Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): Somayeh Naghiloo, Regine Claßen-Bockhoff, Tianzhen Zhang.
The genus Dipsacus is characterized by a remarkable bidirectional flowering sequence and a rare phyllotactic pattern. Considering that flower initiation and flowering sequence may be interconnected, we document the development of the head meristem in Dipsacus fullonum. Our results indicate a gradual change in the geometry of the head meristem beginning with a dome shaped stage, continuing with a remarkable widening in the middle part of the head meristem and ending in a spindle-like form. Quantitative data confirm that meristem expansion is higher in the middle part than at the base of the meristem. Likewise, the size of the flower primordia in the middle part of the young head is significantly larger than at the base soon after initiation. We conclude that the change in the geometry of the meristem and the availability of newly generated space result in the promotion of the middle flowers and the bidirectional flowering sequence at anthesis. Our investigation on phyllotactic patterns reveals a high tendency (30%) of the head meristem to insert or lose parastichies. This finding can also be attributed to changes in the expansion rate of the meristem. Dependent on the spatio-temporal relation between meristem expansion and primordia initiation, either flower primordia are promoted or additional parastichies appear. Our results emphasize the important role of geometry in flower development and phyllotactic pattern formation.
The wild teasel, Dipsacus fullonum L., is a perennial to biennial herb of the family Caprifoliaceae-Dipsocoideae . The genus is mainly recognized by the presence of stem prickles, egg-shaped flower-heads and spine-tipped bracts. It has long been characterized by its remarkable pattern of flowering . Ordinarily, one would expect that flowering begins at the base and extends towards the tip, but in Dipsacus it begins with the middle flowers and then extends both upward and downward. This gives the impression of two rings of open flowers which ‘migrate’ towards the poles of the head over subsequent days (Fig 1). How this interesting flowering sequence is actually originated is still an unsolved puzzle. The enigma of the flowering pattern in Dipsacus is paralleled by the special phyllotaxis of the head and irregularities in parastichies numbers .
The present study indicates a change in the geometry of the head meristem during development influencing flowering sequence and phyllotactic pattern. Given the impact of growth processes on flower development, integrating developmental studies on inflorescence and flowers will provide a better understanding of the developmental constrains of flowering.