HALF FILLED promotes reproductive tract development and fertilization efficiency in Arabidopsis thaliana.

Successful fertilization in angiosperms requires the growth of pollen tubes through the female reproductive tract as they seek out unfertilized ovules. In Arabidopsis, the reproductive tract begins with the stigma, where pollen grains initially adhere, and extends through the transmitting tract of the style and ovary. In wild-type plants, cells within the transmitting tract produce a rich extracellular matrix and undergo programmed cell death to facilitate pollen movement. Here, we show that the HAF, BEE1 and BEE3 genes encode closely related bHLH transcription factors that act redundantly to specify reproductive tract tissues. These three genes are expressed in distinct but overlapping patterns within the reproductive tract, and in haf bee1 bee3 triple mutants extracellular matrix formation and cell death fail to occur within the transmitting tract. We used a minimal pollination assay to show that HAF is necessary and sufficient to promote fertilization efficiency. Our studies further show that HAF expression depends on the NTT gene and on an auxin signaling pathway mediated by the ARF6, ARF8 and HEC genes.