Hypotheses for PSV formation tested in this work. A, PSV form de novo, briefly coexisting with the EV to eventually become the dominant structure in mature seeds. During the transition, PSV-specific tonoplast markers (green outline) appear alongside the EV tonoplast markers (red outline). B, PSV arise through reprogramming of the EV by the accumulation of seed storage proteins in the EV lumen (green pattern fill). PSV and EV tonoplast markers coexist (yellow outline) while the EV is converted to a PSV.