HISTONE DEACETYLASE 19 REGULATESSHOOT MERISTEMLESSEXPRESSION IN THE CARPEL MARGIN MERISTEM CONTRIBUTING TO OVULE NUMBER DETERMINATION AND TRANSMITTING TRACT DIFFERENTIATION

Abstract The gynoecium is critical for the reproduction of flowering species as it contains the ovules and the tissues required for pollen germination and guidance. These tissues are collectively known as the reproductive tract (ReT) and comprise stigma, style and transmitting tract (TT). The ovules and the ReT originate from a meristem within the pistil named carpel margin meristem (CMM). SHOOT MERISTEMLESS (STM) is a key transcription factor required for meristem formation and maintenance. In all above-ground meristems, including the CMM, STM has to be locally downregulated to allow proper organ differentiation. However, how this downregulation is achieved in the CMM is unknown. In this work, we have studied HISTONE DEACETYLASE 19 (HDA19) role in ovule and ReT differentiation, based on the observation that hda19-3 mutant displays reduced ovule number and fails to properly differentiate the TT. Fluorescence activated cell sorting (FACS) coupled with RNA-seq revealed that in the CMM of hda19-3 mutant, genes promoting organ development are downregulated while meristematic markers, including STM , are upregulated. We found that HDA19 is fundamental to downregulate STM in the CMM, allowing ovule formation and TT differentiation. STM is ectopically expressed in hda19-3 at intermediate stages of pistil development, and its downregulation by RNA interference alleviated hda19 -3 phenotypic defects. Furthermore, chromatin immunoprecipitation assays indicated that STM is a direct target of HDA19 during pistil development and that SEEDSTICK ( STK) is required for the histone acetylation-mediated regulation of STM . Our results have led to the identification of the factors required for STM silencing in the gynoecium allowing organogenesis and tissue differentiation from the CMM.