Abiotic stresses such as salt and drought represent adverse environmental conditions that significantly damage plant growth and agricultural productivity. In this study, the mechanism of the plant growth-promoting rhizo-bacteria (PGPR)-stimulated tolerance against abiotic stresses has been explored. Results suggest that PGPR strains, Arthrobacter protophormiae (SA3) and Dietzia natronolimnaea (STR1), can facilitate salt stress tolerance in wheat crop, while Bacillus subtilis (LDR2) can provide tolerance against drought stress in wheat. These PGPR strains enhance photosynthetic efficiency under salt and drought stress conditions. Moreover, all three PGPR strains increase indole-3-acetic acid (IAA) content of wheat under salt and drought stress conditions. The SA3 and LDR2 inoculations counteracted the increase of abscisic acid (ABA) and 1-aminocyclopropane-1-carboxylate (ACC) under both salt and drought stress conditions, whereas STR1 had no significant impact on the ABA and ACC content. The impact of PGPR inoculations on these physiological parameters were further confirmed by gene expression analysis as we observed enhanced levels of the TaCTR1 gene in SA3-, STR1- and LDR2-treated wheat seedlings as compared to uninoculated drought and salt stressed plants. PGPR inoculations enhanced expression of TaDREB2 gene encoding for a transcription factor, which has been shown to be important for improving the tolerance of plants to abiotic stress conditions. Our study suggest that PGPR confer abiotic stress tolerance in wheat by enhancing IAA content, reducing ABA/ACC content, modulating expression of a regulatory component (CTR1) of ethylene signaling pathway and DREB2 transcription factor.

Plant growth‐promoting rhizobacteria enhance wheat salt and drought stress tolerance by altering endogenous phytohormone levels and TaCTR1/TaDREB2 expression

Plant secondary metabolites (PSMs) play many roles including defense against pathogens, pests, and herbivores; response to environmental stresses, and mediating organismal interactions. Similarly, plant microbiomes participate in many of the above-mentioned processes directly or indirectly by regulating plant metabolism. Studies have shown that plants can influence their microbiome by secreting various metabolites and, in turn, the microbiome may also impact the metabolome of the host plant. However, not much is known about the communications between the interacting partners to impact their phenotypic changes. In this article, we review the patterns and potential underlying mechanisms of interactions between PSMs and plant microbiomes. We describe the recent developments in analytical approaches and methods in this field. The applications of these new methods and approaches have increased our understanding of the relationships between PSMs and plant microbiomes. Though the current studies have primarily focused on model organisms, the methods and results obtained so far should help future studies of agriculturally important plants and facilitate the development of methods to manipulate PSMs-microbiome interactions with predictive outcomes for sustainable crop productions.

Linking Plant Secondary Metabolites and Plant Microbiomes: A Review

Plant growth promoting Curtobacterium albidum strain SRV4: An agriculturally important microbe to alleviate salinity stress in paddy plants

Indole-3-acetic acid (IAA) plays an important role in plant microbe interactions. Current study explored the role of indole-3-acetic acid (IAA) as a signalling molecule for chemical dialogue between endophytic fungus and host plant roots. The endophytic fungus was isolated from the leaves of drought stressed Withenia somnifera and was identified as Aspergillus awamori Wl1 through ITS region of 18 S rDNA sequence. The isolated Wl1 strain was capable of producing important secondary metabolites, including IAA, phenols and sugars. Culture filtrate of the strain contained 24.2, 275.4 and 127.4 μg/mL of IAA, phenols and sugars, respectively. The strain has efficiently colonized the maize roots and enhanced the growth of host plant. In order to determine the role of IAA in root colonization by endophyte, we inhibited the biosynthesis of IAA. Inhibition of IAA production by foliar application of yucasin effectively reduced the colonization of endophyte in maize roots by 52%, whereas root application of yucasin has decreased the colonization frequency by 66%. However, the application of IAA restored the ability of Aspergillus awamori to colonize maize roots and significantly improved various growth parameters. From these observations, it is concluded that IAA plays a vital role in initiating a crosstalk between the two partners.

