Open and closed inflorescences: more than simple opposites

Abstract

The absence of a terminal flower in inflorescences ('open inflorescences') is currently explained by the maintenance of putative stem-cells in the central zone (CZ) of the inflorescence meristem (IM) governed by the CLAVATA-WUSCHEL regulatory loop. Disruption of this regulatory pathway, as in Arabidopsis TERMINAL FLOWER LOCUS 1 mutants, leads to terminal flower production. However, recent studies in other taxa reveal novel mechanisms of inflorescence termination; for example, the SEPALLATA-like MADS-box floral identity gene GERBERA REGULATOR OF CAPITULUM DEVELOPMENT 2 in Gerbera excludes the retention of a CZ as an ontogenetic cause for the openness of these inflorescences. Moreover, comparative histological studies show that the retention of a CZ in the IM, mostly a feature of the 'typical open families', is absent in open inflorescences of other families. Concerning these groups, new evidence suggests that spatial constraints at the IM could play a role at the time when terminal flower production (or not) is determined. This indicates that the multiple loss and re-gain of the terminal flower in angiosperms is necessarily based on more than one ontogenetic pathway.

The unique pseudanthium of Actinodium (Myrtaceae) - morphological reinvestigation and possible regulation by CYCLOIDEA-like genes

“…The outer flower primordia develop into monosymmetric ray florets while the tubular central florets are polysymmetric. The inflorescence meristem is completely consumed by flower production permitting no further growth, as known for “mantle-core” or “open II-type” inflorescence meristems [35,36]. …”

“…Second, it can proliferate after flowering. Third, it is an inflorescence of the open I-type with a central-zone meristem [35]. Fourth, the rays are not outer flowers, but branched short shoots, which, fifth, originate below the inflorescence and develop in a basipetal order.…”

“…Comparative developmental studies in open inflorescences have termed these inflorescences as open I [35,46], in contrast to the meristematic organization of daisy heads, which do not show either central zone or proliferation capacity in the wild type at least.…”

Evolution and developmental genetics of floral display—A review of progress

“…AM, apical meristem; IM, inflorescence meristem; FM, floral meristem. The figure shows the integrated information reviewed in Galli & Gallavotti (2016) and Bull-Hereñu & Claßen-Bockhoff (2011). + [Correction added on 10 November 2017, after first online publication: 'C, During the development of an indeterminate…' has been changed to ' C, During the development of a determinate…'].…”

Diversity and Evolution of Inflorescences in Celastrales

“…Such models of flowering shoots rebuilding was suggested by Parkin [18]. Transitions between opened and closed types of inflorescences can be stipulated by different changes in net of genetic regulation inflorescence development [19] and more than one ontogenetic pathway [20]. Analysis of inflorescence diversity in a molecular phylogenetic context (according to Ref.…”