FIGURE 7.

Proposed model for ABAP1 role in the differentiation of male and female gametophytes. ABAP1, associated with transcription factors partners, might sense environmental and endogenous signals, triggering changes in gene expression patterns that will regulate male and female gametophytes differentiation. The molecular mechanism operating in the male gametophyte is similar to those previously described in leaves. In the first asymmetric cell division after meiosis, ABAP1 associates with TCP16, represses CDT1b transcription, controlling CDT1b homoeostasis. Lower levels of CDT1b might affect pre-RC functioning, limiting DNA replication. A direct association of ABAP1 with pre-RC members is still not determined. However, as described in leaves, it could also affect pre-RC assembly and DNA replication in the male gametophyte. In this way ABAP1 would regulate the timing of cell cycle progression at G1 to S. The mechanism of action in the female gametophyte involves association with another partner, ADAP, during embryo sac maturation. ABAP1-ADAP also represses gene transcription of EDA24-like that is necessary for polar nuclei fusion. Thus, ABAP1 might regulate the timing of polar nuclei fusion to form the central cell, that is crucial in pollen tube guidance. The involvement of DNA replication in the mechanism that regulates female differentiation is still not determined.