The role of isoflavone metabolism in plant protection depends on the rhizobacterial MAMP that triggers systemic resistance against Xanthomonas axonopodis pv. glycines in Glycine max (L.) Merr. cv. Osumi

A plant's perception of growth-promoting bacteria and their metabolites

Many recent studies have highlighted the importance of plant growth-promoting (rhizo)bacteria (PGPR) in supporting plant's development, particularly under biotic and abiotic stress. Most focus on the plant growth-promoting traits of selected strains and the latter's effect on plant biomass, root architecture, leaf area, and specific metabolite accumulation. Regarding energy balance, plant growth is the outcome of an input (photosynthesis) and several outputs (i.e., respiration, exudation, shedding, and herbivory), frequently neglected in classical studies on PGPR-plant interaction. Here, we discuss the primary evidence underlying the modifications triggered by PGPR and their metabolites on the plant ecophysiology. We propose to detect PGPR-induced variations in the photosynthetic activity using leaf gas exchange and recommend setting up the correct timing for monitoring plant responses according to the specific objectives of the experiment. This research identifies the challenges and tries to provide future directions to scientists working on PGPR-plant interactions to exploit the potential of microorganisms' application in improving plant value.

Comparative Transcriptome Profiling Unfolds a Complex Defense and Secondary Metabolite Networks Imparting Corynespora cassiicola Resistance in Soybean (Glycine max (L.) Merrill)

Target spot is caused by Corynespora cassiicola, which heavily affects soybean production areas that are hot and humid. Resistant soybean genotypes have been identified; however, the molecular mechanisms governing resistance to infection are unknown. Comparative transcriptomic profiling using two known resistant genotypes and two susceptible genotypes was performed under infected and control conditions to understand the regulatory network operating between soybean and C. cassiicola. RNA-Seq analysis identified a total of 2571 differentially expressed genes (DEGs) which were shared by all four genotypes. These DEGs are related to secondary metabolites, immune response, defense response, phenylpropanoid, and flavonoid/isoflavonoid pathways in all four genotypes after C. cassiicola infection. In the two resistant genotypes, additional upregulated DEGs were identified affiliated with the defense network: flavonoids, jasmonic acid, salicylic acid, and brassinosteroids. Further analysis led to the identification of differentially expressed transcription factors, immune receptors, and defense genes with a leucine-rich repeat domain, dirigent proteins, and cysteine (C)-rich receptor-like kinases. These results will provide insight into molecular mechanisms of soybean resistance to C. cassiicola infection and valuable resources to potentially pyramid quantitative resistance loci for improving soybean germplasm.

Flavonoids Are Intra- and Inter-Kingdom Modulator Signals

Flavonoids are a broad class of secondary metabolites with multifaceted functionalities for plant homeostasis and are involved in facing both biotic and abiotic stresses to sustain plant growth and health. Furthermore, they were discovered as mediators of plant networking with the surrounding environment, showing a surprising ability to perform as signaling compounds for a multitrophic inter-kingdom level of communication that influences the plant host at the phytobiome scale. Flavonoids orchestrate plant-neighboring plant allelopathic interactions, recruit beneficial bacteria and mycorrhizal fungi, counteract pathogen outbreak, influence soil microbiome and affect plant physiology to improve its resilience to fluctuating environmental conditions. This review focuses on the diversified spectrum of flavonoid functions in plants under a variety of stresses in the modulation of plant morphogenesis in response to environmental clues, as well as their role as inter-kingdom signaling molecules with micro- and macroorganisms. Regarding the latter, the review addresses flavonoids as key phytochemicals in the human diet, considering their abundance in fruits and edible plants. Recent evidence highlights their role as nutraceuticals, probiotics and as promising new drugs for the treatment of several pathologies.

Exogenous genistein enhances soybean resistance to Xanthomonas axonopodis pv. glycines

BACKGROUND

Xanthomonas axonopodis pv. glycines (Xag) is the causal agent of bacterial pustule disease and results in enormous losses in soybean production. Although isoflavones are known to be involved in soybean resistance against pathogen infection, the effects of exogenous isoflavones on soybean plants remain unexplored.

