Plant growth-promoting rhizobacterial biofertilizers for crop production: The past, present, and future

The Co-Inoculation Effect on Triticum aestivum Growth with Synthetic Microbial Communities (SynComs) and Their Potential in Agrobiotechnology

The use of rhizospheric SynComs can be a new and sustainable strategy in the agrobiotechnology sector. The objective of this study was to create the most appropriate SynCom composition; examine the ability to dissolve natural rock phosphate (RP) from Morocco in liquid-modified NBRIP medium; determine organic acids, and phytohormones; and verify plant growth promoting and nutrition uptake effect in the pot experiments of winter wheat (Triticum aestivum). A total of nine different microorganisms were isolated, which belonged to three different genera: Bacillus, Pseudomonas, and Streptomyces. Out of the 21 treatments tested, four SynComs had the best phosphate-dissolving properties: IJAK-27+44+91 (129.17 mg L-1), IIBEI-32+40 (90.95 µg mL-1), IIIDEG-45+41 (122.78 mg L-1), and IIIDEG-45+41+72 (120.78 mg L-1). We demonstrate that these SynComs are capable of producing lactic, acetic, gluconic, malic, oxalic, citric acids, and phytohormones such as indole-3-acetic acid, zeatin, gibberellic acid, and abscisic acid. In pot experiments with winter wheat, we also demonstrated that the designed SynComs were able to effectively colonize the plant root rhizosphere and contributed to more abundant plant growth characteristics and nutrient uptake as uninoculated treatment or uninoculated treatment with superphosphate (NPK 0-19-0). The obtained results show that the SynCom compositions of IJAK-27+44+91, IIBEI-32+40, IIIDEG-45+41, and IIIDEG-45+41+72 can be considered as promising candidates for developing biofertilizers to facilitate P absorption and increase plant nutrition.

Engineering plant-microbe communication for plant nutrient use efficiency

Nutrient availability and efficient use are critical for crop productivity. Current agricultural practices rely on excessive chemical fertilizers, contributing to greenhouse gas emissions and environmental pollution. Rhizosphere microbes facilitate plant nutrient acquisition and contribute to nutrient use efficiency. Thus, engineering plant-microbe communication within the rhizosphere emerges as a promising and sustainable strategy to enhance agricultural productivity. Recent advances in plant engineering have enabled the development of plants capable of selectively enriching beneficial microbes through root exudates. At the same time, synthetic biology techniques have produced microbes capable of improving nutrient availability and uptake by plants. By engineering plant-microbe communication, researchers aim to harness beneficial soil microbes, thereby offering a targeted and efficient approach to optimizing plant nutrient use efficiency.

Long-term detection of Hartmannibacter diazotrophicus on winter wheat and spring barley roots under field conditions revealed positive correlations on yield parameters with the bacterium abundance

Monitoring of bioinoculants once released into the field remains largely unexplored; thus, more information is required about their survival and interactions after root colonization. Therefore, specific primers were used to perform a long-term tracking to elucidate the effect of Hartmannibacter diazotrophicus on wheat and barley production at two experimental organic agriculture field stations. Three factors were evaluated: organic fertilizer application (with and without), row spacing (15 and 50 cm), and bacterial inoculation (H. diazotrophicus and control without bacteria). Hartmannibacter diazotrophicus was detected by quantitative polymerase chain reaction on the roots (up to 5 × 105 copies g-1 dry weight) until advanced developmental stages under field conditions during two seasons, and mostly in one farm. Correlation analysis showed a significant effect of H. diazotrophicus copy numbers on the yield parameters straw yield (increase of 453 kg ha-1 in wheat compared to the mean) and crude grain protein concentration (increase of 0.30% in wheat and 0.80% in barley compared to the mean). Our findings showed an apparently constant presence of H. diazotrophicus on both wheat and barley roots until 273 and 119 days after seeding, respectively, and its addition and concentration in the roots are associated with higher yields in one crop.

