Genetic and Molecular Control of Floral Organ Identity in Cereals

Abstract

Grasses represent a major family of monocots comprising mostly cereals. When compared to their eudicot counterparts, cereals show a remarkable morphological diversity. Understanding the molecular basis of floral organ identity and inflorescence development is crucial to gain insight into the grain development for yield improvement purposes in cereals, however, the exact genetic mechanism of floral organogenesis remains elusive due to their complex inflorescence architecture. Extensive molecular analyses of Arabidopsis and other plant genera and species have established the ABCDE floral organ identity model. According to this model, hierarchical combinatorial activities of A, B, C, D, and E classes of homeotic genes regulate the identity of different floral organs with partial conservation and partial diversification between eudicots and cereals. Here, we review the developmental role of A, B, C, D, and E gene classes and explore the recent advances in understanding the floral development and subsequent organ specification in major cereals with reference to model plants. Furthermore, we discuss the evolutionary relationships among known floral organ identity genes. This comparative overview of floral developmental genes and associated regulatory factors, within and between species, will provide a thorough understanding of underlying complex genetic and molecular control of flower development and floral organ identity, which can be helpful to devise innovative strategies for grain yield improvement in cereals.

Keywords: ABCDE model; MADS-box genes; cereals; evolutionary relationships; floral speciation; flower organ identity.

Figure 1

Graphical representation of inflorescences, phase transition and transverse flowers. (A) Structural configuration of inflorescence in Arabidopsis, rice, maize, Brachypodium and wheat. Color codes. Green line: rachis; yellow line: primary branch; blue line: secondary branch; green circles: spikelet/spikelet pair meristems; maroon circle/oval: terminal spikelet; orange circle: floral meristems. (B) Regulation of meristem transition in Arabidopsis, rice, maize, Brachypodium, and wheat. Green arrow: multiple meristems formation; blue arrow: single meristem formation; orange dashed arrow: abortion of floral meristems. (C) Schematic representation of transverse spikelet/flower. Color codes: Blue: palea; dark orange: lemma; gold: lodicules; green: sepal; green circle: rachis; gray: glume; pink: pistil; red: petal; yellow: stamen. Abbreviations: BR: branch; BM: branch meristem; FM: floral meristem; GL: glume; IM: inflorescence meristem; LE: lemma; LO: lodicule; LS: lateral spikelet; LSM: lateral spikelet meristem; PA: palea; PB: primary branch; PE: petal; PI: pistil; PBM: primary branch meristem; RA: rachis; Ra: rachilla; SB: secondary branch; SE: sepal; SBM: secondary branch meristem; SM: spikelet meristem: SPM: spikelet pair meristem; ST: stamen; TS: terminal spikelet.

Figure 2

ABCDE models of floral organ identity. Revised floral organ identity models in Arabidopsis, rice, maize, Brachypodium, and wheat. Class (A)-genes indicated in green, class B in red, class C in dark blue, class D in light blue, class E yellow, and non-MADS in purple. Solid colors show functional data, color gradients represent expression analysis data, while color patterns indicate hypothesized functions. Antagonistic interactions are indicated with barred lines, black arrows illustrate positive regulation of the corresponding genes, and a comma symbolizes duplicated gene interaction. Abbreviations: CA: Carpel; LO: Lodicule; OV: Ovule; PA: Palea; PE: Petal; SE: Sepal; ST: Stamen. For gene abbreviations see text.

Figure 3

Evolutionary relationships among MADS-box genes. Phylogenetic tree constructed from the deduced amino acid sequences of Arabidopsis, Brachypodium, maize, rice, and wheat genes obtained from NCBI database. (Sequence ID information can be seen in supplementary file) The tree was inferred after amino acid sequence alignment by Clustal Omega [137], using the neighbor-joining method [138] and visualized in topology-only mode. Only bootstrap values >50%, as calculated from 100 replicates, are shown. Phylogenetic analysis was conducted in MEGA version 6 [139]. Markers: Diamond: Arabidopsis; triangle: rice; circle: maize; green filled square: wheat; hollow square: Brachypodium genes.