María Evangelina Carezzano

The formulation of rhizobial bioinoculants remains a critical bottleneck for the large-scale deployment of biological nitrogen fixation in sustainable agriculture, mainly due to limitations in the stability and viability of conventional liquid products. In this study, a spray-drying-based process was developed and optimized to produce a stable and functional bioinoculant using Ensifer meliloti Rm8530, an EPS II–producing strain with enhanced stress tolerance. Strain robustness was evaluated through thermal and osmotic stress assays, together with growth performance across relevant temperature and pH ranges. Six carrier-based formulations combining polysaccharides and proteins were then tested under controlled spray-drying conditions. Process performance was assessed in terms of powder recovery, residual moisture, bacterial survival, yield, and storage stability over 16 weeks. The morphological integrity of spray-dried particles and rehydrated cells was analyzed by scanning electron microscopy. The biological functionality of selected formulations was subsequently validated in planta using alfalfa as a host model. Among the formulations tested, a mixed alginate–gum Arabic matrix showed the best overall balance between process efficiency, post-drying viability, long-term stability, and symbiotic performance. Spray-dried cells retained the ability to induce nodulation and support early plant responses under the conditions evaluated. These results demonstrate that spray drying, combined with appropriate strain selection and formulation design, constitutes a viable and scalable platform for producing stable, functional rhizobial bioinoculants.

Wheat (Triticum aestivum L.) is one of the most important cereals worldwide, but is affected by various bacterial diseases, including bacterial leaf blight caused by Pseudomonas syringae, that decrease productivity significantly. The indiscriminate use of synthetic pesticides for disease management has led to environmental contamination, development of resistance and health risks, highlighting the need for alternative and environmentally friendly solutions. In this study, phytopathogenic Pseudomonas were isolated from symptomatic wheat leaves collected in Córdoba province (Argentina). The isolates were phenotypically identified using biochemical tests and the LOPAT scheme, confirming their classification within Group I of P. syringae. Pathogenicity tests verified their ability to induce typical leaf blight symptoms, and nine isolates were positive for the production of syringomycin, a phytotoxin associated with host tissue necrosis. The antimicrobial activity of different phytochemical preparations obtained from Thymus vulgaris and Origanum vulgare (decoctions, alcoholic, hexanic, chloroformic and propanolic extracts, as well as essential oils (EOs)) against the isolated and reference strains of P. syringae was evaluated. Disk diffusion assays revealed that thyme and oregano EOs exhibited the strongest inhibitory effects compared to the other phytochemical preparations tested. Antimicrobial activity assays by broth microdilution technique confirmed the antibacterial potency of thyme EO (MIC = 1.43–22.99 mg/mL) and oregano EO (MIC = 2.89–23.13 mg/mL). These results demonstrated the efficacy of T. vulgaris and O. vulgare EOs as natural antimicrobial agents, suggesting their potential application as sustainable alternatives to synthetic pesticides for the control of bacterial leaf blight in wheat crops.

Plants are constantly exposed to a wide range of environmental factors that cause different kinds of stress, such as drought, salinity, heat, frost, and low nutrient availability. There are also biotic sources of stress, which include pathogens (bacteria, viruses, pests), herbivores, and plant competitors. These various types of stress affect normal plant physiology and development, and may lead to significantly lower yields. However, certain microorganisms (MOs), known as plant growth-promoting rhizobacteria (PGPR), can interact with and benefit plants in stressful environments. They do so through a series of mechanisms which contribute to minimizing the negative effects of plants’ responses to stress. This review summarizes current knowledge about those mechanisms, with a focus on the production of exopolysaccharides (EPSs). These compounds can act as osmoprotectants, promote the production of phytohormones, prevent the entry of pathogens through roots, bioremediate metals, and improve soil structure and permeability, among many other beneficial effects. This makes them suitable alternatives to guarantee food security while reducing the excessive use of chemical agricultural inputs and their harmful consequences for the environment.