RESULTS

Irrigation of soybean plants with isoflavone genistein inhibited plant growth for short periods, probably by inhibiting the tyrosine (brassinosteroids) kinase pathway, and increased disease resistance against Xag. The number of lesions was reduced by 59%-63% when applying 50 μg ml-1 genistein. The effects on disease resistance were observed for 15 days after treatment. Genistein also enhanced the disease resistance of soybean against the fungal pathogen Sclerotinia sclerotiorum. Exogenous genistein increased antioxidant capacity, decreased H2 O2 level and promoted the accumulation of phenolics in Xag-infected soybean leaves. Exogenous genistein reduced the amounts of endogenous daidzein, genistein and glycitein and increased the concentration of genistin, which was found to show strong antibacterial activity against the pathogen and to reduce the expression of virulence factor yapH, and flagella formation gene flgK. The expression of several soybean defense genes, such as chalcone isomerase, glutathione S-transferase and 1-aminocyclopropane-1-carboxylate oxidase 1, was upregulated after genistein treatment.

CONCLUSIONS

The effects of exogenous genistein on soybean plants were examined for the first time, revealing new insights into the roles of isoflavones in soybean defense and demonstrating that irrigation with genistein can be a suitable method to induce disease resistance in soybean plants. © 2022 Society of Chemical Industry.

Antibacterial mechanism of Biochanin A and its efficacy for the control of Xanthomonas axonopodis pv. glycines in soybean

BACKGROUND

Xanthomonas axonopodis pv. glycines (Xag) is a hazardous pathogen able to cause bacterial pustule disease in soybean, reducing crop yield and quality. Although flavonoids rutin and genistein are known to play an important role in soybean defence, soybean is only able to produce Biochanin A in low concentration.

RESULTS

In this work, Biochanin A was found to produce higher antibacterial activity against Xag in comparison with genistein (minimum inhibitory concentration < 100 μg/mL). Biochanin A was able to inhibit DNA synthesis and flagella formation in Xag, and altered the composition of the bacterial membrane. These effects reduced swimming motility, extracellular protease activity and biofilm formation. Further, Biochanin A was tested for the control of Xag in soybean leaves, showing similar, or even higher, inhibitory ability in comparison with some products commonly used for the control of this pathogen.

CONCLUSIONS

The antibacterial properties of Biochanin A against Xag have been studied for the first time, revealing new insights on the potential applications of this isoflavonoid for the management of bacterial pustule disease. © 2020 Society of Chemical Industry.

Tomato Bio-Protection Induced by Pseudomonas fluorescens N21.4 Involves ROS Scavenging Enzymes and PRs, without Compromising Plant Growth

Aims: to discover the interrelationship between growth, protection and photosynthesis induced by Pseudomonas fluorescens N21.4 in tomato (Lycopersicum sculentum) challenged with the leaf pathogen Xanthomonas campestris, and to define its priming fingerprint. Methods: Photosynthesis was determined by fluorescence; plant protection was evaluated by relative disease incidence, enzyme activities by specific colorimetric assays and gene expression by qPCR. Changes in Reactive Oxygen Species (ROS) scavenging cycle enzymes and pathogenesis related protein activity and expression were determined as metabolic and genetic markers of induction of systemic resistance. Results: N21.4 significantly protected plants and increased dry weight. Growth increase is supported by significant increases in photochemical quenching together with significant decreases in energy dissipation (Non-Photochemical Quenching, NPQ). Protection was associated with changes in ROS scavenging cycle enzymes, which were significantly increased on N21.4 + pathogen challenged plants, supporting the priming effect. Superoxide Dismutase (SOD) was a good indicator of biotic stress, showing similar levels in pathogen- and N21.4-treated plants. Similarly, the activity of defense-related enzymes, ß-1,3-glucanase and chitinase significantly increased in post-pathogen challenge state; changes in gene expression were not coupled to activity. Conclusions: protection does not compromise plant growth; N21.4 priming fingerprint is defined by enhanced photochemical quenching and decreased energy dissipation, enhanced chlorophylls, primed ROS scavenging cycle enzyme activity, and glucanase and chitinase activity.

