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NONEXPRESSOR OF PR GENES1 (NPR1) is a key regulator of the plant defense response known as systemic acquired resistance. Accumulation of the signal molecule salicylic acid (SA) leads to a change in intracellular redox potential, enabling NPR1 to enter the nucleus and interact with TGACG sequence-specific binding protein (TGA) transcription factors, which in turn bind to SA-responsive elements in the promoters of defense genes. Here, we show that two NPR1-like genes, BLADE-ON-PETIOLE1 (BOP1) and BOP2, function redundantly to control growth asymmetry, an important aspect of patterning in leaves and flowers. Phenotypes in the double mutant include leafy petioles, loss of floral organ abscission, and asymmetric flowers subtended by a bract. We demonstrate that BOP2 is localized to both the nucleus and the cytoplasm, but unlike NPR1, it is highly expressed in young floral meristems and in yeast interacts preferentially with the TGA transcription factor encoded by PERIANTHIA (PAN). In support of a biological relevance for this interaction, we show that bop1 bop2 and pan mutants share a pentamerous arrangement of first whorl floral organs, a patterning defect that is retained in bop1 bop2 pan triple mutants. Our data provide evidence that BOP proteins control patterning via direct interactions with TGA transcription factors and demonstrate that a signaling mechanism similar to that formally associated with plant defense is likely used for the control of developmental patterning. [ABSTRACT FROM AUTHOR]