LaALMT1 mediates malate release from phosphorus-deficient white lupin root tips and metal root to shoot translocation

Under phosphorus (P) deficiency, Lupinus albus (white lupin) releases large amounts of organic acid anions from specialized root structures, so-called cluster or proteoid roots, to mobilize and acquire sparingly soluble phosphates from a restricted soil volume. The molecular mechanisms underlying this release and its regulation are, however, poorly understood. Here, we identified a gene belonging to the aluminium (Al)-activated malate transporter (ALMT) family that specifically contributes to malate, but not citrate release. This gene, LaALMT1, was most prominently expressed in the root apices under P deficiency, including those of cluster roots and was also detected in the root stele. Contrary to several ALMT homologs in other species, the expression was not stimulated, but moderately repressed by Al. Aluminium-independent malate currents were recorded from the plasma membrane localized LaALMT1 expressed in Xenopus oocytes. In composite lupins with transgenic roots, LaALMT1 was efficiently mutated by CRISPR-Cas9, leading to diminished malate efflux and lower xylem sap malate concentrations. When grown in an alkaline P-deficient soil, mutant shoot phosphate concentrations were similar, but iron and potassium concentrations were diminished in old leaves, suggesting a role for ALMT1 in metal root to shoot translocation, a function that was also supported by growth in hydroponics.