Patterning and polarity in ovule development

Ovules are the female reproductive structures that develop into seeds. Angiosperm ovules include one, or more commonly two, integuments that cover the nucellus and female gametophyte. In Arabidopsis, mutations in KANADI (KAN) and YABBY (YAB) transcription factor genes result in amorphous or arrested integument growth, suggesting that abaxial polarity determinants play key roles in ovule development. We show that the HD-ZIPIII genes CORONA (CNA), PHABULOSA (PHB), and PHAVOLUTA (PHV) are expressed adaxially in the inner integument during ovule development, independent of ABERRANT TESTA SHAPE (ATS), which encodes a KAN transcription factor. Loss and gain of HD-ZIPIII function in ovules affects laminar growth of both integuments in a similar manner. Additionally, loss of HD-ZIPIII activity can partially compensate for loss of ATS activity in the ats cna phb phv quadruple mutant. Based on these studies we propose a model in which a balance between the relative levels of adaxial/abaxial activities, rather than only the maintenance of boundaries of expression domains, is necessary to support laminar growth of the two integuments.;A yeast two-hybrid screen for potential protein partners of ATS identified several proteins involved in gene regulation, including ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3). Physical interaction between ATS and ETT was confirmed using bimolecular fluorescence complementation. ATS and ETT were found to share an overlapping expression pattern during ovule development, providing abaxial identity in the inner integument. Loss of either gene results in integument fusion and abnormal seed shape. We hypothesize that in wild-type ovules the physical interaction between ATS and ETT in the inner integument mediates a transcriptional response the auxin and is required to maintain the boundary between the two integuments. Based on the transcriptional profiling results from our lab, we chose to examine two other ARF genes, ARF11 and ARF18, through reverse genetic analyses because these genes could provide an additional link between auxin signaling and ovule development. ARF11 and ARF18 are expressed in the endothelium and synergid cells and are required for proper embryo sac formation and fertilization. Taken together these studies contribute to our understanding of the molecular mechanisms underlying ovule development.