Syringa oblata maintains rhizosphere r-strategists and microbial functions by regulating exudates in root tip zone under drought

Abstract

How plant regulate exudates in different root parts under drought stress, and their implications for microbial drought resistance in the rhizosphere, remain inadequately understood. We focused on the native shrub Syringa oblata in Loess Plateau, and conducted a 3-year experimental drought experiment to determine the changes in the root exudates and rhizosphere microbiome using microbiome and metabolomic analyses. (1) Root tip zone exhibited higher regulatory capacity of exudation than mature root zone under drought, with the increasing exudation of specific carbohydrates, organic acids, amino acids, fatty acids, and secondary metabolites induced by changes in soil nutrients availability. (2) In bulk soil, drought stress resulted in noticeable shift in microbial communities toward K-strategy, and a reduction of symbiotic fungi, and bacterial nitrogen (N) cycling functions. However, the variation in exudation composition in the rhizosphere of root tip zone contributed to the enrichment of symbiotic fungi and bacterial N cycling functions under drought; the enhanced C flux and chemodiversity of exudates sustained r-strategy and stability of microbial community, respectively. (3) In mature root zone, the exudation composition varied slightly, whereas the C-exudation substantially increased from 26.6 to 85.8 μg C g⁻¹ root hr⁻¹ with decreasing soil water content and contributed to the enrichment of fungal r-strategy species under drought. Our results contribute to the understanding of plant-microbial interactions under drought at a finer scale, and emphasized the significant role of root tip zone in enhancing potential rhizosphere C and N dynamics and plant drought resistance.