Light and Dark Conditions Reveal Distinct Components of the Phosphate Starvation Response in Soybean Roots

Phosphate (P_i) deficiency causes broad transcriptional changes that enhance P_i uptake and utilization. Because sucrose transport from shoot to root mediates long-distance P_i-deficiency signaling, we asked whether soybean can still respond to –P when sucrose supply is restricted. To limit sucrose availability, we exposed hydroponically grown soybean plants to +P or –P conditions for 30 h under light or darkness, with darkness suppressing photosynthetic carbon production. Root transcriptomes were profiled using Oxford Nanopore cDNA sequencing. Across four biological replicates per treatment, ~90 million reads were generated, with >90% mapping to ~25,300 expressed genes (~54% of annotated coding genes). Principal component analysis showed that light accounted for most of the variance, while P_i status contributed a smaller component. Under light, P_i deficiency activated a strong transcriptional program, including canonical phos-phate-starvation markers such as PHT1, PHO1 and PTEN2α. In darkness, however, only a small number of genes were induced, including a MYB-domain transcription factor, and none overlapped with the light-responsive set. Together, these findings suggest that light is required for broad transcriptional responses to P_i deficiency, whereas a local Pi-responsive program becomes detectable only when systemic sucrose signaling is suppressed.