In vitro production of IAA by endophytic fungus Aspergillus awamori and its growth promoting activities in Zea mays

For nearly eight millennia, opium poppy (Papaver somniferum) has been bred and cultivated for therapeutic purposes. The medicinal properties of the plant are conferred by specialized metabolites known as benzylisoquinoline alkaloids (BIAs), comprising the narcotic analgesics morphine and codeine, the antimicrobial agent sanguinarine, and the potential anticancer drug noscapine. In addition, naturally occurring thebaine is used for the semi-synthesis of widely prescribed pain-relievers (e.g., oxycodone and hydrocodone), valuable drugs used in the treatment of opioid addiction (i.e., naltrexone), or antidotes for opioid overdose (i.e., naloxone). The complex stereochemistry of many opiates hinders their chemical synthesis and opium poppy remains the sole commercial source of these important pharmaceuticals. For decades, opium poppy has served as a model plant for research aimed at a comprehensive understanding of BIA metabolism. Recent progress in functional genomics has enabled the discovery of a nearly complete collection of BIA biosynthetic genes, many of which are clustered in the opium poppy genome. Advances in synthetic biology have facilitated the successful reconstitution of several BIA biosynthetic pathways in heterologous hosts such as Saccharomyces cerevisiae and Escherichia coli, although the initially low production levels suggest that commercial scale-up will present additional challenges. This review provides an update of key molecular and biochemical aspects of BIA metabolism in opium poppy, including recent biosynthetic gene discoveries, genomic organization, novel BIA transporters, metabolic regulation, and major efforts in the engineering of pathways in plants and microbes.

Benzylisoquinoline alkaloid biosynthesis in opium poppy: an update

Tissue specific biosynthesis of secondary metabolites is a distinguished feature of medicinal plants. Withania somnifera, source of pharmaceutically important withanolides biosynthesizes withaferin-A in leaves and withanolide-A in roots. To increase the in planta withanolides production, a sustainable approach needs to be explored. Here, we isolated endophytes from different parts of W. somnifera plants and their promising role in in planta withanolide biosynthesis was established in both in-vivo grown as well in in-vitro raised composite W. somnifera plants. Overall, the fungal endophytes improved photosynthesis, plant growth and biomass, and the root-associated bacterial endophytes enhanced the withanolide content in both in-vivo and in-vitro grown plants by modulating the expression of withanolide biosynthesis genes in leaves and roots. Surprisingly, a few indole-3-acetic acid (IAA)-producing and nitrogen-fixing root-associated endophytes could induce the biosynthesis of withaferin-A in roots by inducing in planta IAA-production and upregulating the expression of withanolide biosynthesis genes especially MEP-pathway genes (DXS and DXR) in roots as well. Results indicate the role of endophytes in modulating the synthesis and site of withanolides production and the selected endophytes can be used for enhancing the in planta withanolide production and enriching roots with pharmaceutically important withaferin-A which is generally absent in roots.

Endophytes of Withania somnifera modulate in planta content and the site of withanolide biosynthesis

Aims
The aim of this study was to determine whether ACC deaminase-containing bacterial treatments could enhance the tolerance of poppy (Papaver somniferum L.) plants against biotic stress of downy mildew caused by Peronospora spp.

Methods and Results
Three different genotypes of P. somniferum i.e. Sampada, J-16 and I-14 were included in the experiment. The ACC deaminase-containing bacteria Pseudomonas putida (WPTe) reduced the downy mildew disease severity and significantly improved the growth and yield of P. somniferum plants. The chlorophyll content, photosynthetic rate, stomatal conductance and transpiration rate were modulated upon WPTe treatments in the poppy plants. We observed reduced synthesis of ethylene precursor (ACC) and abscisic acid (ABA), and enhanced production of indole acetic acid (IAA) in P. somniferum plants upon WPTe treatments. Moreover, WPTe treatment reduced proline and lipid peroxidation in plant leaves.

Conclusion
These results highlight that the ACC deaminase-containing plant growth promoting rhizobacteria enhance the tolerance of P. somniferum plant against downy mildew.

