Protein storage vacuoles originate from remodelled pre-existing vacuoles in Arabidopsis thaliana

Protein storage vacuoles (PSV) are the main repository of protein in dicotyledonous seeds, but little is known about the origins of these transient organelles. PSV are hypothesised to either arise de novo or to originate from the pre-existing embryonic vacuole (EV) during seed maturation. Here, we tested these hypotheses by studying PSV formation in Arabidopsis (Arabidopsis thaliana) embryos at different stages of seed maturation and recapitulated this process in Arabidopsis leaves reprogrammed to an embryogenic fate by inducing expression of the LEAFY COTYLEDON2 transcription factor. Confocal and immunoelectron microscopy indicated that both storage proteins and tonoplast proteins typical of PSV were delivered to the pre-existing EV in embryos or to the lytic vacuole in reprogrammed leaf cells. In addition, sectioning through embryos at several developmental stages using serial block face scanning electron microscopy revealed the 3D architecture of forming PSV. Our results indicate that the pre-existing EV is reprogrammed to become a PSV in Arabidopsis.