Belkis Morales-Mena

Rice is a priority crop in Cuba, and bacterial inoculants could help sustain productivity under reduced mineral fertilizer inputs. However, evidence describing the main stages involved in developing these bioproducts in the country remains limited. This study aimed to evaluate the suitability of a bacterial strain as the active microbial ingredient of a prototype rice inoculant for application under reduced nitrogen fertilization. Three promising bacterial strains were evaluated for their effects on rice yield components under field conditions at 60% of the recommended N fertilization rate. The taxonomic affiliation of the selected strain was assessed by multilocus phylogenetic analysis of the recA, rpoB, and glnII genes. The microbiological stability of inoculant formulations supplemented with CaCl₂ or sodium alginate was evaluated during storage at 4 °C and room temperature for 231 days. Inoculant production was scaled up to working volumes of 30 and 300 L, and the resulting prototype was evaluated in a second field experiment at 30% of the recommended N fertilization rate. Among the inoculated treatments, strain Rpd16 produced the highest numbers of panicles per square meter and total and filled grains per panicle. This strain was selected as the active microbial ingredient and was phylogenetically affiliated with the Agrobacterium pusense lineage. After 231 days at 4 °C, all formulations contained viable cell concentrations above 10⁸ CFU mL⁻¹. Compared with the uninoculated treatment receiving the same N fertilization rate, seed bio-priming with the prototype inoculant, provisionally named Azofert-A, was associated with higher tiller number (5.6%), plant height (10.7%), flag leaf length (9.9%), flag leaf dry weight (41.2%), and grain yield (20.4%), providing preliminary evidence of its agronomic potential under reduced N fertilization. This study integrates the main stages involved in developing a prototype bacterial inoculant for rice under Cuban conditions.

Plant growth-promoting bacteria (PGPB)-based inoculants represent a promising alternative to mineral fertilizers. However, their application may be limited by constraints associated with the use of living microorganisms, particularly under field conditions. The objective of this study was to select a bacterial strain and a suitable carrier for the inoculation of rice cv. Selección 1. The effect of inoculation with 3 Rhizobium spp. strains on rice growth was evaluated under greenhouse conditions, enabling selection of the most promising strain. This strain was further characterized based on its motility, production of indolic compounds in the presence of tryptophan, and antagonistic activity against 3 rice phytopathogenic fungi. In addition, the effects of culture media based on aqueous extracts of soybean and rice seeds on bacterial growth and chemotactic response were evaluated, along with the shelf-life stability of the resulting inoculant formulations. Rhizobium sp. strain 5P1 significantly increased plant height (33%), root length (21%), shoot dry weight (30%), and root dry weight (17%) of rice cultivar Selección 1 under greenhouse conditions. The strain exhibited motility predominantly via swarming and twitching, produced indolic compounds (23.9 ± 0.8 µg mL⁻¹), and showed antagonistic activity against Magnaporthe oryzae (32.5% radial growth inhibition at 16 days), Curvularia oryzae (20.0%), and Bipolaris oryzae (6.6%) under in vitro conditions. Culture media based on molasses and soybean or rice seed extracts did not enhance bacterial growth relative to the conventional medium; however, they elicited a stronger chemotactic response. Formulations supplemented with sodium alginate and carboxymethylcellulose maintained cell viability above 10⁸ CFU mL⁻¹ after 105 days of storage at 4 °C. These findings propose Rhizobium sp. strain 5P1 and a molasses-based carrier formulation as strong candidates for the development of an effective bioinoculant for rice in Ferric Gleysol soils.