The HD-Zip II genes HAT3 and ATHB4 control adaxial-abaxial patterning in leaves

The Arabidopsis genome encodes for 10 Homeodomain-Leucine Zipper II (HD-Zip II) proteins. ATHB2, HAT1, HAT2 (HD-Zip II ? clade), HAT3 and ATHB4 (HD-Zip II ? clade) are all induced by changes in the Red/Far Red ratio of the light environment. However, the HD-Zip II ? and ? genes are also tightly regulated during plant development with both distinct and overlapping patterns (Ciarbelli et al., 2008). To assess the role of the light-regulated HD-Zip II genes in plant development, double and multiple mutants within ? and ? clades were constructed by genetic crosses. Here we show that simultaneous lack of HAT3 and ATHB4 transcription factors causes developmental defects in a white light environment. Young hat3 athb4 seedlings show varying degrees of defects in cotyledon and leaf expansion, up to completely radialized organs. The pattern of vascular development is also profoundly altered, in a manner that is tightly linked to lamina expansion. Fully radialized leaves lack procambial cells whereas trumpet shaped leaves show groups of phloem cells in adaxial position, a feature that is found in the vasculature of abaxialized leaves. In situ and GUS/GFP reporter analyses of the HD-Zip II ? genes show that they are expressed in the adaxial side of cotyledons and leaves. Taken together, these data demonstrate that HAT3 and ATHB4 are required to specify adaxial identity in leaf morphogenesis. We are presently studying the genetic and molecular interactions between the HD-Zip II ? genes and members of the HD-Zip III family, such as PHB, PHV and REV, key determinants of adaxial identity in leaves.