Significance and impact of the study
ACC deaminase-containing plant growth promoting rhizobacteria may be used against phytopathogens which apart from protecting the plants from the disease could also be useful in reducing ethylene-induced damages in the event of abiotic stresses.

This article is protected by copyright. All rights reserved.

https://sfamjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/jam.13417

ACC deaminase‐containing plant growth‐promoting rhizobacteria protect Papaver somniferum from downy mildew

Secondary metabolite biosynthesis in medicinal plants is multi-step cascade known to be modulated by associated endophytes. While a single endophyte is not able to upregulate all biosynthetic steps, limiting maximum yield achievement. Therefore to compliment the deficient characteristics in an endophyte we tried consortium of endophytes to achieve maximum yield. Here, efforts were made to maximize the in planta morphine yield, using consortium of two endophytes; SM1B (Acinetobacter sp.) upregulating most of the genes of morphine biosynthesis except T6ODM and CODM, and SM3B (Marmoricola sp.) upregulating T6ODM and CODM in alkaloid-less Papaver somniferum cv. Sujata. Consortium-inoculation significantly increased morphine and thebaine content, and also increased the photosynthetic efficiency of poppy plants resulted in increased biomass, capsule weight, and seed yields compared to single-inoculation. The increment in morphine content was due to the modulation of metabolic-flow of key intermediates including reticuline and thebaine, via upregulating pertinent biosynthetic genes and enhanced expression of COR, key gene for morphine biosynthesis. This is the first report demonstrating the endophytic-consortium complimenting the functional deficiency of one endophyte by another for upregulating multiple genes of a metabolic pathway similar to transgenics (overexpressing multiple genes) for obtaining enhanced yield of pharmaceutically important metabolites.

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Endophytic Consortium With Diverse Gene-Regulating Capabilities of Benzylisoquinoline Alkaloids Biosynthetic Pathway Can Enhance Endogenous Morphine Biosynthesis in Papaver somniferum.

Downy mildew is one of the most serious diseases of Papaver somniferum. Endophytes isolated from different parts of P. somniferum were screened for their ability to enhance resistance against downy mildew caused by the obligate biotrophic oomycete Peronospora meconopsidis. Two endophytes (SMR1 and SMR2) reduced the downy mildew on three P. somniferum genotypes (Sampada, J-16, and I-14). SMR1 (Microbacterium sp.) also enhanced the resistance of P. somniferum against downy mildew under field conditions. The biochemical markers of plant susceptibility under biotic stresses (proline and malondialdehyde) were found to be reduced in P. somniferum upon SMR1 treatment. To understand the mechanisms underlying the enhanced resistance to downy mildew in SMR1 endophyte-treated P. somniferum genotype J-16, we compared the expression profiles using the next-generation RNA sequencing approach between P. somniferum pretreated with SMR1 and untreated endophyte-free control plants following exposure to downy mildew pathogen. Comparative transcriptome analysis revealed differential expression of transcripts belonging to broad classes of signal transduction, protein modification, disease/defense proteins, transcription factors, and phytohormones in SMR1-primed P. somniferum after infection with downy mildew pathogen. Furthermore, enhanced salicylic acid content was observed in SMR1-primed P. somniferum after exposure to downy mildew pathogen. This study sheds light on molecular mechanisms underlying enhanced resistance to downy mildew in SMR1-primed P. somniferum. Finally, we propose that the SA-dependent defense pathway, the hallmark of systemic acquired resistance, is activated in SMR1-primed P. somniferum, triggering the endophyte-induced resistance.

Tania Ray

Papers

Endophytes of Withania somnifera modulate in planta content and the site of withanolide biosynthesis

ACC deaminase‐containing plant growth‐promoting rhizobacteria protect Papaver somniferum from downy mildew

Endophytic Consortium With Diverse Gene-Regulating Capabilities of Benzylisoquinoline Alkaloids Biosynthetic Pathway Can Enhance Endogenous Morphine Biosynthesis in Papaver somniferum.

Molecular insights into enhanced resistance of Papaver somniferum against downy mildew by application of endophyte bacteria Microbacterium sp. SMR1.