Getting to the root of root-microbe interactions

Abstract

Microbial relationships with roots influence many ecosystem functions and nutrient fluxes, including their sometimes-profound effects on plant health and productivity. Fine roots were often classified with a diameter less than 2 mm, but fine roots under that size perform distinct functional roles in the environment. Importantly, two broad functional categories of fine roots are absorptive and transportive, with absorptive fine roots acting as metabolic hotspots for root activity. In two of our recent studies, we have shown that several microbial community characteristics differ between absorptive and transportive fine roots, including composition, abundance, and function, as well as the root metabolome. This highlights a growing recognition within microbial ecology that we must consider fine-scale environmental variability, such as root physiology and morphology, when interpreting microbial patterns. In this commentary, we summarize the findings of our latest article, further speculate on some of these patterns, and suggest future studies for examining decomposition and applying cutting-edge single-cell sequencing techniques.

Keywords: Root morphology; fine roots; root functionality; root microbiome; spatial microbiome.

Figure 1.

Conceptual figure of root–microbe interactions on (A) transportive and (B) absorptive fine roots and potential questions. MF is mycorrhizal fungi. Bacteria in (A) are root surface-associated bacteria (i.e. the rhizoplane). The blue and orange haze indicates the potential zone of influence each fine root exerts on the environment, and multiple colors in (B) indicate the diversity of exudate resources released by absorptive fine roots. A scale size is provided to indicate fine roots are typically below 2 mm in diameter, but for some tree species many fine roots may be less than 1 mm or 0.5 mm in diameter. For anatomical descriptions and illustrations for absorptive and transportive fine roots, see Freschet et al. (2021), McCormack et al. (2015), and Pregitzer (2002).^, Text within the figure links to speculation throughout the commentary article, including rhizodeposition from transportive fine roots, differences in N-cycling and decomposition, and applying cutting-edge sequencing techniques to spatially link root–microbial interactions.