A nuclear transcription factor related to plastid ribosome biogenesis is synthesised early during germination and priming.

Germination is a short developmental process during which many new proteins are synthesised. We have chosen the previously characterised RPL21 gene encoding plastid-localised ribosomal proteins RPL21, to analyse activation of gene expression during germination. Transcription activation occurs at the P1 promoter during the first hours following imbibition and coincides with the appearance of a trans-acting factor that we named AUBE1. AUBE1 binds specifically to a short DNA fragment that encompasses the P1 promoter of the RPL21 gene. The protein has a size of 28-30 kDa and is transiently expressed during the early phase of germination. Using the properties of primed seeds we show that AUBE1 is maintained after desiccation of primed seeds. We conclude that AUBE1 can be used as a marker in spinach seed priming.