Making leaves.

Leaves are determinate organs that develop from the flanks of the shoot apical meristem through founder cell recruitment, establishment of proximodistal, dorsoventral and mediolateral axes, and subsequent growth, expansion and differentiation along these axes. Maintenance of the shoot apical meristem and production of leaves requires balanced partitioning of cells between pluripotent and differentiation fates. Hormones have a significant role in this balance but it is becoming apparent that additional intrinsic and extrinsic inputs influence hormone signalling to control meristem function and leaf initiation. As leaves develop, temporal and spatial regulation of growth and maturation determines leaf shape and complexity. Remarkably genes involved in leaf development in the context of the shoot apical meristem are also involved in elaboration of the leaf shape to generate subtle marginal serrations, more prominent lobes or a dissected compound leaf. Potentially these common regulatory modules represent a fundamental means of setting up boundaries separating discrete zones of growth. Defining gene networks involved in leaf shape variation and exploring interspecies differences between such networks is enabling exciting insight into changes that contribute to natural variation of leaf form.