible to both Foxy-2 and FK3, while the other class was susceptible to either Foxy-2 or FK3. This manifestation correlated with nucleotide mutations at certain loci. Thus, genomic variation in S. hermonthica is a superior determinant of the inconsistent effectiveness of Fos isolates, rather than the S. hermonthica sampling zone. The second research examined the impact of coinoculating Fos and a PGPR (B. subtilis isolate GB03) into a S. hermonthica-sorghum parasitic system. Notwithstanding the colocalization of Fos and GB03 in common ecological niches of diseased S. hermonthica shoot (mainly in flavonoid-rich regions), GB03 thwarted Fos suppressive activity against S. hermonthica. Interestingly, a novel, alternative Fos entry route into S. hermonthica (through the trichome) was discovered. The coinoculation of Fos and GB03 presented no added advantage for S. hermonthica control. Finally, the third research screened a set of highly phytotoxic Fusarium exometabolites against S. hermonthica seed germination (in vitro) and incidence (in planta). This was to identify the most potent/efficient Fusarium exometabolite for S. hermonthica biocontrol. Among the tested exometabolites, diacetoxyscirpenol (DAS) was the most potent/efficient to completely suppress S. hermonthica both in vitro and in planta. Fos, however, did not produce DAS, due to underexpression of key genes necessary for Fusarium trichothecene biosynthesis. In conclusion, owing to the obligate outcrossing mating system in S. hermonthica, genomic variation is an inevitable phenomenon. This, therefore, plays a crucial role in the variable susceptibility of S. hermonthica to Fos. The newly discovered Fos (direct) entry route into S. hermonthica (trichome entry), elucidates a novel paradigm to the infection mechanism occurring under the S. hermonthica (host)–Fos (pathogen) interaction, in addition to the previously reported indirect, rhizosphere-transmission. Thus, this novel phyllosphere-transmission, paves the way for further research that exploit this alternative Fos infection route for better S. hermonthica biocontrol. Lastly, considering the potency and broadscale efficacy against diverse S. hermonthica populations, the exometabolite DAS could serve as a new agent for a more efficient S. hermonthica biocontrol. Though, further examination of its specific mode of action against the target weed (S. hermonthica), as opposed to non-target organisms, is required.