Mutations in epidermis-specific HD-ZIP IV genes affect floral organ identity in Arabidopsis thaliana

Development of the epidermis involves members of the class-IV homeodomain-leucine zipper (HD-ZIPIV) transcription factors. The Arabidopsis HD-ZIPIV family consists of 16 members, among which PROTODERMAL FACTOR2 (PDF2) and ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1) play an indispensable role in the differentiation of shoot epidermal cells; however, the functions of other HD-ZIPIV genes that are also expressed specifically in the shoot epidermis remain to be fully elucidated. We constructed double mutant combinations of these HD-ZIPIV mutant alleles and found that the double mutants of pdf2-1 with homeodomain glabrous1-1 (hdg1-1), hdg2-3, hdg5-1 and hdg12-2 produced abnormal flowers with sepaloid petals and carpelloid stamens in association with the reduced expression of the petal and stamen identity gene APETALA3 (AP3). Expression of another petal and stamen identity gene PISTILATA (PI) was less affected in these mutants. We confirmed that AP3 expression in pdf2-1hdg2-3 was normally induced at the initial stages of flower development, but was attenuated both in the epidermis and internal cell layers of developing flowers. As the expression of PDF2 and these HD-ZIPIV genes during floral organ formation is exclusively limited to the epidermal cell layer, these double mutations may have non-cell-autonomous effects on AP3 expression in the internal cell layers. Our results suggest that cooperative functions of PDF2 and other members of the HD-ZIPIV family in the epidermis are crucial for normal development of floral organs in Arabidopsis.