Sepal Identity of the Pappus and Floral Organ Development in the Common Dandelion (Taraxacum officinale; Asteraceae)

The dry one-seeded fruits (cypselae) of the Asteraceae are often crowned with a pappus, an appendage of hairs or scales that assists in dispersal. It is generally assumed, but little investigated, that the pappus represents the outer floral whorl where the sepals are usually located. We analysed pappus–sepal homology in dandelions using micromorphological and floral gene expression analyses. We show that the pappus initiates from a ring primordium at the base of the corolla, heterochronic to the petals. Pappus parts form from this ring, with those in the alternipetalaous position usually being ahead in growth, referring to sepal identity. Tof-APETALLA1 expression increased during floret development and was highest in mature pappus. Tof-PISTILLATA expression was high and confined to the floral tissues containing the petals and stamens, consistent with expectations for sepals. Apart from the pappus, the dispersal structure of dandelion consists of the upper part of the fruit, the beak, which originates from the inner floral whorl. Thus, our results support the homology of the pappus with the sepals, but show that it is highly derived. Together with our floral stage definitions and verified qPCR reference genes, they provide a basis for evolution and development studies in dandelions and possibly other Asteraceae.

PETAL LOSS gene regulates initiation and orientation of second whorl organs in the Arabidopsis flower

PETAL LOSS is a new class of flower development gene whose mutant phenotype is confined mostly to the second whorl. Two properties are disrupted, organ initiation and organ orientation. Initiation is frequently blocked, especially in later-formed flowers, or variably delayed. The few petals that arise occupy a wider zone of the flower primordium than normal. Also, a minority of petals are trumpet-shaped, thread-like or stamenoid. Studies of ptl combined with homeotic mutants have revealed that the mutant effect is specific to the second whorl, not to organs with a petal identity. We propose that the PTL gene normally promotes the induction of organ primordia in specific regions of the second floral whorl. In ptl mutants, these regions are enlarged and organ induction is variably reduced, often falling below a threshold. A dominant genetic modifier of the ptl mutant phenotype was found in the Landsberg erecta strain that significantly boosts the mean number of petals per flower, perhaps by reinforcing induction so that the threshold is now more often reached. The second major disruption in ptl mutants relates to the orientation adopted by second whorl organs from early in their development. In single mutants the full range of orientations is seen, but when B function (controlling organ identity) is also removed, most second whorl organs now face outwards rather than inwards. Orientation is unaffected in B function single mutants. Thus petals apparently perceive their orientation within the flower primordium by a mechanism requiring PTL function supported redundantly by that of B class genes.

Floral-specific polypeptides in Silene coeli-rosa

In Silene coeli-rosa (L.) Godron given a 7-long-day inductive treatment, scanning electron microscopy and two-dimensional minigel electrophoreses of total proteins were used to characterize the polypeptide pattern of each type of floral organ through early differentiation and to research the changes that occurred in the reproductive apex that initiated each new floral whorl. During early differentiation of each whorl, some polypeptides no longer expressed in the subsequent whorls were distinguished as unique to sepals (24), petals (7), and stamens (4). Newly expressed polypeptides were also observed that occasionally persisted in the subsequent whorl. However, qualitative changes were only between 1.2 and 3.8% of all the detected spots, and common spots remained the most numerous, even if a modulation of their expression in the various types of floral organs was observed. Comparison between leaves and differentiating floral organs showed that sepals shared 57% of their polypeptide spots with leaves, whereas petals, stamens, or carpels shared only 14.6, 10.5, and 7.7%, respectively. In the reproductive apex, polypeptides newly detected or unique to a particular whorl were expressed at the time of initiation of this whorl. However, some of these spots were also detected before, in the apex that initiated the preceding whorl, or they persisted later, in the apex that initiated the following whorl. Key words: floral organs, polypeptides, reproductive apex, Silene coeli-rosa.