SEB Bulletin March 2005
She loves me, she loves me not…
Flowers are romantically plucked by lovers to solve this dilemma, according to whether they have an odd or even number of petals. But what is it, at a molecular level, that determines the characteristics of a flower; the shape, the structure, the number of petals and, hence, the romantic fate of couples?
Flower of an Arabidopsis thaliana plant
homozygous for a loss-of-function
mutation in the AGAMOUS gene.
Photo courtesy of José Luis Reichmann
Scientists are keen to answer such questions because flowers could be adapted to increase crop yield by controlling the timing of flowering, the flower's architecture (shape and size), or their persistence beyond flowering. At the Plant Gene Expression centre in California Professor Sarah Hake1 studies flower development. “Understanding how the flowers of grasses are organised may lead to clues that help improve these important crops”, she says. “We aim to improve crop yields by understanding plant architecture”.
The development of flowers is studied using a variety of techniques. Mutants, commonly of the model plants Arabidopsis and Antirrhinum, that have flowering defects often reveal genes which encode proteins that are involved in the process of flower formation. It is not surprising that, for such a complicated and diverse structure, there are many genes that play a role in initiating and patterning a flower2. Further players can be identified by screening for proteins that interact with known developmental control proteins3 to build up a picture of the complex pathways that together define a flower.
Wild-type Antirrhinum majus flower
and the double-mutant
(CYCLOIDEA and DICHOTOMA).
Photo courtesy of John Innes Centre.
The sequence of events preceding the appearance of a flower in Arabidopsis can be broken down into defined stages. First the plant is prompted to flower, by a 'floral inductive cue', so that the shoot apical meristem (SAM) converts to reproduction as an inflorescence meristem4. The SAM is a group of cells at the growing tip of the plant that constantly divide and has the potential to differentiate into a variety of different cell types (like stem cells in animals5). In turn, this inflorescence meristem sprouts floral meristems, but these are determinate, i.e. they are only able to make a flower. Cells on the surface of this floral meristem are the precursors of the four floral organs; sepals, petals, stamens and carpels. The group of Professor Nick Battey at the University of Reading6 are working on the floral meristem of Impatiens. “We have found that, in Impatiens, reversion to a non-flowering shoot apical meristem from the floral meristem occurs because leaves fail to supply a permanent floral signal and the meristem does not commit completely to flowering. This means that when these plants are moved from conditions that induce flowering to non-inductive conditions they are able to stop flowering, whereas in other species of plants, once flowering is induced it cannot be reversed”, says Tinashe Chiurugwi, a researcher in the Reading lab.
To flower or not to flower ….
The gene TERMINAL FLOWER 1 (TFL1) is needed for the establishment and maintenance of an inflorescence meristem; removing TFL1 results in a plant with only one short inflorescence meristem that has just a single terminal flower7. An enhancer (tfl2) of the tfl1 mutant phenotype helps the plant decide when to flower (the subject of flowering time is dealt with more in another SEB Bulletin8). TFL1, is found to be expressed at the correct time (2-3 days onwards) and in an appropriate place (the SAM) for a protein that is involved in maintaining an indeterminate inflorescence meristem. Therefore in order to actually progress to the determinate and sole fate of flowering TFL1 needs to be excluded from the SAM.
Two proteins, LEAFY (LFY) and APETALA1 (AP1), encoded by floral meristem identity genes are responsible for inhibiting TFL1. Overexpressing either LFY or AP1 results in a phenotype similar to that seen when TFL1 is mutated. The unique transcription factor LFY acts upstream of AP1 in the floral developmental pathway. AP1 is then responsible for preventing TFL1 transcription in the meristem but TFL1 antagonises the effects of LFY and AP1 so that a constant war is being waged by these genes, the outcome of which is determined by the relative timing and position of expression9. Another important protein involved in specifying the floral meristem and controlled by LFY is AGAMOUS (AG)10 . This protein is also expressed in the apex of the floral meristem and promotes flowering by making the floral meristem determinate so that a set number of organs are produced and then further proliferation is prevented
Easy as ABC
Only by specifying the position and type of floral organ on the floral meristem can a flower arise. Without these molecular instructions for making a flower some beautifully strange mutants are formed. In these 'homeotic' mutants one organ is substituted by another organ, prompting researchers Coen and Meyerowitz11 to postulate the ABC model of floral development. This simple model proposes 3 classes of genes (A, B and C) with overlapping expression in whorls so that A alone encodes sepals, B overlapping with A encodes petals, B overlapping with C encodes stamens, while C alone encodes the carpel. A and C expression is confined to distinct whorls since these genes negatively regulate each other's expression.
