Moderate Elevation of Ambient Temperature Alleviates Growth Reduction Caused by Phosphorus Deficiency in Lotus japonicus Under Hydroponic Conditions

High-temperature stress driven by global warming and phosphorus (P) deficiency resulting from the depletion of phosphate rock reserves pose serious threats to modern agriculture. However, our understanding of the combined effects of these stresses on plants remains limited. In this study, we employed a hydroponic culture system to investigate the physiological and transcriptomic responses of the model legume Lotus japonicus to simultaneous moderate high-temperature and low-P conditions. Both elevated temperature and low-P individually reduced shoot biomass. Under low-P conditions, shoot biomass remained significantly lower than in P sufficient plants. However, the relative reduction was smaller under elevated temperature than under favorable temperature. L. japonicus exhibited enhanced root-secreted acid phosphatase activity in under low-P alone and displayed an increased organic acid (e.g. malate and citrate) exudation rates under elevated temperature, irrespective of the P supply. Root transcriptome analysis identified up-regulation of the PEPC1 gene, encoding phosphoenolpyruvate carboxylase, suggesting its potential involvement in organic acid root exudation. Additionally, elevated temperatures promoted metal ion translocation and uptake, increased anthocyanin accumulation, and enhanced carbohydrate transport in shoots. Transcriptomic analysis further revealed that low-P stress altered heat shock protein expression, potentially associated with reduced ATP availability. These findings demonstrate that L. japonicus employs distinct and overlapping strategies to adapt to the combined stresses of elevated temperatures and P deficiency. These strategies include the adjustment of root exudation patterns, nutrient transport, and stress-responsive gene expression.