Several plant species adapted to low-phosphorus (P) conditions develop cluster roots, specialized structures that release organic acids and acid phosphatases (APases) to increase inorganic phosphate (Pi) availability. White lupin ( Lupinus albus L.) is used as a model for studying cluster root function. Using a positron-emitting tracer imaging system (PETIS), we previously observed spot-like carbon (C) secretion patterns in the cluster roots of white lupin, amounts of which differed widely among spots, suggesting variation in secretion activity among cluster roots. Here, we combined PETIS with RNA-Seq to investigate transcriptomic differences between cluster roots with varying secretion activities. We identified 564 genes positively correlated and 135 genes negatively correlated with secretion levels. Among the positively correlated genes, we found three aluminum-activated malate transporter genes and two multi-drug and toxic compound extrusion genes, likely involved respectively in malate and citrate secretion. Two APase genes encoding putative secreted enzymes were also upregulated. All Pi transporter genes except PHO1;6H were stably expressed, whereas PHO1;6H was significantly upregulated in high-C-secreting roots. We also identified seven transcription factors potentially involved in organic acid transporter and APase gene expression. Our findings provide key candidates for genes involved in Pi mobilization, offering insights into plant adaptation to P deficiency. (200/200 words)