Apoplast-localized beta-glucosidase facilitates isoflavone secretion into soybean rhizosphere

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 in a field and analyzed transcriptome of leaves and roots, and isoflavone contents in leaves, roots, and rhizosphere soil. Although a principal component analysis on transcriptomes showed no distinct difference between ichg mutants and wild-types, isoflavone aglycone contents were decreased considerably in the mutant roots. The lack of ICHG activity also significantly reduced not only isoflavone aglycones but total isoflavones in the rhizosphere. Despite the decrease of isoflavones in the root and rhizosphere of ichg mutants, root and rhizosphere microbiomes and the nodulation capacity of ichg mutants did not differ from those of wild-types. Taken together, our results revealed a predominant contribution of ICHG for isoflavone supply into the soybean rhizosphere and highlight deglycosylation in root apoplast as a crucial reaction for facilitating PSM secretion into the rhizosphere.