Soybean mosaic disease, caused by the soybean mosaic virus (SMV), is responsible for major losses in yield and seed quality worldwide. Although resistant cultivars are used for its prevention and control, an alternative strategy could consist of applying environmentally friendly antimicrobial agents, such as extracts and essential oils (EOs) of aromatic plants. This study assessed an extract of Achyrocline satureioides and EOs of Minthostachys verticillata, Origanum vulgare, and Thymus vulgaris in terms of their phytotoxicity in soybean. Since all the concentrations tested were found to be safe, the activity of each product against SMV was then assayed in vivo, i.e., in experimentally infected soybean plants. The parameters measured were plant height, wet weight, and virus titer. All the treated plants had a greater height and weight than those in the viral control group. The EOs of M. verticillata (0.80 mg/mL) and T. vulgaris (0.71 mg/mL) inhibited the production of viral antigens, as determined by an ELISA test. These findings could encourage further studies aimed at developing an effective biopesticide against SMV.

Phytopathogenic bacteria not only affect crop yield and quality but also the environment. Understanding the mechanisms involved in their survival is essential to develop new strategies to control plant disease. One such mechanism is the formation of biofilms; i.e., microbial communities within a three-dimensional structure that offers adaptive advantages, such as protection against unfavorable environmental conditions. Biofilm-producing phytopathogenic bacteria are difficult to manage. They colonize the intercellular spaces and the vascular system of the host plants and cause a wide range of symptoms such as necrosis, wilting, leaf spots, blight, soft rot, and hyperplasia. This review summarizes up-to-date information about saline and drought stress in plants (abiotic stress) and then goes on to focus on the biotic stress produced by biofilm-forming phytopathogenic bacteria, which are responsible for serious disease in many crops. Their characteristics, pathogenesis, virulence factors, systems of cellular communication, and the molecules implicated in the regulation of these processes are all covered.

As a follow-up to previous studies, the effects of Thymus vulgaris essential oil on selected virulence factors (growth, sessile cell survival, swimming, swarming, and exopolysaccharide production) were evaluated in phytopathogenic Pseudomonas syringae strains isolated from soybean fields in Argentina; reference strains Pseudomonas savastanoi pv. glycinea B076 and Pseudomonas aeruginosa PAO1. P. syringae are responsible for bacterial blight, a disease that affects crops worldwide. Plant bacterioses are usually treated with antibiotics and copper compounds, which may contribute to the development of resistance in pathogens and damage the environment. For these reasons, eco-friendly alternatives are necessary. Although aromatic plants are a natural source of antimicrobial substances, the effects of these substances on phytopathogenic bacteria remain largely unexplored. Subinhibitory concentrations of the oil significantly reduced the slope and rate of bacterial growth. In addition, biofilm and exopolysaccharide (EPS) production were inhibited, with swimming and swarming motility patterns being affected at all of the oil concentrations tested. Therefore, TEO could potentially be a highly efficient antipseudomonal agent for treating plant infections caused by P. syringae.

Introduction: Paenibacillus larvae is a spore-forming bacillus, the most important bacterial pathogen of honeybee larvae and the causative agent of American foulbrood (AFB). Control measures are limited and represent a challenge for both beekeepers and researchers. For this reason, many studies focus on the search for alternative treatments based on natural products. Aim: The objective of this study was to determine the antimicrobial activity of the hexanic extract (HE) of Achyrocline satureioides on P. larvae and the inhibitory activity on some mechanisms related to pathogenicity. Material and methods: The Minimum Inhibitory Concentration (MIC) of the HE was determined by the broth microdilution technique and the Minimum Bactericidal Concentration (MBC) by the microdrop technique. Swimming and swarming motility was evaluated in plates with 0.3 and 0.5% agar, respectively. Biofilm formation was evaluated and quantified by the Congo red and crystal violet method. The protease activity was evaluated by the qualitative technique on skim milk agar plates. Results: It was determined that the MIC of the HE on four strains of P. larvae ranged between 0.3 and 9.37 µg/ml and the MBC between 1.17 and 150 µg/ml. On the other hand, sub-inhibitory concentrations of the HE were able to decrease swimming motility, biofilm formation and the proteases production of P. larvae.

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.