SEEDSTICK is a Master Regulator of Development and Metabolism in the Arabidopsis Seed Coat

Abstract

The role of secondary metabolites in the determination of cell identity has been an area of particular interest over recent years, and studies strongly indicate a connection between cell fate and the regulation of enzymes involved in secondary metabolism. In Arabidopsis thaliana, the maternally derived seed coat plays pivotal roles in both the protection of the developing embryo and the first steps of germination. In this regard, a characteristic feature of seed coat development is the accumulation of proanthocyanidins (PAs - a class of phenylpropanoid metabolites) in the innermost layer of the seed coat. Our genome-wide transcriptomic analysis suggests that the ovule identity factor SEEDSTICK (STK) is involved in the regulation of several metabolic processes, providing a strong basis for a connection between cell fate determination, development and metabolism. Using phenotypic, genetic, biochemical and transcriptomic approaches, we have focused specifically on the role of STK in PA biosynthesis. Our results indicate that STK exerts its effect by direct regulation of the gene encoding BANYULS/ ANTHOCYANIDIN REDUCTASE ( BAN/ ANR), which converts anthocyanidins into their corresponding 2,3-cis-flavan-3-ols. Our study also demonstrates that the levels of H3K9ac chromatin modification directly correlate with the active state of BAN in an STK-dependent way. This is consistent with the idea that MADS-domain proteins control the expression of their target genes through the modification of chromatin states. STK might thus recruit or regulate histone modifying factors to control their activity. In addition, we show that STK is able to regulate other BAN regulators. Our study demonstrates for the first time how a floral homeotic gene controls tissue identity through the regulation of a wide range of processes including the accumulation of secondary metabolites.

Author Summary

Plant secondary metabolites accumulate in seeds to protect the developing embryo. Using an RNA sequencing approach in conjunction with enrichment analyses we identified the homeotic MADS-domain gene SEEDSTICK ( STK) as a regulator of metabolic processes during seed development. We analyzed the role of STK as a key regulator of the production of proanthocyanidins, compounds which are important for the pigmentation of the seed. STK directly regulates a network of metabolic genes, and is also implicated in changes occurring in the chromatin landscape. Our work demonstrates that a key homeotic transcription factor not only determines the identity of ovules but also controls metabolic processes that occur subsequent to the initial identity determination process, thus suggesting a link between identity determination and cell-specific (metabolic) processes.