Endophytic diazotrophic communities from rice roots are diverse and weakly associated with soil diazotrophic community composition and soil properties.

AIM: Bacteria that promote plant growth, such as diazotrophs, are valuable tools for achieving a more sustainable production of important non-legume crops like rice. Different strategies have been used to discover new bacteria capable of promoting plant growth. This work evaluated the contribution of soil diazotrophs to the endophytic communities established in the roots of rice seedlings cultivated on seven representative soils from Uruguay. METHODS AND RESULTS: The soils were classified into two groups according to the C and clay content. qPCR, terminal restriction fragment length polymorphism (T-RFLP), and 454-pyrosequencing of the nifH gene were used for analyzing diazotrophs in soil and plantlets' roots grown from seeds of the same genotype for 25 days under controlled conditions. A similar nifH abundance was found among the seven soils, roots, or leaves. The distribution of diazotrophs was more uneven in roots than in soils, with dominance indices significantly higher than in soils (nifH T-RFLP). Dominant soils' diazotrophs were mainly affiliated to Alphaproteobacteria and Planctomycetota. Conversely, Alpha, Beta, Gammaproteobacteria, and Bacillota were predominant in different roots, though undetectable in soils. Almost no nifH sequences were shared between soils and roots. CONCLUSIONS: Root endophytic diazotrophs comprised a broader taxonomic range of microorganisms than diazotrophs found in soils from which the plantlets were grown and showed strong colonization patterns.

Bacterial community analysis of the water surface layer from a rice-planted and an unplanted flooded field

The bacterial communities in floodwater, from a rice-planted and an unplanted field were characterized at the beginning (flooding stage) and at the end (harvest stage) of the rice cropping cycle. Most probable number estimations and plate counts of aerobic and anaerobic heterotrophic bacteria and of several metabolic bacterial groups (methanogens, sulfate-reducers, anaerobic sulfur and nonsulfur phototrophs, denitrifiers and ammonifiers) were similar in rice and unplanted floodwater at both sampling times. The analysis of denitrifiers and methanogens by fluorescent in situ hybridization revealed a shift in the phylogenetic affiliation only of the former group in the rice-planted floodwater. Terminal restriction fragment length polymorphism of 16S rRNA gene amplicons indicated that the bacterial communities of the rice-planted and unplanted soils were consistently diverse and strongly influenced by the season.