CUP-SHAPED COTYLEDON1 transcription factor activates the expression of LSH4 and LSH3, two members of the ALOG gene family, in shoot organ boundary cells

The establishment of organ boundaries is a fundamental process for proper morphogenesis in multicellular organisms. In plants, the shoot meristem repetitively forms organ primordia from its periphery, and boundary cells are generated between them to separate their cellular fates. The genes CUP-SHAPED COTYLEDON1 (CUC1) and CUC2, which encode plant-specific NAC transcription factors, play central roles in establishment of the shoot organ boundaries in Arabidopsis thaliana. Here we show that CUC1 protein activates expression of LIGHT-DEPENDENT SHORT HYPOCOTYLS 4 (LSH4) and its homolog LSH3 in shoot organ boundary cells. Both genes encode nuclear proteins of the Arabidopsis LSH1 and Oryza G1 (ALOG) family, the members of which are widely conserved in land plants. Expression of LSH4 and LSH3 is detected in the boundary cells of various shoot organs, such as cotyledons, leaves and floral organs, and requires the activity of CUC1 and CUC2. Experiments using the glucocorticoid receptor system indicate that transcription of LSH4 and LSH3 is directly up-regulated by CUC1. Constitutive expression of LSH4 in the shoot apex causes inhibition of leaf growth in the vegetative phase, and formation of extra shoots or shoot organs within a flower in the reproductive phase. Together, our results indicate that CUC1 directly activates transcription of the nuclear factor genes LSH4 and LSH3, which may suppress organ differentiation in the boundary region.