Abstract
Floral organ formation has been the subject of intensive study for over 20 years, particularly in the model dicot species Arabidopsis thaliana. These studies have led to the establishment of a general model for the development of floral organs in higher plants, the so-called ABCDE model, in which floral whorl-specific combinations of class A, B, C, D, or E genes specify floral organ identity. In Arabidopsis, class A, B, C, D, E genes encode MADS-box transcription factors except for the class A gene APETALA2. Mutation of these genes induces floral organ homeosis. In this review, I focus on the roles of these homeotic genes in bread wheat (Triticum aestivum), particularly with respect to the ABCDE model. Pistillody, the homeotic transformation of stamens into pistil-like structures, occurs in cytoplasmic substitution (alloplasmic) wheat lines that have the cytoplasm of the related wild species Aegilops crassa. This phenomenon is a valuable tool for analysis of the wheat ABCDE model. Using an alloplasmic line, the wheat ortholog of DROOPING LEAF (TaDL), a member of the YABBY gene family, has been shown to regulate pistil specification. Here, I describe the current understanding of the ABCDE model for floral organ formation in wheat.
Highlights
The ABCDE model for flower development proposes that floral organ identity is defined by five classes of homeotic genes, named A, B, C, D and E [1]
Three AP1-like MADS-box genes have been identified in the rice genome, namely, OsMADS14/RAP1B, OsMADS15/RAP1A and OsMADS18, which are all derived from the FRUITFULL
Studies on transgenic plants suggested that OsMADS14 is involved in promoting flowering and in determining the identity of the floral meristem [23]
Summary
The ABCDE model for flower development proposes that floral organ identity is defined by five classes of homeotic genes, named A, B, C, D and E [1]. The diversification of MADS-box genes during evolution has contributed to the wide variation of flower forms in angiosperms [12] They are not included in the conventional ABCDE model, the AGAMOUS LIKE 6 (AGL6)-clade genes AGL6 and AGL13, may play a role in floral organ formation, probably in ovule formation [13]. The spikelet is composed of florets that join the axis (rachilla) alternately on opposite sides, and is encompassed by two glumes (Figures 1 and 2). A palea, lodicules, stamens and a pistil are wheat floral organs developed in the whorl 1, 2, 3, and 4, respectively. The reproductive organs, pistil (Pi) and stamens (St) are enveloped by two leaf-like structures, the lemma (Le) and the palea (Pa). Each floret is composed of a lemma, a palea, two lodicules, three stamens and a pistil. gl, glume; le, lemma; pa, palea; lo, lodicule; st, stamen; pi, pistil
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