Floral initiation and inflorescence architecture: a comparative view

Abstract

Background: A huge variety of plant forms can be found in nature. This is particularly noticeable for inflorescences, the region of the plant that contains the flowers. The architecture of the inflorescence depends on its branching pattern and on the relative position where flowers are formed. In model species such as Arabidopsis thaliana or Antirrhinum majus the key genes that regulate the initiation of flowers have been studied in detail and much is known about how they work. Studies being carried out in other species of higher plants indicate that the homologues of these genes are also key regulators of the development of their reproductive structures. Further, changes in these gene expression patterns and/or function play a crucial role in the generation of different plant architectures.

Scope: In this review we aim to present a summarized view on what is known about floral initiation genes in different plants, particularly dicotyledonous species, and aim to emphasize their contribution to plant architecture.

Fig. 1.

Diagrams of different types of inflorescences. (A–C) Indeterminate inflorescences: (A) the simple raceme of Arabidopsis thaliana and (B) Antirrhinum majus, and (C) the compound raceme of pea. (D–G) Determinate inflorescences: (D) the dichasium of Silene latifolia; (E) the tobacco cyme; (F) the sympodium of tomato; and (G) a panicle. Open circles represent flowers and arrows represent indeterminate shoots. Grey triangles in (C) represent stubs.

Fig. 2.

Meristem identity genes in arabidopsis. (A) Inflorescence of the wild type (wt) and of lfy, ap1 and tfl1 mutants. In the inflorescences of lfy and ap1, flowers (open circles) are replaced by structures with shoot characteristics (indicated by arrowheads in the photographs), while in the tfl1 mutant solitary flowers replace shoots in the axils of cauline leaves (arrowheads). The inflorescences of the wild type, lfy and ap1 show indeterminate growth but the inflorescence of tfl1 is determinate and forms a terminal flower (arrow in the photograph). Filled circles in the diagrams represent abnormal flowers with shoot traits. (B) Complementary expression of TFL1 (blue) and LFY/AP1/CAL (red) genes in the arabidopsis inflorescence shoot apex. While LFY and AP1/CAL specify floral identity, TFL1 is required to maintain the inflorescence identity of all shoot meristems.

Fig. 3.

Tree of the phylogenetic relationships among the species cited in this review. The tree is based on Soltis and Soltis (2003), Leebens-Mack et al. (2005) and the ‘Tree of life web project’ (http://www.tolweb.org/tree/).

Fig. 4.

Inflorescence and floral meristem mutants in pea, an example of a compound raceme. Inflorescences of the wild-type (wt) pea plant and of uni, det and veg1 mutants. In a wt plant, the vegetative meristem (V) after the floral transition becomes a primary inflorescence meristem (I1) that generates secondary inflorescence meristems (I2) that produce the flowers (F). The secondary inflorescences are formed in the axil of leaves and produce 1–2 flowers before generating a rudimentary stub (indicated by arrows in the photographs). In the uni mutant the I2, instead of flowers, generate other I2s that keep proliferating indefinitely (I*2). In the det mutant, the I1 is prematurely transformed into an I2 that produces one or two flowers before terminating into a stub (arrow). In the veg1 mutant, the I2s are transformed into vegetative I1 meristems (indicated by arrowheads in the photograph), generating a plant that never flowers.