In praise of bracteoles

Abstract

There are currently few articles dedicated to bracteoles even though they play a unique role in the development of inflorescences and flowers. What functions do they perform? How do cymes develop without bracteoles?

Eichler (1875) hypothesized that bracteole position influences sepal arrangement. Our phyllotaxis model, based on the influence of inhibition fields on primordia initiation patterns, gives a theoretical basis to Eichler's hypothesis: we used our model to examine the influence of bracteoles on calyx arrangement in different angiosperms with diverse inflorescence and bracteole configuration.

In Ranunculales, bracteoles play a significant role in cymose branching, but are also sometimes found in racemose inflorescences, for instance in Delphinium. Members of the core eudicot subfamily Faboideae give us the opportunity to compare the arrangement of calyces within racemose inflorescences with and without bracteoles (either absent or aborted during development).

Cincinnus are highly complex cymes that evolved independently in monocots and dicots. In Solanaceae, the spiral initiation of the calyx depends on either two lateral bracteoles, as in Petunia, or a single bracteole, as in some Nicotiana species. When bracteoles are absent, as in Solanum, other peripheral organs play a role in determining calyx arrangement. Evolution has allowed the production of increasingly compact inflorescences. Boraginaceae, and in particular the genus Myosotis, illustrate the complexity of these inflorescence structures.

In conclusion, we discuss the evolution of bracteole function, a key element of the diversity of inflorescences and for the arrangement of floral organs.