The arrangement of leaves and flowers around the stem typically follows a stereotyped phyllotactic pattern, with the location of a new leaf defined by the positions of previously specified ones. This is controlled by the plant hormone auxin. In the meristem epidermis, auxin accumulates at the site of future primordia, generating intervening regions of auxin minima where no new organ can form. Auxin-dependent phyllotactic patterns can be computationally modelled in two dimensions, considering only the epidermal layer. However, auxin flows into lower layers of the primordium, where the incipient midvein is thought to drain auxin away from the meristem. The contribution of this auxin drainage to leaf development has been hard to assess, but Didier Reinhardt and co-workers (p. ) have employed sophisticated live imaging and cell ablation techniques to study the role of the midvein in organ formation and positioning. The authors specifically ablate the future midvein, while leaving overlying layers intact. This leads to a transient auxin accumulation and consequent widening of the primordium. Although the damage is rapidly healed and subsequent leaf development is normal, phyllotaxis is disrupted, spacing of adjacent primordia is aberrant – revealing the importance of the incipient midvein in phyllotaxis.
