C4 cereal and biofuel crop microbiomes.

Microbiomes provide multiple life-support functions for plants, including nutrient acquisition and tolerance to abiotic and biotic stresses. Considering the importance of C4 cereal and biofuel crops for food security under climate change conditions, more attention has been given recently to C4 plant microbiome assembly and functions. Here, we review the current status of C4 cereal and biofuel crop microbiome research with a focus on beneficial microbial traits for crop growth and health. We highlight the importance of environmental factors and plant genetics in C4 crop microbiome assembly and pinpoint current knowledge gaps. Finally, we discuss the potential of foxtail millet as a C4 model species and outline future perspectives of C4 plant microbiome research.

Coupling Root Diameter With Rooting Depth to Reveal the Heterogeneous Assembly of Root-Associated Bacterial Communities in Soybean.

Root diameter and rooting depth lead to morphological and architectural heterogeneity of plant roots; however, little is known about their effects on root-associated microbial communities. Bacterial community assembly was explored across 156 samples from three rhizocompartments (the rhizosphere, rhizoplane, and endosphere) for different diameters (0.0-0.5 mm, 0.5-1.0 mm, 1.0-2.0 mm, and>2.0 mm) and depths (0-5 cm, 5-10 cm, 10-15 cm, and 15-20 cm) of soybean [Glycine max (L.) Merrill] root systems. The microbial communities of all samples were analyzed using amplicon sequencing of bacterial 16S rRNA genes. The results showed that root diameter significantly affected the rhizosphere and endosphere bacterial communities, while rooting depth significantly influenced the rhizosphere and rhizoplane bacterial communities. The bacterial alpha diversity decreased with increasing root diameter in all three rhizocompartments, and the diversity increased with increasing rooting depth only in the rhizoplane. Clearly, the hierarchical enrichment process of the bacterial community showed a change from the rhizosphere to the rhizoplane to the endosphere, and the bacterial enrichment was higher in thinner or deeper roots (except for the roots at a depth of 15-20 cm). Network analysis indicated that thinner or deeper roots led to higher bacterial network complexity. The core and keystone taxa associated with the specific root diameter class and rooting depth class harbored specific adaptation or selection strategies. Root diameter and rooting depth together affected the root-associated bacterial assembly and network complexity in the root system. Linking root traits to microbiota may enhance our understanding of plant root-microbe interactions and their role in developing environmentally resilient root ecosystems.

Effect of Root Diameter on the Selection and Network Interactions of Root-Associated Bacterial Microbiomes in Robinia pseudoacacia L.

The high plasticity of root morphology, physiology, and function influences root-associated microbiomes. However, the variation in root-associated microbiome diversity and structures in response to root diameter at different root depths remains poorly understood. Here, we selected black locust (Robinia pseudoacacia L.) as a model plant to investigate the selection and network interactions of rhizospheric and root endophytic bacterial microbiomes associated with roots of different diameters (1, 1-2, and > 2 mm) among root depths of 0-100 cm via the Illumina sequencing of the 16S rRNA gene. The results showed that the alpha diversity of the root-associated bacterial communities decreased with increasing root diameters among different root depths; fewer orders with higher relative abundance, especially in the endosphere, were enriched in association with coarse roots (> 2 mm) than fine roots among root depths. Furthermore, the variation in the enriched bacterial orders associated with different root diameters was explained by bulk soil properties. Higher co-occurrence network complexity and stability emerged in the rhizosphere microbiomes of fine roots than those of coarse roots, in contrast to the situation in the endosphere microbiomes. In particular, the endosphere of roots with a diameter of 1-2 mm exhibited the lowest network complexity and stability and a high proportion of keystone taxa (e.g., Cytophagia, Flavobacteriia, Sphingobacteriia, ß-Proteobacteria, and γ-Proteobacteria), suggesting a keystone taxon-reliant strategy in this transitional stage. In summary, this study indicated that root diameter at different root depths differentially affects rhizospheric and endophytic bacterial communities, which implies a close relationship between the bacterial microbiome, root function, and soil properties.