Leslie Sieburth and colleagues first identified the gene BYPASS 1 (BPS1) in a screen for Arabidopsis thaliana mutants with leaf-vein defects. BPS1 encodes a novel protein needed for proper shoot and root development; however, when BPS1 is mutated, root defects become apparent first, hinting that the shoot phenotype might be a secondary effect of faulty root development, owing to disrupted water or mineral uptake. But this was ruled out when mutants were grown in liquid culture — removing the need for uptake through the roots — as the same phenotype was seen.
To explore the relationship between the root and shoot defects further, the authors carried out grafting experiments. First, the apical parts of a bps1 mutant were grafted onto the rootstock of a wild-type plant to test where BPS1 function is required. This partially rescued the shoot defects, although the effect was only transient. An intriguing result came from control experiments: grafting a mutant apex onto a mutant rootstock also gave a partial rescue, implying that separating the developing shoot from the roots prevents the effects of bps1 mutation. One explanation is that the roots produce a signal that inhibits shoot growth unless functional BPS1 is present. This was supported by the fact that continuous removal of roots during development rescues the bps1 phenotype and that bps1 roots are sufficient to arrest wild-type leaf growth in grafting experiments.
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