Roles of polarity determinants 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. Mutations in the Arabidopsis KANADI (KAN) and YABBY polarity genes result in amorphous or arrested integument growth, suggesting that polarity determinants play key roles in ovule development. We show that the class III homeodomain leucine zipper (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, also known as KANADI4) activity. Loss of function of these genes leads to aberrant integument growth. Additionally, over-expression of PHB or PHV in ovules is not sufficient to repress ATS expression, and can produce phenotypes similar to those of the HD-ZIPIII loss-of-function lines. The absence of evidence of mutual negative regulation by KAN and HD-ZIPIII transcription factors is in contrast to known mechanisms in leaves. Loss of HD-ZIPIII activity can partially compensate for loss of ATS activity in the ats cna phb phv quadruple mutant, showing that CNA/PHB/PHV act in concert with ATS to control integument morphogenesis. In a parallel pathway, ATS acts with REVOLUTA (REV) to restrict expression of INNER NO OUTER (INO) and outer integument growth. Based on these expression and genetic studies, we propose a model in which a balance between the relative levels of adaxial/abaxial activities, rather than maintenance of boundaries of expression domains, is necessary to support laminar growth of the two integuments.