Beneficial features of plant growth-promoting rhizobacteria for improving plant growth and health in challenging conditions: A methodical review

New eco-friendly approaches are required to improve plant biomass production. Beneficial plant growth-promoting (PGP) bacteria may be exploited as excellent and efficient biotechnological tools to improve plant growth in various - including stressful - environments. We present an overview of bacterial mechanisms which contribute to plant health, growth, and development. Plant growth promoting rhizobacteria (PGPR) can interact with plants directly by increasing the availability of essential nutrients (e.g. nitrogen, phosphorus, iron), production and regulation of compounds involved in plant growth (e.g. phytohormones), and stress hormonal status (e.g. ethylene levels by ACC-deaminase). They can also indirectly affect plants by protecting them against diseases via competition with pathogens for highly limited nutrients, biocontrol of pathogens through production of aseptic-activity compounds, synthesis of fungal cell wall lysing enzymes, and induction of systemic responses in host plants. The potential of PGPR to facilitate plant growth is of fundamental importance, especially in case of abiotic stress, where bacteria can support plant fitness, stress tolerance, and/or even assist in remediation of pollutants. Providing additional evidence and better understanding of bacterial traits underlying plant growth-promotion can inspire and stir up the development of innovative solutions exploiting PGPR in times of highly variable environmental and climatological conditions.

Multiplex CRISPR/Cas9-mediated metabolic engineering increases soya bean isoflavone content and resistance to soya bean mosaic virus

Isoflavonoids, which include a variety of secondary metabolites, are derived from the phenylpropanoid pathway and are distributed predominantly in leguminous plants. These compounds play a critical role in plant-environment interactions and are beneficial to human health. Isoflavone synthase (IFS) is a key enzyme in isoflavonoid synthesis and shares a common substrate with flavanone-3-hydroxylase (F3H) and flavone synthase II (FNS II). In this study, CRISPR/Cas9-mediated multiplex gene-editing technology was employed to simultaneously target GmF3H1, GmF3H2 and GmFNSII-1 in soya bean hairy roots and plants. Various mutation types and frequencies were observed in hairy roots. Higher mutation efficiencies were found in the T0 transgenic plants, with a triple gene mutation efficiency of 44.44%, and these results of targeted mutagenesis were stably inherited in the progeny. Metabolomic analysis of T0 triple-mutants leaves revealed significant improvement in isoflavone content. Compared with the wild type, the T3 generation homozygous triple mutants had approximately twice the leaf isoflavone content, and the soya bean mosaic virus (SMV) coat protein content was significantly reduced by one-third after infection with strain SC7, suggesting that increased isoflavone content enhanced the leaf resistance to SMV. The isoflavone content in the seeds of T2 triple mutants was also significantly increased. This study provides not only materials for the improvement of soya bean isoflavone content and resistance to SMV but also a simple system to generate multiplex mutations in soya bean, which may be beneficial for further breeding and metabolic engineering.

Elicitation with Bacillus QV15 reveals a pivotal role of F3H on flavonoid metabolism improving adaptation to biotic stress in blackberry

The aim of this study is to determine the involvement of the flavonol-anthocyanin pathway on plant adaptation to biotic stress using the B.amyloliquefaciens QV15 to trigger blackberry metabolism and identify target genes to improve plant fitness and fruit quality. To achieve this goal, field-grown blackberries were root-inoculated with QV15 along its growth cycle. At fruiting, a transcriptomic analysis by RNA-Seq was performed on leaves and fruits of treated and non-treated field-grown blackberries after a sustained mildew outbreak; expression of the regulating and core genes of the Flavonol-Anthocyanin pathway were analysed by qPCR and metabolomic profiles by UHPLC/ESI-qTOF-MS; plant protection was found to be up to 88%. Overexpression of step-controlling genes in leaves and fruits, associated to lower concentration of flavonols and anthocyanins in QV15-treated plants, together with a higher protection suggest a phytoanticipin role for flavonols in blackberry; kempferol-3-O-rutinoside concentration was strikingly high. Overexpression of RuF3H (Flavonol-3-hidroxylase) suggests a pivotal role in the coordination of committing steps in this pathway, controlling carbon flux towards the different sinks. Furthermore, this C demand is supported by an activation of the photosynthetic machinery, and boosted by a coordinated control of ROS into a sub-lethal range, and associated to enhanced protection to biotic stress.

