In flowering plants, sperm cells are delivered to the embryo sac by a pollen tube guided by female signals. Both the gametic and synergid cells contribute to pollen tube attraction. Synergids secrete peptide signals that lure the tube, while the role of the gametic cells is unknown. Previously, we showed that CENTRAL CELL GUIDANCE (CCG) is essential for pollen tube attraction in Arabidopsis thaliana, but the molecular mechanism is unclear. Here, we identified CCG BINDING PROTEIN1 (CBP1) and demonstrated that it interacts with CCG, Mediator subunits, RNA polymerase II (Pol II), and central cell-specific AGAMOUS-like transcription factors. In addition, CCG interacts with TATA-box Binding Protein 1 and Pol II as a TFIIB-like transcription factor. CBP1-knockdown ovules are defective in pollen tube attraction. Expression profiling revealed that cysteine-rich peptide (CRP) transcripts were downregulated in ccg ovules. CCG and CBP1 coregulate a subset of CRPs in the central cell and the synergids, including the attractant LURE1. CBP1 is extensively expressed in multiple vegetative tissues and specifically in the central cell in reproductive growth. We propose that CBP1, via interaction with CCG and the Mediator complex, connects transcription factors and the Pol II machinery to regulate pollen tube attraction.

Significance: Double fertilization is a key innovation of flowering plants. The central cell is the second female gamete, and its fertilization gives rise to the endosperm, the nurse of a seed. However, how this cell is specified and evolved in angiosperm remains unknown. This study reveals a transcriptional repression mechanism to specify the central cell in Arabidopsis and provides insight into the origination of this mechanism in Brassicaceae.

The endosperm in cereal grains is instrumental in determining grain yield and seed quality, as it controls starch and seed storage protein (SSP) production. In this study, we identified a specific nuclear factor-Y (NF-Y) trimeric complex in wheat (Triticum aestivum L.), consisting of TaNF-YA3-D, TaNF-YB7-B, and TaNF-YC6-B, and exhibiting robust expression within the endosperm during grain filling. Knockdown of either TaNF-YA3 or TaNF-YC6 led to reduced starch but increased gluten protein levels. TaNF-Y indirectly boosted starch biosynthesis genes by repressing TaNAC019, a repressor of cytosolic small ADP-glucose pyrophosphorylase 1a (TacAGPS1a), sucrose synthase 2 (TaSuS2), and other genes involved in starch biosynthesis. Conversely, TaNF-Y directly inhibited the expression of Gliadin-γ-700 (TaGli-γ-700) and low molecular weight-400 (TaLMW-400). Furthermore, TaNF-Y components interacted with SWINGER (TaSWN), the histone methyltransferase subunit of Polycomb repressive complex 2 (PRC2), to repress TaNAC019, TaGli-γ-700, and TaLMW-400 expression through trimethylation of histone H3 at lysine 27 (H3K27me3) modification. Notably, weak mutation of FERTILIZATION INDEPENDENT ENDOSPERM (TaFIE), a core PRC2 subunit, reduced starch but elevated gliadin and LMW-GS contents. Intriguingly, sequence variation within the TaNF-YB7-B coding region was linked to differences in starch and SSP content. Distinct TaNF-YB7-B haplotypes affect its interaction with TaSWN-B, influencing the repression of targets like TaNAC019 and TaGli-γ-700. Our findings illuminate the intricate molecular mechanisms governing TaNF-Y–PRC2-mediated epigenetic regulation for wheat endosperm development. Manipulating the TaNF-Y complex holds potential for optimizing grain yield and enhancing grain quality.