Introduction

Molecular genetic studies in the last fifteen years have been enormously successful in elucidating some aspects of flower development but many questions remain. We still have little insight into one of the most basic questions in plant biology. What is responsible for the wide diversity of floral morphologies? What genetic mechanisms control this variety? Differences in floral morphologies are significant as they are driven by heterogeneity among environments (primarily pollinators) and have likely contributed to the enormous evolutionary success of flowering plants. My lab is interested in the fundamental genetic processes that govern this variety. We are studying the molecular basis of several floral features (organ size, organ shape and organ fusion) that contribute to morphological diversity.

Arabidopsis thaliana flowers

We study flower development in Arabidopsis thaliana, because of the many tools available for use with this model plant. Arabidopsis flowers are composed of four different organ types that arise in a characteristic pattern within concentric rings called whorls. Four sepals (se) are present in the outermost whorl (whorl one). Four petals (pe) develop in whorl two at locations between the sepals. Six stamens (st) are found in whorl three and two fused carpels (ca) are located in the innermost fourth whorl.

ABCE model for flower development

The updated ABCE model for flower development proposes that four classes of floral organ identity genes function in overlapping domains to specify different organs types. In Arabidopsis, the A class genes APETALA1 (AP1) and APETALA2 (AP2) act to specify sepal and petal development in whorls one and two, the B class genes APETALA3 (AP3) and PISTILLATA (PI) act to specify petal and stamen development in whorls two and three, and the C class gene AG acts to specify stamen and carpel development in whorls three and four. Proteins encoded by the E class SEPALLATA genes (SEP1-SEP4) work to specify sepal, petal, stamen, and carpel development as cofactors with A, B, and C class proteins. Most of the ABCE genes encode MADS domain transcription factors that are expressed in spatially restricted regions of the floral meristem consistent with their domains of organ identity activities. Alterations in the expression patterns of these genes result in homeotic changes in organ identity.