Metabolomic applications for understanding complex tripartite plant-microbes interactions: Strategies and perspectives

Phytopathogens from the Alternaria sp., Fusarium sp., Penicillium sp., and Pseudomonas sp. and their toxigenic metabolites - alternariol, fumonisin, citrinin, and coronatine respectively, negatively impact crop yields and sales by eliciting plant diseases and/or causing human and veterinary toxicoses upon the consumption of contaminated food. These phytopathogens and their associated toxins, however, are present and most likely in undetectable concentrations pre-harvest and post-harvest of many major staple crops. Metabolomic approaches have been used extensively for better characterizing and diagnosing human disease, plant disease and, their etiological agents. Their use in agro-industrial research focusing specifically on tripartite (plant - toxicogenic microbe - beneficial microbe) interactions is, however, limited. Since new approaches for eradicating food-borne pathogens, increasing crop productivity and improving agro-international trade are being sought worldwide, the consequent integration of metabolomic approaches and perspectives in crop protection strategies for better understanding plant - toxicogenic microbe - beneficial microbe interaction in tandem is discussed.

Phytobiome metabolism: beneficial soil microbes steer crop plants' secondary metabolism

Crops are negatively affected by abiotic and biotic stresses, however, plant-microbe cooperation allows prompt buffering of these environmental changes. Microorganisms exhibit an extensive metabolic capability to assist plants in reducing these burdens. Interestingly, beneficial microbes may also trigger, at the host side, a sequence of events from signal perception to metabolic responses leading to stress tolerance or protection against biotic threats. Although plants are well known for their vast chemical diversity, plant-microbial interactions often stimulate the production of a rich and different repertoire of metabolites in plants. The targeted microbial-plant interactions reprogramming plant metabolism represent potential means to foster various pest managements. However, the molecular mechanisms of microbial modulation of plant metabolic plasticity are still poorly understood. Here, we review an increasing amount of reports providing evidence for alterations to plant metabolism caused by beneficial microbial colonization. In addition, we highlight the vital importance of these metabolic reprograms for plants under stress erratic conditions. © 2019 Society of Chemical Industry.

Biotic elicitation as a tool to improve strawberry and raspberry extract potential on metabolic syndrome-related enzymes in vitro

BACKGROUND

Raspberry and strawberry are high value-added food products that can contribute to human health due to the abundance of polyphenols that they contain. Polyphenols are secondary metabolites and therefore devoted to improve plant adaptation, these polyphenol profile can be induced applying different stimuli, such as certain bacteria. The aim of this study was twofold: (i) to evaluate the ability of two bacterial strains to modulate secondary metabolisms in strawberry and raspberry, and (ii) to explore the ability of plant extracts to modify enzyme activities related to metabolic syndrome.

RESULTS

Total phenolic and anthocyanin content was higher in strawberries than in raspberries, despite similar antioxidant capacities. Strawberry extracts performed better on the tested enzymes, except on α-glucosidase inhibition capacity. Bacillus amyloliquefaciens stabilized the effects of extracts at different points in time, and Pseudomonas fluorescens modified plant metabolism after more inoculations (spring) in both species, improving the effects of raspberry extracts on α-glucosidase, COX1, and COX2, and of strawberry on α-amylase and COX1.

CONCLUSION

It is good to include these two fruits in the diet because they improve the activity of metabolic syndrome-related enzymes. Applying either strain during plant growth modifies the bioactive profile of the plants, improving the effects of the fruit extracts on human health. © 2018 Society of Chemical Industry.

A Multi-Omics Analysis of Glycine max Leaves Reveals Alteration in Flavonoid and Isoflavonoid Metabolism Upon Ethylene and Abscisic Acid Treatment

Phytohormones are central to plant growth and development. Despite the advancement in our knowledge of hormone signaling, downstream targets, and their interactions upon hormones action remain largely fragmented, especially at the protein and metabolite levels. With an aim to get new insight into the effects of two hormones, ethylene (ET) and abscisic acid (ABA), this study utilizes an integrated proteomics and metabolomics approach to investigate their individual and combined (ABA+ET) signaling in soybean leaves. Targeting low-abundance proteins, our previously established protamine sulfate precipitation method was applied, followed by label-free quantification of identified proteins. A total of 4129 unique protein groups including 1083 differentially modulated in one (individual) or other (combined) treatments were discerned. Functional annotation of the identified proteins showed an increased abundance of proteins related to the flavonoid and isoflavonoid biosynthesis and MAPK signaling pathway in response to ET treatment. HPLC analysis showed an accumulation of isoflavones (genistin, daidzein, and genistein) upon ET treatment, in agreement with the proteomics results. A metabolome analysis assigned 79 metabolites and further confirmed the accumulation of flavonoids and isoflavonoids in response to ET. A potential cross-talk between ET and MAPK signaling, leading to the accumulation of flavonoids and isoflavonoids in soybean leaves is suggested.

