Globoids in roots accumulate phosphorus to extremely high concentrations following a sudden increase in phosphorus availability

Crops with enhanced uptake of fertiliser-phosphorus (P) would confer environmental benefits. We examined how roots of P-sufficient 6-week-old soil-grown plants and 2-week-old seedlings in solution culture of Trifolium subterraneum stored P following P addition. We expected cell vacuoles to store excess P. After 1 week, microanalysis showed cell vacuole [P] remained low ( 3000 mmol kg–1), potassium, magnesium, sodium and sulfur. Globoid [P] in seedlings increased three-fold post-P addition and NMR spectroscopy showed roots accumulated inorganic P. Seedling root differentially expressed genes were measured by RNAseq and mapped (T. subterraneum reference assembly). Gene ontology analysis showed enrichment in transmembrane transport and localisation post-P addition, but no change in expression of PHT5 vacuolar P transporter homologs. Amongst the most upregulated genes 4 h post-P addition was TSub_g9430.t1 which is similar to known plastid envelope P transporters (PHT4;1, PHT4;4). TEM revealed large plastids. Sub-cellular P accumulation in specialised structures is a previously unsuspected mechanism for P homeostasis under high P availability and aids seedling roots high in P from seed reserves to safely store fertiliser-P.