Plant growth promoting rhizobacteria (PGPR) hold promising future for sustainable agriculture. Here, we demonstrate a carotenoid producing halotolerant PGPR Dietzia natronolimnaea STR1 protecting wheat plants from salt stress by modulating the transcriptional machinery responsible for salinity tolerance in plants. The expression studies confirmed the involvement of ABA-signalling cascade, as TaABARE and TaOPR1 were upregulated in PGPR inoculated plants leading to induction of TaMYB and TaWRKY expression followed by stimulation of expression of a plethora of stress related genes. Enhanced expression of TaST, a salt stress-induced gene, associated with promoting salinity tolerance was observed in PGPR inoculated plants in comparison to uninoculated control plants. Expression of SOS pathway related genes (SOS1 and SOS4) was modulated in PGPR-applied wheat shoots and root systems. Tissue-specific responses of ion transporters TaNHX1, TaHAK, and TaHKT1, were observed in PGPR-inoculated plants. The enhanced gene expression of various antioxidant enzymes such as APX, MnSOD, CAT, POD, GPX and GR and higher proline content in PGPR-inoculated wheat plants contributed to increased tolerance to salinity stress. Overall, these results indicate that halotolerant PGPR-mediated salinity tolerance is a complex phenomenon that involves modulation of ABA-signalling, SOS pathway, ion transporters and antioxidant machinery.

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.

Not much is known about the mechanism of endophyte-mediated induction of secondary metabolite production in Catharanthus roseus. In the present study two fungal endophytes, Curvularia sp. CATDLF5 and Choanephora infundibulifera CATDLF6 were isolated from the leaves of the plant that were found to enhance vindoline content by 229–403%. The isolated endophytes did not affect the primary metabolism of the plant as the maximum quantum efficiency of PSII, net CO2 assimilation, plant biomass and starch content of endophyte-inoculated plants was similar to endophyte-free control plants. Expression of terpenoid indole alkaloid (TIA) pathway genes, geraniol 10-hydroxylase (G10H), tryptophan decarboxylase (TDC), strictosidine synthase (STR), 16-hydoxytabersonine-O-methyltransferase (16OMT), desacetoxyvindoline-4-hydroxylase (D4H), deacetylvindoline-4-O-acetyltransferase (DAT) were upregulated in endophyte-inoculated plants. Endophyte inoculation upregulated the expression of the gene for transcriptional activator octadecanoid-responsive Catharanthus AP2-domain protein (ORCA3) and downregulated the expression of Cys2/His2-type zinc finger protein family transcriptional repressors (ZCTs). The gene for the vacuolar class III peroxidase (PRX1), responsible for coupling vindoline and catharanthine, was upregulated in endophyte-inoculated plants. These endophytes may enhance vindoline production by modulating the expression of key structural and regulatory genes of vindoline biosynthesis without affecting the primary metabolism of the host plant.

Endophytes reside in different parts of the poppy plant and perform the tissue-specific functions. Most leaf endophytes modulate photosynthetic efficiency, plant growth, and productivity while capsule endophytes modulate alkaloid biosynthesis. Endophytes promote plant growth, provide protection from environmental stresses and are the source of important secondary metabolites. Here, we established that the endophytes of opium poppy Papaver somniferum L. may play a role in the modulation of plant productivity and benzylisoquinoline alkaloid (BIA) biosynthesis. A total of 22 endophytes isolated from leaves, roots, capsules and seeds of the poppy plants were identified. Isolated endophytes were used to inoculate the endophytes free poppy seeds and screened for their ability to improve plant productivity and BIA production. It was evident that the endophytes from leaf were involved in improving photosynthetic efficiency, and thus crop growth and yield and the endophytes from capsule were involved in enhancing BIA biosynthesis. Capsule endophytes of alkaloid-rich P. somniferum cv. Sampada enhanced BIA production even in alkaloid-less cv. Sujata. Expression study of the genes involved in BIA biosynthesis conferred the differential regulation of their expression in the presence of capsule endophytes. The capsule endophyte SM1B (Acinetobacter) upregulated the expression of the key genes for the BIA biosynthesis except thebaine 6-O-demethylase (T6ODM) and codeine O-demethylase (CODM). On the other hand, another capsule endophyte SM3B (Marmoricola sp.) could upregulate both T6ODM and CODM. Colonization of poppy plant by endophytes isolated from leaves, roots and capsules found to be higher in their respective plant parts confirmed their tissue-specific role. Overall, the results demonstrate the specific role of endophytes in the modulation of host plant productivity and BIA production.

Chlorophyll b is synthesized by the oxidation of a methyl group on the B ring of a tetrapyrrole molecule to a formyl group by chlorophyllide a oxygenase (CAO). The full-length CAO from Arabidopsis (Arabidopsis thaliana) was overexpressed in tobacco (Nicotiana tabacum) that grows well at light intensities much higher than those tolerated by Arabidopsis. This resulted in an increased synthesis of glutamate semialdehyde, 5-aminolevulinic acid, magnesium-porphyrins, and chlorophylls. Overexpression of CAO resulted in increased chlorophyll b synthesis and a decreased chlorophyll a/b ratio in low light-grown as well as high light-grown tobacco plants; this effect, however, was more pronounced in high light. The increased potential of the protochlorophyllide oxidoreductase activity and chlorophyll biosynthesis compensated for the usual loss of chlorophylls in high light. Increased chlorophyll b synthesis in CAO-overexpressed plants was accompanied not only by an increased abundance of light-harvesting chlorophyll proteins but also of other proteins of the electron transport chain, which led to an increase in the capture of light as well as enhanced (40%-80%) electron transport rates of photosystems I and II at both limiting and saturating light intensities. Although the quantum yield of carbon dioxide fixation remained unchanged, the light-saturated photosynthetic carbon assimilation, starch content, and dry matter accumulation increased in CAO-overexpressed plants grown in both low- and high-light regimes. These results demonstrate that controlled up-regulation of chlorophyll b biosynthesis comodulates the expression of several thylakoid membrane proteins that increase both the antenna size and the electron transport rates and enhance carbon dioxide assimilation, starch content, and dry matter accumulation.