Global Transcriptome Analyses Reveal Differentially Expressed Genes of Six Organs and Putative Genes Involved in (Iso)flavonoid Biosynthesis in Belamcanda chinensis

Belamcanda chinensis (L.) DC., a perennial herb of the family Iridaceae, is rich in a variety of (iso)flavonoids with significant organ-specific distribution and has a swollen rhizome that is widely used in East Asia as a traditional medicine. In the present study, comprehensive transcriptomes of six organs (root, rhizome, aerial stem, leaf, flower, and young fruit) of B. chinensis were obtained by high-throughput RNA-sequencing and de novo assembly. A total of 423,661 unigenes (mean length = 618 bp, median length = 391 bp) were assembled and annotated in seven databases: Non-redundant protein sequences, Nucleotide sequences, Swiss-Prot, Protein family database, euKaryotic Ortholog Groups, Kyoto Encyclopedia of Genes and Genomes (KEGG), and Gene Ontology (GO). A total of 4995 transcription factors were identified, including 408 MYB, 182 bHLH, 277 AP2/ERF, and 228 WRKY genes. A total of 129 cytochrome P450 unigenes belonging to 10 divergent clans were identified and grouped into clades in a phylogenetic tree that showed their inferred evolutionary relationship. Differentially expressed unigenes among the six organs were subjected to GO and KEGG enrichment analysis to profile the functions of each organ. Unigenes associated with (iso)flavonoid biosynthesis were then profiled by expression level analysis. Additionally, the complete coding sequences of six predicted enzymes essential to the (iso)flavonoid pathway were obtained, based on the annotated unigenes. This work reveals clear differences in expression patterns of genes among the six organs and will provide a sound platform to understand the (iso)flavonoid pathways in B. chinensis.

Gibberellins Producing Endophytic Fungus Porostereum spadiceum AGH786 Rescues Growth of Salt Affected Soybean

In the pursuit of sustainable agriculture through environment and human health friendly practices, we evaluated the potential of a novel gibberellins (GAs) producing basidiomycetous endophytic fungus Porostereum spadiceum AGH786, for alleviating salt stress and promoting health benefits of soybean. Soybean seedlings exposed to different levels of NaCl stress (70 and 140 mM) under greenhouse conditions, were inoculated with the AGH786 strain. Levels of phytohormones including GAs, JA and ABA, and isoflavones were compared in control and the inoculated seedlings to understand the mechanism through which the stress is alleviated. Gibberellins producing endophytic fungi have been vital for promoting plant growth under normal and stress conditions. We report P. spadiceum AGH786 as the ever first GAs producing basidiomycetous fungus capable of producing six types of GAs. In comparison to the so for most efficient GAs producing Gibberella fujikuroi, AGH786 produced significantly higher amount of the bioactive GA3. Salt-stressed phenotype of soybean seedlings was characterized by low content of GAs and high amount of ABA and JA with reduced shoot length, biomass, leaf area, chlorophyll contents, and rate of photosynthesis. Mitigation of salt stress by AGH786 was always accompanied by high GAs, and low ABA and JA, suggesting that this endophytic fungus reduces the effect of salinity by modulating endogenous phytohormones of the seedlings. Additionally, this strain also enhanced the endogenous level of two isoflavones including daidzen and genistein in soybean seedlings under normal as well as salt stress conditions as compared to their respective controls. P. spadiceum AGH786 boosted the NaCl stress tolerance and growth in soybean, by modulating seedlings endogenous phytohormones and isoflavones suggesting a valuable contribution of this potent fungal biofertilizer in sustainable agriculture in salt affected soils.

