Quality control of Bradyrhizobium inoculant strains: detection of nosZ and correlation of symbiotic efficiency with soybean leaf chlorophyll levels

BRIGNOLI, DAMIÁN; FRICKEL-CRITTO, EMILIA; SANDOBAL, TAMARA J.; BALDA, ROCÍO S.; CASTELLS, CECILIA B.; MONGIARDINI, ELÍAS J.; PÉREZ-GIMÉNEZ, JULIETA; LODEIRO, ANÍBAL R.

Frontiers in Agronomy

Frontiers in agronomy

Año: 2024 vol. 6

Greenhouse gas emissions, such as N2O from excessive N-fertilizer use, areof concern. Symbiotic N2-fixation by pulses such as soybean might mitigatethis issue, for which inoculants carrying locally adapted Bradyrhizobiumstrains are recommended. In the frame of this goal, enhancing the qualitycontrol of these inoculants is required on two key aspects: determining thepresence of nosZ, for the strains being able to reduce N2O, and assessing N2-fixation potential. Previously it was demonstrated that, in soybean leaves, Ncontents are well correlated with chlorophyll contents. However, no suchcorrelations were made with either N obtained from N2-fixation or withnodules mass, which is an indicator of nodular activity. Here we aimed toleverage the correlation between N and chlorophyll levels to develop asimple and non-destructive laboratory method to be applied in qualitycontrol of inoculants, able to assess the N2-fixing capacity of rhizobialstrains. To establish such correlations, we cultivated soybeans invermiculite with N-free nutrient solution, and inoculated them with variousBradyrhizobium field isolates that displayed a range of symbiotic N2-fixingcapacities. Subsequently, we measured chlorophyll with a portablechlorophyllometer, and correlated these measures with symbioticparameters. Moreover, we tested for the presence of nosZ by PCR. Weobserved significant correlations between chlorophyll and shoot nitrogenobtained from N2-fixation and, in addition, we corroborated that chlorophyllcontents were significantly correlated also with nodules mass. Two B.diazoefficiens strains stood out and possessed nosZ. In contrast, B. elkaniiand B. japonicum isolates displayed lower chlorophyll and symbiotic performance, and lacked nosZ. Our findings highlight the potential ofmeasuring chlorophyll contents and testing for the presence of nosZ astwo straightforward techniques that may enhance laboratory tests for qualitycontrol, enabling selection of superior and safe locally isolated strains forsoybean inoculants without increased production costs.