Spatial pattern formation in the flower of Arabidopsis thaliana: Mathematical modeling

Abstract

The research into the genetic control of flower formation is a rapidly evolving field of plant developmental biology. An ABC model of the genetic control of floral morphogenesis based on the studies of Arabidopsis thaliana mutants [1] envisaged the flower structure as a pattern comprising four organ whorls, with the particular organ development in each whorl specified by the combined activities of several genes. The expression of the A class genes APETALA1 ( AP1 ) and APETALA2 ( AP2 ) determines for the development of sepals (whorl 1). The development of carpels (whorl 4) is specified by the C class gene AGAMOUS ( AG ). The combined activities of the A and C genes together with the B genes APETALA3 ( AP 3 ) and PISTILLATA ( PI ) specify the development of petals and stamens (whorls 2 and 3, respectively). The ABC model postulates that the mutations in these genes would change the organ specificity in particular whorls. In several ABC mutants, the number of flower organs is changed; therefore a hypothesis was put forward that these genes specify both the identity of flower organs and flower organ initiation [2]. However, the mechanism of such specification is poorly understood. When working out the mathematical model of floral development based on the ABC-model postulates, we found that the existing evidence on the functions of the ABC genes did not sufficiently clarify the nature of changes in the arrangement of flower organs in the mutants. It follows that the processes that determine spatial pattern formation in the flower must be studied in more detail [3]. There are two hypotheses describing the process of spatial pattern formation in the developing flower. The first one presumes that floral development proceeds acropetally, from periphery towards the center [4]. The development of each whorl is triggered by a signal from the whorl on the outside. The second hypothesis maintains that in floral development, the formation of organ spatial pattern precedes the initiation of corresponding organs [5]. Recently Chub and Penin [6] suggested that the spatial patterning may advance both acropetally and basipetally. The present study aimed at elucidating, by mathematic modeling, the basic aspects of spatial flower patterning in order to recognize the effects of the ABC genes on flower organ arrangement. The modeling demonstrated that organ spatial pattern formation in flowers was regularly scheduled and advanced in two directions, acropetally in the perianth and basipetally in the stamens and carpels. We demonstrated that AP2 (class A) and AG (class C) both specified the particular flower organs and also determined the acropetal and basipetal patterning zones in the floral meristem (APZ and BPZ, respectively).