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

Kitty Vijverberg,Barbara Gravendeel,Marjan Kraaij,Monique Welten,Bertie Joan Van Heuven,Erik Smets

doi:10.3390/plants10081682

Kitty Vijverberg, Barbara Gravendeel + Show 4 more

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https://doi.org/10.3390/plants10081682

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Abstract

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.

Highlights

We investigated the development of the pappus in dandelion in detail, aiming to develop the use of dispersal capacity for urban evolution studies in plants
The majority of species studied by Harris [68], showed a pappus ring preceding the origin of many pappus part primordia on this ring, as we found in T. officinale
Micro-morphological and molecular data provide ample evidence for the more generally accepted, but little investigated, hypothesis that the pappus in Asteraceae is homologous to the whorl of sepals

Summary

Introduction

We investigated the development of the pappus in dandelion in detail, aiming to develop the use of dispersal capacity for urban evolution studies in plants. The pappus plays an important role in the dispersal of the seeds and fruits of the Asteraceae [1,2], and variation in the pappus may directly lead to differences in dispersal ability [3,4] allowing plants to anticipate changing environments. To measure such changes and determine how quickly they occur in response to changing environments, we should first characterize pappus development and investigate pappus–sepal homology. Our results serve as a reference for future studies on the development of dandelion florets and evolution of dispersal capacity

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