A key question in plant development concerns what controls leaf development and leaf shape. On , Hay et al. report that interactions between the hormone auxin and the AS1 and KNOX transcription factors control leaf development in Arabidopsis. In higher plants, the specification of leaf initials at the tip of the shoot apical meristem (a slowly dividing stem cell population) is facilitated by mutual repression between AS1 (which promotes leaf fate) and KNOX (which promotes meristem activity). Now, Hay and colleagues show that auxin activity (which is transported towards leaf initials, where it accumulates via the efflux facilitator PIN1) acts with AS1 to repress expression of the KNOX gene BREVIPEDICELLUS and thus promote leaf fate. They also show that PIN1-regulated auxin gradients control leaf shape in a KNOX-independent manner, but that ectopic KNOX expression in leaves perturbs these gradients and so alters leaf shape. Thus, the researchers suggest, regulation of auxin gradients by KNOX proteins may underlie natural variations in leaf form.
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