Lotus japonicus plants of the Gifu B-129 ecotype subjected to alkaline stress improve their Fe(2+) bio-availability through inoculation with Pantoea eucalypti M91

Inoculation assays with Pantoea eucalypti M91 were performed on Lotus japonicus ecotype Gifu. Under alkaline conditions, this ecotype is characterized by the development of interveinal chlorosis of the apical leaves due to low mobilization of Fe(2+). Inoculation with P. eucalypti M91, a plant growth-promoting bacterial strain capable of producing pyoverdine-like and pyochelin-like siderophores under alkaline growth conditions, alters the root, resulting in a herringbone pattern of root branching. Additional features include improvement in Fe(2+) transport to the shoots, acidification of the hydroponic solution of the plant cultures, and an accompanying increase in the efficiency of the PSII parameters. In addition, there was an increase in the expression of the FRO1 and IRT1 genes, accompanied by a significant increase in FRO activity. Results showed that P. eucalypti M91 has a beneficial effect on the Fe acquisition machinery of Strategy I, as described for non-graminaceous monocots and dicots, suggesting its potential as an inoculant for legume crops cultivated in alkaline soils.

Phosphate-solubilization mechanism and in vitro plant growth promotion activity mediated by Pantoea eucalypti isolated from Lotus tenuis rhizosphere in the Salado River Basin (Argentina)

AIMS

To isolate and characterize phosphate-solubilizing strains from a constrained environment such as the Salado River Basin and to assess their phosphate-solubilizing mechanisms, to further selection of the most promising strains to inoculate and improve the implantation and persistence of Lotus tenuis in the most important area devoted to meat-cow production in Argentina.

METHODS AND RESULTS

Fifty isolates were obtained and through BOX-PCR analysis, 17 non-redundant strains were identified. Subsequently, they were found to be related to Pantoea, Erwinia, Pseudomonas, Rhizobium and Enterobacter genera, via 16S rRNA gene sequence analysis. This was in agreement with the clusters obtained by antibiotic resistance analysis. All isolates were tested for their phosphate-solubilizing activity and selected strains were inoculated onto L. tenuis plants. The most efficient isolate, was identified as Pantoea eucalypti, a novel species in terms of plant growth-promoting rhizobacteria.

CONCLUSIONS

The isolates obtained in this study showed a significant in vitro plant-growth promoting activity onto Lotus tenuis and the best of them solubilizes phosphate mainly via induction of the metabolism through secretion and oxidation of gluconic acid.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of these bacteria as bioinoculants, alone or in combination with nitrogen-fixing micro-organisms, could be a sustainable practice to facilitate the nutrient supply to Lotus tenuis plants and preventing negative side-effects such as eutrophication.

Comparative symbiotic performance of native rhizobia of the Flooding Pampa and strains currently used for inoculating Lotus tenuis in this region

The Flooding Pampa (FP) is the most important area for cattle breeding in Argentina. In this region, persistence and yield of typical forage legumes are strongly limited by soil salinity and alkalinity, which affect around 30% of the total area. Instead, naturalized Lotus tenuis is the main forage legume in this region. Rhizobial strains currently used for inoculating L. tenuis in the FP are exotic or native from non-saline soils of this region, their taxonomic identity being unknown. Assuming that rhizobia native from the most restrictive environments are well adapted to adverse conditions, the use of such isolates could improve the productivity of L. tenuis in the FP. Hence, the goal of this study was to evaluate the symbiotic efficiency of selected L. tenuis rhizobia native from the FP, as compared with strains currently used for field inoculation of this legume. Under non-stressing conditions, the symbiotic performance of native strains of FP exceeded those ones currently used for L. tenuis. Moreover, the symbiotic performance of the native strain ML103 was considerably high under salt stress, compared with strains currently used as inoculants. Analysis of 16S rRNA gene sequencing revealed that unclassified rhizobia currently used for field inoculation of L. tenuis and native strains grouped with the genus Mesorhizobium. As a whole, results obtained demonstrate that soils of the FP are a source of efficient and diverse rhizobia that could be used as a sustainable agronomic tool to formulate inoculants that improve forage yield of L. tenuis in this region.