Marcela Susana Montecchia

Bacterial diseases are major constraints in tomato production, particularly bacterial canker caused by Clavibacter michiganensis and bacterial spot caused by different Xanthomonas species. The limited effectiveness of chemical bactericides, the emergence of resistant strains and environmental concerns highlight the need for ecofriendly, sustainable strategies to control these phytopathogens. This study aimed to evaluate the biocontrol potential of a heat‐inactivated culture of Burkholderia gladioli strain BNM349 against major bacterial pathogens of tomato. Antibacterial activity of B. gladioli BNM349 culture‐derived preparations, including heat‐inactivated culture, cell‐free supernatant and butanol extract, was first assessed in vitro against C. michiganensis , X. vesicatoria and X. euvesicatoria pv. perforans . All BNM349‐derived preparations inhibited pathogen growth, although the extent of inhibition varied depending on the preparation type and pathogenic bacterial species. Greenhouse experiments further demonstrated that soil application of the heat‐inactivated culture significantly reduced bacterial canker severity by 40.1%, decreasing both disease progression and stem necrosis. The same treatment also reduced bacterial spot severity caused by X. vesicatoria by 48%. In contrast, the cell‐free supernatant had no significant effect on disease severity. These results indicate that heat‐inactivated cultures of B. gladioli BNM349 retain bioactive compounds capable of suppressing bacterial diseases in tomato, highlighting the potential of non‐viable microbial preparations as an environmentally friendly strategy for crop disease management.

Blueberry roots are inefficient in taking up water and nutrients, a fact partially related to their scarcity of root hairs, but they improve nutrient uptake by associating with ericoid mycorrhizal and endophytic fungi. However, the benefits of this association are both cultivar- and fungus-dependent. Our objective was to assess the effect of inoculation with three native fungal strains (Oidiodendron maius A, O. maius BP, and Acanthomyces lecanii BC) on plantlet growth, plantlet survival, and nitrogen (N) absorption of the southern highbush blueberry (SHB) cultivars Biloxi and Misty. The fungal strains were inoculated into the peat-based substrate for growing blueberry cultivars, and plantlets produced by micropropagation were transplanted and grown for four months. The three inoculated strains positively affected the survival percentage in at least one of the cultivars tested, whereas O. maius BP positively affected plant biomass, N derived from fertilizer absorption, N content, and plant N recovery (%) in both Biloxi and Misty. Our results show that the O. maius BP strain may prove useful as a bio-inoculant to improve blueberry production during the nursery stage.

Burkholderia ambifaria T16 is a bacterium isolated from the rhizosphere of barley plants that showed a remarkable antifungal activity. This strain was also able to degrade fusaric acid (5-Butylpyridine-2-carboxylic acid) and detoxify this mycotoxin in inoculated barley seedlings. Genes and enzymes responsible for fusaric acid degradation have an important biotechnological potential in the control of fungal diseases caused by fusaric acid producers, or in the biodegradation/bio catalysis processes of pyridine derivatives. In this study, the complete genome of B. ambifaria T16 was sequenced and analyzed to identify genes involved in survival and competition in the rhizosphere, plant growth promotion, fungal growth inhibition, and degradation of aromatic compounds. The genomic analysis revealed the presence of several operons for the biosynthesis of antimicrobial compounds, such as pyrrolnitrin, ornibactin, occidiofungin and the membrane-associated AFC-BC11. These compounds were also detected in bacterial culture supernatants by mass spectrometry analysis. In addition, this strain has multiple genes contributing to its plant growth-promoting profile, including those for acetoin, 2,3-butanediol and indole-3-acetic acid production, siderophores biosynthesis, and solubilisation of organic and inorganic phosphate. A pan-genomic analysis demonstrated that the genome of strain T16 possesses large gene clusters that are absent in the genomes of B. ambifaria reference strains. According to predictions, most of these clusters would be involved in aromatic compounds degradation. One genomic region, encoding flavin-dependent monooxygenases of unknown function, is proposed as a candidate responsible for fusaric acid degradation.

Tropical and subtropical ecosystems are widely affected by the expansion of agriculture over pristine lands. Despite research efforts, knowledge of the impact of land‐use change on soil is still limited by intrinsic variability, inconsistent results and inadequate replication. This study aimed to better understand the consequences of land‐use change by focusing on long‐term effects on both soil biotic and abiotic parameters. For this purpose, we selected three productive farms under similar management, each of them with pristine forest sites and agricultural sites that had been deforested for ~15 and ~30 years. In each site, we analysed soil microbiological (phospholipid fatty acids [PLFAs], biomass and activity) and physicochemical parameters. Long‐term land‐use change caused a detriment in soil microbial biomass, activity and fungal abundance, but only small changes in PLFA composition. In fact, PLFA composition was more affected by soil physicochemical properties such as carbon‐to‐nutrient ratios and labile carbon than by land use. Some physicochemical parameters (e.g., organic carbon and nutrients) were also negatively affected by land‐use change and were more sensitive to time under agricultural use than microbiological parameters. The lower sensitivity of microbiological parameters could be the result of severe drought conditions at sampling, which may have affected soil microbial communities in both land uses. We were also able to detect associations between specific microbiological and physicochemical parameters. Among these, we identified some that seemed to result from their co‐variation in response to land‐use change and others that seemed to be independent of land use. Overall, our results show that soils can suffer further deterioration several years after deforestation. In order to restore soil health in these degraded lands, we need to keep on investigating the physical, chemical and biological mechanisms responsible for this deterioration. Highlights: Land‐use change affected soil microbiological and physicochemical parameters. Microbiological parameters seemed to stabilize after continuous agriculture. Soil organic C, total N and fine particles were still reduced after long‐term cultivation. Microbiological parameters were mostly associated with C‐to‐nutrient ratios and labile C. Drought conditions may have affected microbial response to land‐use change.

