Inside the gynoecium: at the carpel margin

• Carpel marginal meristem (CMM) gives rise to the marginal tissues.
• Transcription factors and hormones guide its development.
• Auxins, brassinosteroids, and cytokinins are important for CMM and marginal tissue development.
• Hormonal cross-talk may be defining the different regions of the gynoecium.

The gynoecium, which is produced at the center of most flowers, is the female reproductive organ and consists of one or more carpels. The Arabidopsis gynoecium consists of two fused carpels. Its inner tissues possess meristematic characteristics and are called the carpel margin meristem (CMM), because they are located at the margins of the carpels and generate the ‘marginal’ tissues of the gynoecium (placenta, ovules, septum, transmitting tract, style, and stigma). A key question is which factors are guiding the correct development of all these tissues, many of which are essential for reproduction. Besides regulatory genes, hormones play an important part in the development of the marginal tissues, and recent reports have highlighted the role of cytokinins, as discussed in this review.