Apoplast-localized beta-glucosidase facilitates the root-to-soil delivery of isoflavones in soybean

Plant specialized metabolites (PSMs) are often stored as glycosides in cells and secreted from roots with some chemical modifications. Although isoflavones function as a symbiotic signal with rhizobia and affect the composition of the soybean rhizosphere microbiome, the underlying secretion mechanism from roots is poorly understood. In addition to transporter-mediated secretion, a route through the hydrolysis of isoflavone glycosides in the apoplast by an isoflavone conjugate-hydrolyzing beta-glucosidase (ICHG) is proposed but not yet clarified. We isolated two independent mutants defective in ICHG activity from a soybean high-density mutant library. These mutants were backcrossed to wild-type and their progenies were employed for our analyses. We cultivated homozygous ichg mutants and wild alleles under both nitrogen-sufficient and nitrogen-deficient conditions and analyzed the root transcriptome, isoflavone contents, and bacterial communities. Although the difference between ichg mutants and WTs in isoflavone aglycone contents in roots become more apparent under nitrogen-deficient condition, where biosynthetic genes for isoflavones were up-regulated, we observed no significant effects of the ichg mutation in the transcriptome and microbiome in both conditions.