Identification of cis-regulatory sequences that activate transcription in the suspensor of plant embryos

After asymmetrical division of the zygote in many higher plants, the two daughter cells, the apical and basal cell, give rise to two different tissues: the embryo proper and suspensor, respectively, with distinct developmental fates. Different sets of genes become active in the apical and basal cells; however, what DNA sequences and regulatory factors control transcription in these cells are yet to be determined. The suspensor is a terminally differentiated tissue and its cells are direct clonal descendants of the basal cell; therefore, it is possible that the molecular mechanisms that control suspensor-specific transcription are directly linked to the processes that activate transcription in the basal cell.;In order to identify suspensor cis-regulatory sequences, I carried out promoter dissection experiments on the scarlet runner bean G564 gene, which is highly active in the suspensor during early embryo development. It was found that a 54-bp fragment within the G564 upstream region is sufficient for suspensor-specific transcription and contains at least three cis-regulatory sequences. In addition, the scarlet runner bean G564 suspensor cis-regulatory sequences can activate suspensor-specific transcription in tobacco and Arabidopsis, indicating conservation of the suspensor transcriptional machinery among flowering plants. What regulatory factors bind these sequences remain unknown.;Gymnosperms also generate suspensors during embryogenesis, and the conservation of the suspensor transcriptional machinery among flowering plants raised another question as to whether the suspensor transcriptional machinery is conserved among seed plants. I tested the transcriptional activity of the upstream region of the gymnosperm Loblolly pine PtNIP1;1 gene, which activates suspensor-specific transcription in the gymnosperm embryo, in the angiosperm tobacco embryo. The suspensor-specific transcription in the globular-stage tobacco embryo was observed, demonstrating that the suspensor transcriptional machinery recognizing the PtNIP1;1 cis-regulatory sequences is conserved among seed plants, which diverged over 300 million years ago. What sequences in the PtNIP1;1 upstream region are responsible for suspensor-specific transcription and what regulatory factors bind those sequences remain to be determined.