Introduction

The unique ability of plants to initiate organs throughout their lifespan results from the activity of meristems. The shoot apical meristem, a dome of cells at the apex of the plant, provides cells for the initiation of new organ primordia around its periphery while maintaining a central core of pluripotent stem cells that remain undifferentiated. During reproductive development, the shoot apical meristem (also called an inflorescence meristem) initiates flower primordia around its periphery. These flower primordia (called floral meristems) give rise to floral organ primordia that arise in concentric rings called whorls. Floral meristems differ from shoot meristems in that all floral stem cells are used up in the process of organ initiation. My lab is interested in understanding the molecular mechanisms that regulate the transitioning of pluripotent stem cells toward differentiated cell fates. In addition, we are studying early events in flower development that control the positioning, number, size and identity of floral organ primordia. We study these processes in the model plant Arabidopsis thaliana.

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.