Genetic control of inflorescence architecture in legumes

Abstract

The architecture of the inflorescence, the shoot system that bears the flowers, is a main component of the huge diversity of forms found in flowering plants. Inflorescence architecture has also a strong impact on the production of fruits and seeds, and on crop management, two highly relevant agronomical traits. Elucidating the genetic networks that control inflorescence development, and how they vary between different species, is essential to understanding the evolution of plant form and to being able to breed key architectural traits in crop species. Inflorescence architecture depends on the identity and activity of the meristems in the inflorescence apex, which determines when flowers are formed, how many are produced and their relative position in the inflorescence axis. Arabidopsis thaliana, where the genetic control of inflorescence development is best known, has a simple inflorescence, where the primary inflorescence meristem directly produces the flowers, which are thus borne in the main inflorescence axis. In contrast, legumes represent a more complex inflorescence type, the compound inflorescence, where flowers are not directly borne in the main inflorescence axis but, instead, they are formed by secondary or higher order inflorescence meristems. Studies in model legumes such as pea (Pisum sativum) or Medicago truncatula have led to a rather good knowledge of the genetic control of the development of the legume compound inflorescence. In addition, the increasing availability of genetic and genomic tools for legumes is allowing to rapidly extending this knowledge to other grain legume crops. This review aims to describe the current knowledge of the genetic network controlling inflorescence development in legumes. It also discusses how the combination of this knowledge with the use of emerging genomic tools and resources may allow rapid advances in the breeding of grain legume crops.

Keywords: AP1; TFL1; VEG1; inflorescence architecture; legumes; meristem identity; pea.

FIGURE 1

Different types of inflorescence architecture. Images of plant species representative of main inflorescence types (top) and the corresponding diagrams (below) of the architecture of their inflorescences. Open circles represent flowers and arrows represent indeterminate shoots.

FIGURE 2

Meristem identity genes in Arabidopsis. (A) Images of wild-type (WT) and tfl1 mutant plants. While in the WT the main inflorescence and the lateral inflorescences (appearing in the axil of cauline leaves) show indeterminate growth, in the tfl1 mutant the main inflorescence ends into a terminal flower (a fruit in this image) and lateral branches are replaced by solitary flowers. (B) Inflorescence of an ap1 mutant. Individual flowers are replaced by branched structures. (C) Diagrams of meristem identity in the inflorescences of the wild-type and the tfl1 and ap1 mutants. In tfl1, the indeterminate inflorescence apex (I) is replaced by a terminal flower (F) while in ap1, the flowers are replaced by inflorescence-like structures. Arrowheads, indeterminate shoot; open circles, flowers, closed circles, abnormal flowers. (D) Model for specification of meristem identity in the simple inflorescence of Arabidopsis. In the Arabidopsis inflorescence apex, TFL1 expression in the inflorescence meristem (I) and AP1 and LFY expression in the floral meristem (F) are required for these meristems to acquire their identity. Expression of these genes in their correct domains is maintained by mutual repressive interactions.

FIGURE 3

Meristem identity genes in pea. (A) Picture and diagram of a pea WT plant. The main primary inflorescence (I1) shows indeterminate growth (arrowhead). Upper nodes of the plant contain secondary inflorescences (I2) which produce 1–2 flowers (F, open circles) and terminate into a stub (triangles). The inset shows a close up of a secondary inflorescence with two flowers (pods) and the stub (arrowhead). (B) Diagrams of meristem identity of the pim, det, and veg1 mutants. In the pim mutant, flowers are replaced by proliferating I2s with abnormal flowers (closed circles). In the det mutant, the primary inflorescence is replaced by a terminal secondary inflorescence. In the veg1 mutant, the I2s are replaced by vegetative branches with I1 identity. (C) Model for specification of meristem identity in the compound pea inflorescence. In the pea inflorescence apex, DET expression in the primary inflorescence meristem (I2), VEG1 in the secondary inflorescence meristem (I2) and PIM in the floral meristem (F) are required for these meristems to acquire their identity. Expression of these genes in their correct domains is maintained by a network of mutual repressive interactions.

FIGURE 4

Possible modifications of the pea inflorescence architecture. Different plant architectures deriving from modifications of the inflorescence, with potential to improve crop performance in legumes. Open circles represent flowers and arrows represent indeterminate shoots.