Drought-induced changes in ectomycorrhizal association modify the chemical construct of fine roots.

Abstract:

Fine roots (diameter ≤ 2mm) adapt to their soil environment through changes in root morphology and physiology, which is relatively well studied. However, the associated changes in root chemistry are less known. The chemistry of roots also regulates their decomposition that recycles nutrients in senesced tissues and facilitates ecosystem productivity. Thus, a poor understanding of root chemical construction compromises our ability to forecast plant adaptation to environmental stress and their potential for soil C sequestration. Understanding the chemistry of fine roots is critical particularly, in the face of climate change as fine roots contribute to a significant proportion of soil carbon. Here we introduce and explore the novel concept of chemical plasticity, where the chemical construct of fine roots is shaped by their immediate biotic/abiotic soil environments.

We subjected Quercus alba (white oak) and Quercus rubra (red oak) seedlings to controlled drought and ambient soil moisture conditions and explored the diversity and dynamics of small molecules and oligomers in fine root orders as a function of stress treatment and the associated ectomycorrhizal colonization.

Drought increased the ectomycorrhizal association in first and second order roots of Q. alba, which was more adapted to drought than Q. rubra. Our results revealed that the chemical construct of fine roots varied across different root orders which were evident in lignins, tannins and bound phenols across the two species. The lignin content across root orders did not vary with drought, but the overall integrity of the 1+2 root orders with higher nitrogen was maintained by the relative abundance of lignin monomers that result in highly networked lignin than linear lignin. Similarly, despite their similar total content of tannins, Q. alba, the species that was adapted to drought had a lower content of ellagitannins with high antimicrobial activity, an attribute that would facilitate mycorrhizal associations. The above chemical plasticity was evident in the 1+2 root orders but not in the higher-order fine roots. This uniqueness in fine root chemical construct unambiguously points to a sophisticated strategy in plants that maximize the resource uptake functions while concurrently adapting to unfavorable biotic and abiotic soil conditions.