Determination of flower structure in Elaeis guineensis: do palms use the same homeotic genes as other species?

Abstract

Aims: In this article a review is made of data recently obtained on the structural diversity and possible functions of MADS box genes in the determination of flower structure in the African oil palm (Elaeis guineensis). MADS box genes play a dominant role in the ABC model established to explain how floral organ identity is determined in model dicotyledon species such as Arabidopsis thaliana and Antirrhinum majus. In the monocotyledons, although there appears to be a broad general conservation of ABC gene functions, the model itself needs to be adapted in some cases, notably for certain species which produce flowers with sepals and petals of similar appearance. For the moment, ABC genes remain unstudied in a number of key monocot clades, so only a partial picture is available for the Liliopsida as a whole. The aim of this article is to summarize data recently obtained for the African oil palm Elaeis guineensis, a member of the family Arecaceae (Arecales), and to discuss their significance with respect to knowledge gained from other Angiosperm groups, particularly within the monocotyledons.

Scope: The essential details of reproductive development in oil palm are discussed and an overview is provided of the structural and functional characterization of MADS box genes likely to play a homeotic role in flower development in this species.

Conclusions: The structural and functional data provide evidence for a general conservation of the generic 'ABC' model in oil palm, rather than the 'modified ABC model' proposed for some other monocot species which produce homochlamydeous flowers (i.e. with morphologically similar organs in both perianth whorls), such as members of the Liliales. Our oil palm data therefore follow a similar pattern to those obtained for other Commelinid species in the orders Commelinales and Poales. The significance of these findings is discussed.

Fig. 1.

Key stages of pistillate (upper panel) and staminate (lower panel) flower development in oil palm. Developmental stages (indicated at bottom left of each photo) were assigned on the basis of the differentiation of the floral whorls. Stage 1 corresponds to a floral meristem. Stage 2 corresponds to the initiation of perianth organs. Stage 3 corresponds to the development of perianth organs and the initiation of reproductive organs. Stage 4 corresponds to the development of reproductive organs and stage 5 to a mature flower. Photographs are of either PAS/NBR-stained transverse and longitudinal sections or scanning electron micrographs (right-hand photographs, upper and lower panels). Abbreviations: asf1/asf2, accompanying staminate flowers 1 and 2; B, bracteole; BI/BII, bracteoles I and II; c, carpel; ff, pistillate flower; fm, floral triad meristem; Fb, floral triad bract; o, ovule; p, petal; s, sepal; sta, staminodes; c, carpel; m, megaspore mother cell; ps, pollen sac; sta, staminodes; te, tetrads; tg, integuments.

Fig. 2.

Dendrogram illustrating sequence affinities between oil palm MADS box proteins and selected relatives from other angiosperm groups. Subfamilies are designated according to the system of Becker and Theissen (2003). The tree shown is a schematic representation of topologies obtained using the maximum parsimony (MP) method with full-length MADS box amino acid sequences. Abbreviations: euAG, euAGAMOUS clade; PLE, PLENA clade; FBP7, FLORAL BINDING PROTEIN 7 clade; SEP3, SEPALLATA3; SEP1/2, SEPALLATA1/2; LHS1, LEAFY HULL STERILE1; euAP1, euAPETALA1 clade; FUL, FRUITFULL; CAL, calyx; COR, corolla; AN, androecium; GY, gynoecium; OV, ovule.

Fig. 3.

Spatial expression patterns of oil palm MADS box genes in staminate and pistillate flowers: schematic representation of in situ hybridization results. Two different developmental stages are depicted: firstly, the developing androecium and gynoecium stage (Stage 3); and secondly, the stage where gynoecium and androecium have reached full size but not maturity (Stage 4). Observations with mantled pistillate flowers are shown for Stage 4 only. Floral zones in which an in situ hybridization signal was observed are shown in colour, depending on the gene involved (yellow for EgSQUA1, blue for the B class genes EgDEF1 and EgGLO2, red for EgAG2 and green for EgAGL2-1).

Fig. 4.

A model to explain the possible roles of various oil palm genes in the determination of flower structure in oil palm, as based upon the generic eudicot ABC model (Coen and Meyerowitz, 1991; Angenent and Colombo, 1996; Pelaz et al., 2000). In the top left-hand corner is shown a schematic representation of sequence relationships between the oil palm MADS box genes studied. Boxes are colour shaded according to homeotic functions as follows: yellow, A function; blue, B function; orange, C and D functions; green, E functions.

Fig. 5.

Summary of MADS box structural and functional data obtained for monocot orders. The characterization of species putatively following the generic ABCDE model or ‘modified ABC model’ (involving B function in the outer perianth) is indicated where data are available. Orders for which sequences are available but not functional, data are indicated by an asterisk. Note that the tree (not to scale) is for illustrative purposes only and is based on the topology indicated by Savolainen and Chase (2003), which is undergoing revision.