Clavibacter michiganensis subsp. michiganensis (Cmm) causes bacterial wilt and canker in tomato, producing important economic losses worldwide. Its virulence has been related to several putative virulence factors present on a chromosomal pathogenicity island and on plasmids pCM1 and pCM2, in strain NCPPB382. We genotypically characterized a collection of Cmm isolates from the main greenhouse tomato‐producing areas of Argentina by BOX‐PCR fingerprinting and screened for the presence of genes and plasmids involved in pathogenicity by PCR. In addition, we evaluated in vitro cellulolytic activity and virulence in planta of selected strains. BOX‐PCR fingerprinting clustered strains into four groups. Group II was dominant and included the most virulent strains, while Group III was the smallest and had the least virulent strains. All local strains exhibited similar cellulolytic activity. Most of the examined strains carry two plasmids of similar size to those of NCPPB382, although there were strains with one or three plasmids. By PCR amplification of repA , pCM1 was detected only in strains belonging to Group III, which includes local strains closely related to reference strain NCPPB382. All analysed pathogenicity genes were widespread among strains, and so in strains belonging to Groups I and II, celA found on pCM1 in NCPPB382 could be found in the chromosome or in plasmids other than pCM1. This study contributes to a better understanding of the diversity of Cmm genetic profiles and virulence of strains present in Argentina. Such information could be useful for the selection of strains for screening of host resistance and development of resistant tomato varieties.

The genetic diversity among 31 putative Azotobacter isolates obtained from agricultural and non‐agricultural soils was assessed using rep‐PCR genomic fingerprinting and identified to species level by ARDRA and partial 16S rRNA gene sequence analysis. High diversity was found among the isolates, identified as A. chroococcum, A. salinestris, and A. armeniacus. Selected isolates were characterized on the basis of phytohormone biosynthesis, nitrogenase activity, siderophore production, and phosphate solubilization. Indole‐3 acetic‐acid (IAA), gibberellin (GA₃) and zeatin (Z) biosynthesis, nitrogenase activity, and siderophore production were found in all evaluated strains, with variation among them, but no phosphate solubilization was detected. Phytohormones excreted to the culture medium ranged in the following concentrations: 2.2–18.2 μg IAA mL⁻¹, 0.3–0.7 μg GA₃ mL⁻¹, and 0.5–1.2 μg Z mL⁻¹. Seed inoculations with further selected Azotobacter strains and treatments with their cell‐free cultures increased the number of seminal roots and root hairs in wheat seedlings. This latter effect was mimicked by treatments with IAA‐pure solutions, but it was not related to bacterial root colonization. Our survey constitutes a first approach to the knowledge of Azotobacter species inhabiting Argentinean soils in three contrasting geographical regions. Moreover, this phenotypic characterization constitutes an important contribution to the selection of Azotobacter strains for biofertilizer formulations.

Photosynthetic bradyrhizobia are nitrogen-fixing symbionts colonizing the stem and roots of some leguminous plants like Aeschynomene. The effect of oxygen and light on the formation of the photosynthetic apparatus of Bradyrhizobium sp. C7T1 strain is described here. Oxygen is required for growth, but at high concentration inhibits the synthesis of bacteriochlorophyll (BChl) and of the photosynthetic apparatus. However, we show that in vitro, aerobic photosynthetic electron transport occurred leading to ADP photophosphorylation. The expression of the photosynthetic apparatus was regulated by oxygen in a manner which did not agree with earlier results in other photosynthetic bradyrhizobia since BChl accumulation was the highest under microaerobic conditions. This strain produces photosynthetic pigments when grown under cyclic illumination or darkness. However, under continuous white light illumination, a Northern blot analysis of the puf operon showed that, the expression of the photosynthetic genes of the antenna was considerable. Under latter conditions BChl accumulation in the cells was dependent on the oxygen concentration. It was not detectable at high oxygen tensions but became accumulated under low oxygen (microaerobiosis). It is known that in photosynthetic bradyrhizobia bacteriophytochrome photoreceptor (BphP) partially controls the synthesis of the photosystem in response to light. In C7T1 strain far-red light illumination did not stimulate the synthesis of the photosynthetic apparatus suggesting the presence of a non-functional BphP-mediated light regulatory mechanism.