Agronomic strategies to enhance the early vigor and yield of maize part II: the role of seed applied biostimulant, hybrid, and starter fertilization on crop performance

Maize cropping systems need to be re-designed, within a sustainable intensification context, by focusing on the application of high-use efficiency crop practices, such as those that are able to enhance an early plant vigor in the first critical growth stages; such practices could lead to significant agronomic and yield benefits. The aim of this study has been to evaluate the effects of the cultivation of hybrids with superior early vigor, of the distribution of starter fertilizers at sowing, and of the seed application of biostimulants on promoting plant growth and grain yield in full factorial experiments carried out in both a growth chamber and in open fields. The greatest benefits, in terms of plant growth enhancement (plant height, biomass, leaf area) and cold stress mitigation, were detected for the starter fertilization, followed by the use of an early vigor hybrid and a biostimulant seed treatment. The starter fertilization and the early vigor hybrid led to earlier flowering dates, that is, of 2.1 and 2.8 days, respectively, and significantly reduced grain moisture at harvest. Moreover, the early vigor hybrid, the starter NP fertilization, and the biostimulant treatment increased grain yield by 8.5%, 6.0%, and 5.1%, respectively, compared to the standard hybrid and the untreated controls. The combination of all the considered factors resulted in the maximum benefits, compared to the control cropping system, with an increase in the plant growth of 124%, a reduction of the sowing-flowering period of 5 days, and a gain in grain yield of 14%. When choosing the most suitable crop practice, the diversity of each cropping system should be considered, according to the pedo-climatic conditions, the agronomic background, the yield potential, and the supply chain requirements.

Plant beneficial microbiome a boon for improving multiple stress tolerance in plants

Beneficial microbes or their products have been key drivers for improving adaptive and growth features in plants under biotic and abiotic stress conditions. However, the majority of these studies so far have been utilized against individual stressors. In comparison to individual stressors, the combination of many environmental stresses that plants experience has a greater detrimental effect on them and poses a threat to their existence. Therefore, there is a need to explore the beneficial microbiota against combined stressors or multiple stressors, as this will offer new possibilities for improving plant growth and multiple adaptive traits. However, recognition of the multifaceted core beneficial microbiota from plant microbiome under stress combinations will require a thorough understanding of the functional and mechanistic facets of plant microbiome interactions under different environmental conditions in addition to agronomic management practices. Also, the development of tailored beneficial multiple stress tolerant microbiota in sustainable agriculture necessitates new model systems and prioritizes agricultural microbiome research. In this review, we provided an update on the effect of combined stressors on plants and their microbiome structure. Next, we discussed the role of beneficial microbes in plant growth promotion and stress adaptation. We also discussed how plant-beneficial microbes can be utilized for mitigating multiple stresses in plants. Finally, we have highlighted some key points that warrant future investigation for exploring plant microbiome interactions under multiple stressors.

The Biosynthesis and Functions of Polyamines in the Interaction of Plant Growth-Promoting Rhizobacteria with Plants

Plant growth-promoting rhizobacteria (PGPR) are members of the plant rhizomicrobiome that enhance plant growth and stress resistance by increasing nutrient availability to the plant, producing phytohormones or other secondary metabolites, stimulating plant defense responses against abiotic stresses and pathogens, or fixing nitrogen. The use of PGPR to increase crop yield with minimal environmental impact is a sustainable and readily applicable replacement for a portion of chemical fertilizer and pesticides required for the growth of high-yielding varieties. Increased plant health and productivity have long been gained by applying PGPR as commercial inoculants to crops, although with uneven results. The establishment of plant-PGPR relationships requires the exchange of chemical signals and nutrients between the partners, and polyamines (PAs) are an important class of compounds that act as physiological effectors and signal molecules in plant-microbe interactions. In this review, we focus on the role of PAs in interactions between PGPR and plants. We describe the basic ecology of PGPR and the production and function of PAs in them and the plants with which they interact. We examine the metabolism and the roles of PAs in PGPR and plants individually and during their interaction with one another. Lastly, we describe some directions for future research.

Zinc-solubilizing Bacillus spp. in conjunction with chemical fertilizers enhance growth, yield, nutrient content, and zinc biofortification in wheat crop

