AUTHOR=Brignoli Damián , Frickel-Critto Emilia , Sandobal Tamara J. , Balda Rocío S. , Castells Cecilia B. , Mongiardini Elías J. , Pérez-Giménez Julieta , Lodeiro Aníbal R. 

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

JOURNAL=Frontiers in Agronomy

VOLUME=6

YEAR=2024

URL=https://www.frontiersin.org/journals/agronomy/articles/10.3389/fagro.2024.1336433

DOI=10.3389/fagro.2024.1336433

ISSN=2673-3218

ABSTRACT=<p>Greenhouse gas emissions, such as N<sub>2</sub>O from excessive N-fertilizer use, are of concern. Symbiotic N<sub>2</sub>-fixation by pulses such as soybean might mitigate this issue, for which inoculants carrying locally adapted <italic>Bradyrhizobium</italic> strains are recommended. In the frame of this goal, enhancing the quality control of these inoculants is required on two key aspects: determining the presence of <italic>nosZ</italic>, for the strains being able to reduce N<sub>2</sub>O, and assessing N<sub>2</sub>-fixation potential. Previously it was demonstrated that, in soybean leaves, N-contents are well correlated with chlorophyll contents. However, no such correlations were made with either N obtained from N<sub>2</sub>-fixation or with nodules mass, which is an indicator of nodular activity. Here we aimed to leverage the correlation between N and chlorophyll levels to develop a simple and non-destructive laboratory method to be applied in quality control of inoculants, able to assess the N<sub>2</sub>-fixing capacity of rhizobial strains. To establish such correlations, we cultivated soybeans in vermiculite with N-free nutrient solution, and inoculated them with various <italic>Bradyrhizobium</italic> field isolates that displayed a range of symbiotic N<sub>2</sub>-fixing capacities. Subsequently, we measured chlorophyll with a portable chlorophyllometer, and correlated these measures with symbiotic parameters. Moreover, we tested for the presence of nosZ by PCR. We observed significant correlations between chlorophyll and shoot nitrogen obtained from N<sub>2</sub>-fixation and, in addition, we corroborated that chlorophyll contents were significantly correlated also with nodules mass. Two <italic>B. diazoefficiens</italic> strains stood out and possessed <italic>nosZ</italic>. In contrast, <italic>B. elkanii</italic> and <italic>B. japonicum</italic> isolates displayed lower chlorophyll and symbiotic performance, and lacked <italic>nosZ</italic>. Our findings highlight the potential of measuring chlorophyll contents and testing for the presence of <italic>nosZ</italic> as two straightforward techniques that may enhance laboratory tests for quality control, enabling selection of superior and safe locally isolated strains for soybean inoculants without increased production costs.</p>