Floral organ size

One aspect of flower development that is not well understood is organ size control. Arabidopsis plants grown under similar environmental conditions produce floral organs of the same size, suggesting that organ size is largely controlled by intrinsic developmental mechanisms. However, the size of floral organs can vary significantly between closely related plants. Since the size of an organ is dependent upon both the number and size of its constituent cells, the amount of cell growth and cell division within an organ determines its size. Cell division is usually coordinated with cell growth, but the links between these two processes have not been clearly defined. Furthermore, genetic manipulations that alter cell number and/or cell size are often not sufficient to change the final size of an organ. Compensatory mechanisms appear to act within developing organs to maintain overall organ size.

Components of the genetic circuitry underlying organ size regulation in plants are starting to be identified. One gene that plays a critical role in controlling organ size in Arabidopsis is AINTEGUMENTA (ANT). Mutations in ANT result in the production of smaller floral organs, while overexpression of ANT is sufficient to increase organ size. ANT encodes a member of the AP2/ERF transcription factor family, whose members contain either one or two copies of a DNA-binding domain called the AP2 repeat. ANT acts as a transcription factor in plants and is thought to promote growth within developing organs by maintaining cells in a cell-division competent state. We are currently trying to identify downstream targets of ANT.

Seven AP2/ERF genes with sequence similarity to ANT within the AP2 domains are present within the Arabidopsis genome. These AINTEGUMENTA-like (AIL) genes are expressed in young actively dividing tissues of a plant. AIL1, AIL5, AIL6, and AIL7 are expressed in inflorescences in somewhat overlapping but distinct patterns. We are in the process of determining the roles of the four genes during flower development. AIL5, like ANT, may play a role in organ growth, as plants overexpressing AIL5 produce larger floral organs. Our results suggest that AIL genes specify growth competent states within a plant.

Floral organ fusion

The formation of compound organs by the fusion of individual organ primordia can result in quite different organ morphologies (compare the simple distinct petals of an Arabidopsis thaliana flower in a with the tubular corolla of a Silene latifolia flower in b). Changes in floral morphology resulting from organ fusion events are likely to be under strong selection because of their great potential to alter fitness. While organs of primitive flowers were most likely separate entities, fusion of floral organs is now quite common and can occur between organs of the same or different types. While only carpels are fused in wild-type Arabidopsis flowers, mutants that exhibit fusion between other floral organs have been identified.

Currently, we are focused on understanding the development of separate, unfused sepals in the outermost whorl of the flower. Our characterization of the RABBIT EARS (RBE) gene has led us to propose the existence of novel interwhorl signaling pathways that regulate cell proliferation in the outer two floral whorls. These signaling pathways promote petal initiation and growth in the second whorl and the development of separate sepals in the first whorl. Mutations in components of these pathways lead to reduced or altered petal growth and fusion between adjacent sepals. Thus, these pathways are required to maintain boundaries between adjacent sepal primordia, allowing the development of four distinct sepals. Besides RBE, which encodes a SUPERMAN-like protein with a single Cys₂His₂ zinc finger and a EAR-like transcriptional repression motif, two other important factors in these pathways are the trihelix transcription factor PETAL LOSS (PTL) and the F-box protein UNUSUAL FLORAL ORGANS (UFO).