In the years following the formulation of the ABC theory, genes have been identified in several plants that correspond to genes A, B and C. For example in Arabidopsis, the protein encoded by AP2 is confined to the outer and second whorl and so is a representative of the A-class genes. Plants mutated for AP2 have carpels where sepals should be and stamens in place of petals, because the C-class gene function has spread throughout all four whorls. The converse happens when the AG (C-class and floral meristem gene) is mutated; sepals appear in place of carpels and petals in place of stamens. The laboratory of Professor Meyerowitz12 studies the role of the C-class gene. “We wanted to understand in detail one of the things that the C-function gene, AG, does in terms of stamen development. We found out that it turns on a gene, which we show is itself sufficient to activate pollen formation. Thus the C gene is now known to act by selectively activating many downstream regulatory genes, that individually carry out only a subset of the overall C function”.
Scanning electron micrograph of
organ primordia in wild type plants of.
Photo courtesy of Elliot Mayerowitz
Meyerowitz is realistic about our current knowledge of floral patterning and sums it up by saying, “The ABC model describes at a very schematic level the function of a small number of regulatory genes in one thin slice of developmental time. Flower development is much more than organ specification, and organ specification itself has only been described in the broadest terms. The basic issue of pattern formation is only partly answered by the ABC model - we still don't know why the ABC genes only turn on in the right places in the flower primordium, and after they turn on, we don't know how they lead to highly structured organs, with many cell types”.
Symmetry
We have considered scenarios of a plant making a flower versus not making one, or having normal flowers versus having flowers with abnormally arranged organs, but so far we have not considered which genes are involved in creating distinct forms of flowers. Some research done with Arabidopsis shows that certain genes (CLAVATA, WIGGUM, SHAGGY, AINTEGUMENTA, TOUSLED, PETAL LOSS, ETTIN to name a few delightfully titled genes13) are involved in defining the number and position of organs, while others (YABBY, KANADI, REVOLUTA, PRESSED FLOWER14) specify the polarity of plant organs (e.g. the near and far, or inside and outside cells of petals).
Antirrhinum has been used as a representative of plants displaying bilaterally symmetrical flowers. The distinctive 'snapdragon' flower heads have one plane of symmetry and are made up of three types of petal; dorsal, lateral and ventral. Four genes, CYCLOIDEA, DIVARICATA, RADIALIS and DICHOTOMA, are responsible for making asymmetric flower shapes in Antirrhinum. Dr Manuela Costa, a postdoctoral worker in Professor Enrico Coen's group at the John Innes Centre, Norwich15, works on these genes. “We are analysing how these genes, all coding for transcription factors, interact to establish an asymmetric pre-pattern in the Antirrhinum floral meristem and we are exploring the extent to which these processes are conserved in Arabidopsis, a species with radially symmetric flowers”, she says. “Apart from using current molecular biology techniques we make use of a novel imaging method called Optical Projection Tomography. This system was set up by Karen Lee in collaboration with the OPT and Mouse Atlas Groups in Edinburgh16 and allows us to determine the 3D organisation of plant form and gene expression during development”.
A blossoming area of research
Despite all this knowledge there is still so much more to be learnt about flower development. Some of the processes remain enigmatic, for example little is known about which genes are involved in the initial formation of an inflorescence meristem, whereas other processes require integration into the bigger picture. Indeed, Professor Nick Battey suggests that future work should aim for “a better understanding of the molecular and genetic networks that control flowering. This could be achieved by studying a wider range of species, that have diverse flowering behaviours and different flowering forms”. So the message for lovers plucking off petals is to make sure that you select a species or mutant, with the appropriate genes turned on or off, to ensure the correct number of petals for a prophesy of reciprocated love!
Lucy Moore
University of Oxford
An extended version is available on the SEB website at www.sebiology.org/Publications/pageview.asp?S=7&mid=13
References
http://arjournals.annualreviews.org/doi/full
/10.1146/annurev.%20biochem.73.011303.
073950http://www.sebiology.org/Publications/pageview.
asp?S=7& mid=&id=282http://darkwing.uoregon.edu/~uocomm/experts
/faculty-data/Meeks-%20Wagner+_Douglas_Ry.
htmlhttp://www.sebiology.org/Education/pageview.
asp?S=6& mid=&id=239http://www.bio.psu.edu/biology/directory/
homepages/hxm16.actionCoen, E.S., Meyerowitz, E.M., 1991. The war
of the whorls: genetic interactions controlling
flower development. Nature, 353, 6339: 31-7Running, M.P., and Hake, S., 2001. The Role
of Floral Meristems in Patterning. Current
Opinion in Plant Biology, 4: 69-74
