United Kingdom OX1 3RB (S.M.M.)The transition to flowering in many plant species, including Arabidopsis (Arabidopsis thaliana), is marked by the elongation of internodes to make an inflorescence upon which lateral branches and flowers are arranged in a characteristic pattern. Inflorescence patterning relies in part on the activities of two three-amino-acid loop-extension homeodomain transcription factors: BREVIPEDICELLUS (BP) and PENNYWISE (PNY) whose interacting products also promote meristem function. We examine here the genetic interactions between BP-PNY whose expression is up-regulated in stems at the floral transition, and the lateral organ boundary genes BLADE-ON-PETIOLE1 (BOP1) and BOP2, whose expression is restricted to pedicel axils. Our data show that bp and pny inflorescence defects are caused by BOP1/2 gain of function in stems and pedicels. Compatible with this, inactivation of BOP1/2 rescues these defects. BOP expression domains are differentially enlarged in bp and pny mutants, corresponding to the distinctive patterns of growth restriction in these mutants leading to compacted internodes and clustered or downward-oriented fruits. Our data indicate that BOP1/2 are positive regulators of KNOTTED1-LIKE FROM ARABIDOP-SIS THALIANA6 expression and that growth restriction in BOP1/2 gain-of-function plants requires KNOTTED1-LIKE FROM ARABIDOPSIS THALIANA6. Antagonism between BOP1/2 and BP is explained in part by their reciprocal regulation of gene expression, as evidenced by the identification of lignin biosynthetic genes that are repressed by BP and activated by BOP1/2 in stems. These data reveal BOP1/2 gain of function as the basis of bp and pny inflorescence defects and reveal how antagonism between BOP1/2 and BP-PNY contributes to inflorescence patterning in a model plant species.Flowering plants display a remarkable variety of inflorescence architectures selected to optimally display flowers for pollination and seed dispersal. Formation of the aerial parts of a plant is controlled by the shoot apical meristem (SAM), a cluster of pleuripotent stem cells located at the apex of the primary shoot. The SAM produces a series of reiterative modules known as phytomers to generate the aerial parts of the plant.Each phytomer comprises an internode (stem) subtending a node, which is a leaf associated with a potential axillary meristem (Steeves and Sussex, 1989). Elaboration of the different parts of a module (leaves, internodes, and axillary meristems) varies according to the phase of development and between species to generate architectural diversity (Sussex and Kerk, 2001).Arabidopsis (Arabidopsis thaliana) has distinct vegetative and reproductive phases. During vegetative development, the SAM generates leaf primordia on its flanks; both internode and axillary meristem formation are inhibited, resulting in a compact rosette of leaves. At the end of the vegetative phase, endogenous and environmental cues promote the transition to flowering. The SAM responds to floral inductive signals by acquiring infloresce...

ORCID IDs: 0000-0002-0050-4001 (B.C.S.); 0000-0002-3526-7982 (J.L.); 0000-0001-7896-6049 (M.P.); 0000-0001-7144-1274 (D.X.); 0000-0001-5026-095X (S.Ci.); 0000-0002-6496-3792 (S.R.H.); 0000-0003-1808-5172 (V.P.).In the model plant Arabidopsis (Arabidopsis thaliana), endogenous and environmental signals acting on the shoot apical meristem cause acquisition of inflorescence meristem fate. This results in changed patterns of aerial development seen as the transition from making leaves to the production of flowers separated by elongated internodes. Two related BEL1-like homeobox genes, PENNYWISE (PNY) and POUND-FOOLISH (PNF), fulfill this transition. Loss of function of these genes impairs stem cell maintenance and blocks internode elongation and flowering. We show here that pny pnf apices misexpress lateral organ boundary genes BLADE-ON-PETIOLE1/2 (BOP1/2) and KNOTTED-LIKE FROM ARABIDOPSIS THALIANA6 (KNAT6) together with ARABIDOPSIS THALIANA HOMEOBOX GENE1 (ATH1). Inactivation of genes in this module fully rescues pny pnf defects. We further show that BOP1 directly activates ATH1, whereas activation of KNAT6 is indirect. The pny pnf restoration correlates with renewed accumulation of transcripts conferring floral meristem identity, including FD, SQUAMOSA PROMOTER-BINDING PROTEIN LIKE genes, LEAFY, and APETALA1. To gain insight into how this module blocks flowering, we analyzed the transcriptome of BOP1-overexpressing plants. Our data suggest a central role for the microRNA156-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE-microRNA172 module in integrating stress signals conferred in part by promotion of jasmonic acid biosynthesis. These data reveal a potential mechanism by which repression of lateral organ boundary genes by PNY-PNF is essential for flowering.Plant development relies on the activity of the shoot apical meristem (SAM) as a continuous source of founder cells for production of new leaves, shoots, and internodes throughout the life cycle (for review, see Aichinger et al., 2012). A tight balance between the allocation of cells to developing primordia and the perpetuation of pluripotent stem cells in the central zone maintains the SAM at a constant size. In Arabidopsis (Arabidopsis thaliana), the vegetative SAM produces leaves in a spiral phyllotaxy with dormant axillary meristems. In conjunction, internode elongation is repressed, resulting in a basal rosette. The transition to flowering is governed by internal and external signals that converge at the SAM to promote acquisition of inflorescence meristem (IM) fate (for review, see Amasino and Michaels, 2010;Srikanth and Schmid, 2011;Andrés and Coupland, 2012). This process, known as floral evocation, results in new patterns of growth at the shoot apex, including production of flowers, and an increase in stem elongation, called bolting. Lateral organ boundaries are specialized domains of restricted growth that separate meristem and organ compartments and produce axillary meristems (for review, see Aida and Tasaka, 2006;Tian et al., 2014). Early in the transition t...