FILAMENTOUS FLOWER controls lateral organ development by acting as both an activator and a repressor

Of the four Arabidopsis YABs tested in yeast, only FILAMENTOUS FLOWER (FIL) activated reporter gene expression. Similar analysis with Antirrhinum YABs identified the FIL ortholog GRAMINIFOLIA as an activator. Plant-based transactivation assays not only confirmed the potential of FIL to activate transcription, but also extended this property to the FIL paralog YABBY3 (YAB3). Subsequent transcriptomic analysis of lines expressing a steroid-inducible FIL protein revealed groups of genes that responded either positively or negatively to YAB induction. Included in the positively regulated group of genes were the polarity regulators KANADI1 (KAN1), AUXIN RESPONSE FACTOR 4 (ARF4) and ASYMMETRIC LEAVES1 (AS1). We also show that modifying FIL to function as an obligate repressor causes strong yab loss-of-function phenotypes.Collectively these data show that FIL functions as a transcriptional activator in plants and that this activity is involved in leaf patterning. Interestingly, our study also supports the idea that FIL can act as a repressor, as transcriptomic analysis identified negatively regulated FIL-response genes. To reconcile these observations, we propose that YABs are bifunctional transcription factors that participate in both positive and negative regulation. These findings fit a model of leaf development in which adaxial/abaxial patterning is maintained by a regulatory network consisting of positive feedback loops.The YABBY (YAB) family of transcription factors regulates various aspects of vegetative and floral development in flowering plants. First identified in Arabidopsis, YABs have a characteristic structure that includes an N-terminal zinc finger domain and a C-terminal YABBY domain containing a helix-loop-helix motif similar to that found in the high mobility group (HMG) of proteins [1,2]. Analysis of the zinc finger domain has shown that it mediates homo- and heterodimerization between the YABs, as well as interactions with other proteins [3,4]. In contra