Beneficial Microbes for Sustainable Crop Production

Microbial communities represent a major component of cultivated soils and are responsible for the successful production of crops [1,2,3]. The evolution of soil microbiomes in the current agronomic context depends on both soil type and fertility, as well as the cultivated species and technologies, and has demonstrated large variability under the impact of climate change [4,5,6,7]. Numerous and diverse microorganisms are involved in securing nutrients for their host and in sustaining their optimum growth and development processes [8,9,10]. Soil inoculation with microbial consortia represents a viable solution to increase the amount of nutrients required by crops, and their protection against biotic and abiotic stresses, alongside ensuring stability in soil fluxes and organic matter recycling [11,12,13,14].

The Special Issue “Beneficial Microbes for Sustainable Crop Production” is a collection of eight manuscripts that explore the complex interactions between soil, plants, and microbial communities. We want to thank all the contributors for their studies, which were performed in field and controlled environments. All these studies deepen the understanding on microbial performance and functions and sustain the development of future agronomic strategies and applications.

A pot study with six arbuscular mycorrhizal species was conducted on maize [15], with the aim of analyzing the effect of inoculation on multiple plant and soil parameters. The authors assessed plant growth, leaf gas exchange and sugar levels, mycorrhizal colonization rate in roots, glomalin-related soil proteins, and conducted an analysis of SWEET gene expression. Their findings indicate significant species-specific variation in terms of root colonization, improvements in plant traits, and glomalin-related soil protein fractions. The same species-specific effect was observed in reprogrammed ZmSWEET expression, with core ZmSWEET genes mediating sugar accumulation to support symbiosis. The study provides a foundation for further exploration of the interaction between arbuscular mycorrhizal fungi and SWEET genes.

Two commercial cultivars (Pedrosillano and Blanco Lechoso) and twenty Cicer arietinum L. (chickpea) germplasm were tested against seven Mesorhizobium strains for cultivar-specific symbiotic performance [16]. The research was conducted by the authors under growth-chamber and greenhouse conditions, in the presence and absence of drought stress. During the initial screening, three elite strains (ISC11, ISC15, and ISC25) showed superior symbiotic performance and nitrogen activity. The effect of drought was significantly mitigated in several strain–cultivar combinations, maintaining over 70% of shoot biomass compared to controls. Based on the whole-genome sequencing results, diverse taxonomic affiliations were observed—Mesorhizobium ciceri (ISC11), Mesorhizobium mediterraneum (ISC15), and a potential novel species (ISC25). The findings of the authors show the potential of targeted rhizobial inoculants optimized for chickpea cultivars with the aim of improving crop performance under water-limiting conditions.

The bacterial populations in five turmeric (Curcuma) species were analyzed in field and pot trials [17]. The bacterial populations recorded in the rhizosphere, stems, and leaves showed variations across growth stages, with the highest values reached in the rhizosphere in both types of experiments. A compositional distinction between leaf-associated bacterial populations and those from rhizosphere and stems was observed. The results showed that Curcuma growth stage and species have a significant effect on the bacterial community structure. The findings of the authors can facilitate a better understanding of plant–bacteria dynamics and the optimization of microbial inoculation strategies for the development of sustainable agricultural practices.

Seedling P uptake and mycorrhizal colonization in rice and pearl millet was assessed under different arbuscular mycorrhizal fungi inoculation and water regime conditions [18]. The results presented by the authors showed that two new inoculants (I₂ and I₃) had higher propagule numbers and an increase in the infection rate compared to the control seedlings, while another inoculant (I₁) improved root transversal area and shoot growth parameters. All inoculants showed lower infection rates than the indigenous arbuscular mycorrhizal fungi from upland Andosol. For pearl millet, a higher P uptake and shoot dry weight was observed in conditions involving a higher infection rate. The study showed the importance of inoculants for seedling establishment and highlighted the increased P uptake through mycorrhizal mediation in pearl millet than in rice.

An extensive biochemical characterization of Phytobacter sp. isolate, WL65, yielding genomic insights, was conducted to accurately identify the species and their functional potential for plant growth promotion [19]. In addition, the research predicted secondary metabolic pathways through genomic analyses and the potential use of the isolate as biofertilizer for rice cultivation. Based on whole-genome analysis, the identity of Phytobacter palmae WL65 was confirmed, as well as the presence of genes for nitrogen fixation, phosphate solubilization, IAA biosynthesis, siderophore production, and biofilm formation. The application of the isolate in rice increased plant growth and yield, also improving soil fertility. The findings of the study sustain the potential of WL65 as biofertilizer and improve the understanding of plant growth-promotion rhizobacterium mechanism.

Plant growth-promoting rhizobacteria and hydrogel (potassium polyacrylate) were analyzed on Brassica napus L. under drought conditions [20]. Serratia plymuthica was selected from six candidates, based on in vitro and pot experiments, where it showed an enhancement in plant biomass, shoot length, and the number of internodes. The application of hydrogel showed no adverse effects on seed viability and tested bacterial strains. Field experiments showed an increase in the number of siliques, yield, plant height, and the number of branches after the application of S. plymuthica. The application of hydrogel (solely or with bacterium) to the soil delayed seedling emergence and diminished the growth-promoting effect of S. plymuthica. The findings of the study indicate the potential use of S. plymuthica as an ingredient for seed coatings.

Research on the reduction in glucosinolates levels in transgenic rapeseed and the presence of arbuscular mycorrhiza in the rhizosphere was performed to test if the formation of specific fungal structures would be possible [21]. After the establishment of the symbiosis, the main objective was to identify the beneficial effects of the association on seed yield. The conclusion of the study was that inoculation with arbuscular mycorrhiza of transgenic rapeseed plants expressing the Thkel1 gene improves the seed yield and fatty acid composition of oilseed. However, the reduction in glucosinolates levels is not enough to sustain the formation of specific arbuscular mycorrhiza structures—arbuscules and vesicles. The results of the research represent new opportunities in agrobiotechnology for the application of arbuscular mycorrhizal fungi as biofertilizers for Brassicaceae crops.

The effects of individual and combined inoculation with microorganisms on the polyphenol content in Okra (Abelmoschus esculentus L.) were analyzed for the detection of synergistic combinations [22]. The tested microorganism had generally positive effects on plant growth and yield, also leading to an increase in the glutathione-S-transferase enzyme activity of leaves. Individual inoculation showed higher enzyme activities compared to combined treatment. The treatment, involving a combination of F. mosseae and Streptomyces strain K61, led to a significant increase in the coumaric acid content, while Aureobasidium strain DSM 14950 showed a similar effect on quercetin and quercetin-3-diglucoside. The findings of the study showed that targeted inoculation can have a selective influence on the content of specific polyphenol compounds.