Micronutrient deficiency is a serious health issue in resource-poor human populations worldwide, which is responsible for the death of millions of women and underage children in most developing countries. Zinc (Zn) malnutrition in middle- and lower-class families is rampant when daily calorie intake of staple cereals contains extremely low concentrations of micronutrients, especially Zn and Fe. Looking at the importance of the problem, the present investigation aimed to enhance the growth, yield, nutrient status, and biofortification of wheat crop by inoculation of native zinc-solubilizing Bacillus spp. in conjunction with soil-applied fertilizers (NPK) and zinc phosphate in saline soil. In this study, 175 bacterial isolates were recovered from the rhizosphere of wheat grown in the eastern parts of the Indo-Gangetic Plain of India. These isolates were further screened for Zn solubilization potential using sparingly insoluble zinc carbonate (ZnCO₃), zinc oxide (ZnO), and zinc phosphate {Zn₃(PO₄)₂} as a source of Zn under in vitro conditions. Of 175 bacterial isolates, 42 were found to solubilize either one or two or all the three insoluble Zn compounds, and subsequently, these isolates were identified based on 16S rRNA gene sequences. Based on zone halo diameter, solubilization efficiency, and amount of solubilized zinc, six potential bacterial strains, i.e., Bacillus altitudinis AJW-3, B. subtilis ABW-30, B. megaterium CHW-22, B. licheniformis MJW-38, Brevibacillus borstelensis CHW-2, and B. xiamenensis BLW-7, were further shortlisted for pot- and field-level evaluation in wheat crop. The results of the present investigation clearly indicated that these inoculants not only increase plant growth but also enhance the yield and yield attributes. Furthermore, bacterial inoculation also enhanced available nutrients and microbial activity in the wheat rhizosphere under pot experiments. It was observed that the application of B. megaterium CHW-22 significantly increased the Zn content in wheat straw and grains along with other nutrients (N, P, K, Fe, Cu, and Mn) followed by B. licheniformis MJW-38 as compared to other inoculants. By and large, similar observations were recorded under field conditions. Interestingly, when comparing the nutrient use efficiency (NUE) of wheat, bacterial inoculants showed their potential in enhancing the NUE in a greater way, which was further confirmed by correlation and principal component analyses. This study apparently provides evidence of Zn biofortification in wheat upon bacterial inoculation in conjunction with chemical fertilizers and zinc phosphate in degraded soil under both nethouse and field conditions.

Research progress on iron absorption, transport, and molecular regulation strategy in plants

Iron is a trace element essential for normal plant life activities and is involved in various metabolic pathways such as chlorophyll synthesis, photosynthesis, and respiration. Although iron is highly abundant in the earth's crust, the amount that can be absorbed and utilized by plants is very low. Therefore, plants have developed a series of systems for absorption, transport, and utilization in the course of long-term evolution. This review focuses on the findings of current studies of the Fe²⁺ absorption mechanism I, Fe³⁺ chelate absorption mechanism II and plant-microbial interaction iron absorption mechanism, particularly effective measures for artificially regulating plant iron absorption and transportation to promote plant growth and development. According to the available literature, the beneficial effects of using microbial fertilizers as iron fertilizers are promising but further evidence of the interaction mechanism between microorganisms and plants is required.

Studies Using Mutant Strains of Azospirillum brasilense Reveal That Atmospheric Nitrogen Fixation and Auxin Production Are Light Dependent Processes

As the use of microbial inoculants in agriculture rises, it becomes important to understand how the environment may influence microbial ability to promote plant growth. This work examines whether there are light dependencies in the biological functions of Azospirillum brasilense, a commercialized prolific grass-root colonizer. Though classically defined as non-phototrophic, A. brasilense possesses photoreceptors that could perceive light conducted through its host's roots. Here, we examined the light dependency of atmospheric biological nitrogen fixation (BNF) and auxin biosynthesis along with supporting processes including ATP biosynthesis, and iron and manganese uptake. Functional mutants of A. brasilense were studied in light and dark environments: HM053 (high BNF and auxin production), ipdC (capable of BNF, deficient in auxin production), and FP10 (capable of auxin production, deficient in BNF). HM053 exhibited the highest rate of nitrogenase activity with the greatest light dependency comparing iterations in light and dark environments. The ipdC mutant showed similar behavior with relatively lower nitrogenase activity observed, while FP10 did not show a light dependency. Auxin biosynthesis showed strong light dependencies in HM053 and FP10 strains, but not for ipdC. Ferrous iron is involved in BNF, and a light dependency was observed for microbial ⁵⁹Fe²⁺ uptake in HM053 and ipdC, but not FP10. Surprisingly, a light dependency for ⁵²Mn²⁺ uptake was only observed in ipdC. Finally, ATP biosynthesis was sensitive to light across all three mutants favoring blue light over red light compared to darkness with observed ATP levels in descending order for HM053 > ipdC > FP10.

Ascochyta Blight in Chickpea: An Update

Chickpea (Cicer arietinum L.), one of the most cultivated legumes worldwide, is crucial for the economy of several countries and a valuable source of nutrients. Yields may be severely affected by Ascochyta blight, a disease caused by the fungus Ascochyta rabiei. Molecular and pathological studies have not yet managed to establish its pathogenesis, since it is highly variable. Similarly, much remains to be elucidated about plant defense mechanisms against the pathogen. Further knowledge of these two aspects is fundamental for the development of tools and strategies to protect the crop. This review summarizes up-to-date information on the disease's pathogenesis, symptomatology, and geographical distribution, as well as on the environmental factors that favor infection, host defense mechanisms, and resistant chickpea genotypes. It also outlines existing practices for integrated blight management.