THE PLANT CELL, Vol 5, Issue 12 1725-1738, Copyright © 1993 by American Society of Plant Biologists
Transductions for the Expression of Structural Pattern: Analysis in Sunflower
L. F. Hernandez and P. B. Green
Departamento de Agronomia, Universidad Nacional del Sur, Bahia Blanca 8000, Argentina
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.