Additive effects due to biochar and endophyte application enable soybean to enhance nutrient uptake and modulate nutritional parameters

We studied the effects of hardwood-derived biochar (BC) and the phytohormone-producing endophyte Galactomyces geotrichum WLL1 in soybean (Glycine max (L.) Merr.) with respect to basic, macro- and micronutrient uptakes and assimilations, and their subsequent effects on the regulation of functional amino acids, isoflavones, fatty acid composition, total sugar contents, total phenolic contents, and 1,1-diphenyl-2-picrylhydrazyl (DPPH)-scavenging activity. The assimilation of basic nutrients such as nitrogen was up-regulated, leaving carbon, oxygen, and hydrogen unaffected in BC+G. geotrichum-treated soybean plants. In comparison, the uptakes of macro- and micronutrients fluctuated in the individual or co-application of BC and G. geotrichum in soybean plant organs and rhizospheric substrate. Moreover, the same attribute was recorded for the regulation of functional amino acids, isoflavones, fatty acid composition, total sugar contents, total phenolic contents, and DPPH-scavenging activity. Collectively, these results showed that BC+G. geotrichum-treated soybean yielded better results than did the plants treated with individual applications. It was concluded that BC is an additional nutriment source and that the G. geotrichum acts as a plant biostimulating source and the effects of both are additive towards plant growth promotion. Strategies involving the incorporation of BC and endophytic symbiosis may help achieve eco-friendly agricultural production, thus reducing the excessive use of chemical agents.

Volatiles released by endophytic Pseudomonas fluorescens promoting the growth and volatile oil accumulation in Atractylodes lancea

Atractylodes lancea is a well-known, but endangered, Chinese medicinal plant whose volatile oils are its main active components. As the volatile oil content in cultivated A. lancea is much lower than that in the wild herb, the application of microbes or related elicitors to promote growth and volatile oil accumulation in the cultivated herb is an important area of research. This study demonstrates that the endophytic bacterium Pseudomonas fluorescens ALEB7B isolated from the geo-authentic A. lancea can release several nitrogenous volatiles, such as formamide and N,N-dimethyl-formamide, which significantly promote the growth of non-infected A. lancea. Moreover, the main bacterial volatile benzaldehyde significantly promotes volatile oil accumulation in non-infected A. lancea via activating plant defense responses. Notably, the bacterial nitrogenous volatiles cannot be detected in the A. lancea - Pseudomonas fluorescens symbiont while the benzaldehyde can be detected, indicating the nitrogenous volatiles or their precursors may have been consumed by the host plant. This study firstly demonstrates that the interaction between plant and endophytic bacterium is not limited to the commonly known physical contact, extending the ecological functions of endophyte in the phytosphere and deepening the understandings about the symbiotic interaction.

Application of Pseudomonas fluorescens to Blackberry under Field Conditions Improves Fruit Quality by Modifying Flavonoid Metabolism

Application of a plant growth promoting rhizobacterium (PGPR), Pseudomonas fluorescens N21.4, to roots of blackberries (Rubus sp.) is part of an optimised cultivation practice to improve yields and quality of fruit throughout the year in this important fruit crop. Blackberries are especially rich in flavonoids and therefore offer potential benefits for human health in prevention or amelioration of chronic diseases. However, the phenylpropanoid pathway and its regulation during ripening have not been studied in detail, in this species. PGPR may trigger flavonoid biosynthesis as part of an induced systemic response (ISR) given the important role of this pathway in plant defence, to cause increased levels of flavonoids in the fruit. We have identified structural genes encoding enzymes of the phenylpropanoid and flavonoid biosynthetic pathways catalysing the conversion of phenylalanine to the final products including flavonols, anthocyanins and catechins from blackberry, and regulatory genes likely involved in controlling the activity of pathway branches. We have also measured the major flavonols, anthocyanins and catechins at three stages during ripening. Our results demonstrate the coordinated expression of flavonoid biosynthetic genes with the accumulation of anthocyanins, catechins, and flavonols in developing fruits of blackberry. Elicitation of blackberry plants by treatment of roots with P.fluorescens N21.4, caused increased expression of some flavonoid biosynthetic genes and an accompanying increase in the concentration of selected flavonoids in fruits. Our data demonstrate the physiological mechanisms involved in the improvement of fruit quality by PGPR under field conditions, and highlight some of the genetic targets of elicitation by beneficial bacteria.