Catalyzes a salvage reaction resulting in the formation of AMP, that is energically less costly than de novo synthesis. Contributes primarily to the recycling of adenine into adenylate nucleotides, but is also involved in the inactivation of cytokinins by phosphoribosylation. Catalyzes the conversion of cytokinins from free bases (active form) to the corresponding nucleotides (inactive form). AMP + diphosphate = 5-phospho-alpha-D-ribose 1-diphosphate + adenine Optimum pH is 8.8. Purine metabolism; AMP biosynthesis via salvage pathway; AMP from adenine: step 1/1. Homodimer. Male sterility due to pollen abortion after meiosis. Accumulation of free bases of cytokinins and enhanced resistance to exogenous cytokinins. Plants with reduced activity of APT1 and increased levels of cellular adenine show enhanced stress tolerance and improve growth, leading to increases in plant biomass. Belongs to the purine/pyrimidine phosphoribosyltransferase family.