Plant development is regulated by a number of mobile factors. The Arabidopsis BYPASS1 (BPS1) gene was previously shown to control shoot and root development by preventing formation of a mobile compound, but how this compound functions and whether it modulates other signalling pathways is unclear. Now, Leslie Sieburth and colleagues show that Arabidopsis BPS1, as well as two related genes, BPS2 and BPS3, control the production of a mobile factor, the bps signal, which regulates patterning and growth in parallel with auxin signalling (p. 805). By analysing single, double and triple mutants, the researchers show that all three BPS genes control bps signal synthesis. Importantly, bps triple mutants display severe embryogenesis defects, including disruptions to vascular, root and shoot stem cell populations. Finally, bps triple mutants exhibit normal auxin-induced gene expression and localisation of the PIN1 auxin transporter, suggesting that the bps signal functions in an auxin-independent manner. Although the nature of the bps signal remains unknown, these studies identify a novel pathway that regulates multiple stages of plant patterning and growth.

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