Non-linear signaling for pattern formation?

Developmental pathways are often simplified as linear cascades of gene acti vation caused by initial signaling events. Recent data, however, support a more complex view in which non-linear responses to signals are important fo r pattern formation. First, a positive feedback loop may regulate cellular patterning in the Arabidopsis epidermis. Second, an asymmetric auxin distri bution that organizes pattern in the distal root tip may be established by positive feedback.