Lateral-root-order dependent rhizosphere metabolite profiles reveal cultivar-specific responses to phosphorus deficiency in soybean

Soybean adapts to phosphorus (P) deficiency by changing the root system architecture, especially via lateral root development that improves nutrient acquisition. While morphological responses have been documented, no study has evaluated the rhizosphere metabolite profiles of various lateral root orders under P deficiency. This study aimed to examine the metabolite responses to P deficiency in first-, second-, and third-order lateral roots of two soybean cultivars varying in low P tolerance. The low-P-tolerant cultivar Ippon and the low-P-sensitive cultivar Tokachi were grown in Andosol under − P (0 g P kg-1) and + P (0.44 g P kg-1) treatments. Rhizosphere soil from the first (1L), second (2L), and third (3L) order lateral roots were collected at 16 and 23 DAT and analyzed using CE-TOF MS. Ippon sustained root dry weight and P uptake under − P, whereas Tokachi revealed substantial reductions. The number of rhizosphere metabolites detected in Ippon’s 1L, 2L, and 3L was 77, 72, and 79 at 16 DAT, and 58, 80, and 55 at 23 DAT, respectively; in Tokachi, the numbers were 112, 66, and 71 at 16 DAT, and 83, 90, and 81 at 23 DAT. Metabolite profiling showed spatial differentiation across lateral root orders. Several unique metabolites revealed consistent accumulation from 1 to 3L in Ippon, indicating spatially regulated exudation. Contrastingly, Tokachi showed a less structured and cultivar-specific metabolic response. Spatially distinct metabolite exudation along lateral root orders underlies cultivar-specific responses to P deficiency in soybean, emphasizing the role of root developmental context in rhizosphere processes and P acquisition.