Transductions for the Expression of Structural Pattern: Analysis in Sunflower.

The transductions to initiate and propagate pattern were investigated in the sunflower head. The spiral pattern emerges as new florets form in regular centripetal sequence on the flat disc. The spiral lines of florets arise as rows of simple bumps. Each bump splits to become a small bract and a circular disc flower. This topographical progression was described by a scanning electron microscopic technique applied to living tissue. The suitability of various theories to explain the progression was examined. Because no periodic cell specialization was seen by scanning electron microscopy prior to pattern emergence, a mechanism that produces pattern in uniform tissue by spontaneous physical buckling (folding) was examined further. Key configurational changes of development were reproduced in models using the buckling assumptions. In further testing, a young head was physically constrained to cause it to grow as an oval. Pattern was modified as predicted. Unexpectedly, organ character changed as well. In localized regions, the folding was abnormal; the typical dyad floret, bract and flower, was replaced by a single large bract. This anomaly is known in mutants of sunflower; hence, the physical treatment induced a phenocopy. We concluded that (1) buckling is a strong candidate to be the process producing organ pattern, and (2) the accompanying topographical changes can be prerequisite to organ differentiation.