ZUR HOMOLOGIEFRAGE DER BLÜTENORGANE:
LETTER TO THE EDITOR: THE PLANT CELL 6: 574 -577, MAY 1994

Although I appreciate the excellent series of papers on plant reproduction in the October 1993 special issue of The Plant Cell, I would, nevertheless, take exception to some of the basic comments on flower morphology by Coen and Carpenter (1993) in their otherwise informative article.

The authors argue, with Goethe (1790), that flowers and shoots are fundamentally equivalent, that is, that the different parts of a flower (sepals, petals, stamens, carpels) are equivalent to the leaves of a shoot. Because internodes in the flower are "so short as to be barely visible" (p. 1175) or, in contrast to floricaula, "undetectable" (p. 1176), a shortening of the internodes is envisioned. The phyllotaxy of floral organs is usually different from that of leaves, and flower growth is determinate (both apically and laterally) instead of indeterminate .Thus, organ identity, internode length, phyllotaxy, and determinacy have all been changed. These changes are thought to be "simply different modifications of a common growth plan", produced by "different permutations of a few key features of plant growth" (p. 1175)

In positing this equivalence theory of the flower and stem, the authors have, however, not considered the most important aspect of flower morphology: sexuality or sexual reproduction, i.e., that aspect of the flower for which it exists at all. In fact, Goethe, seeing that his theory ("everything is leaf") did not correspond to the sexual aspect of plant reproduction, later denied sexuality in plants (Goethe, 1820), together with the botanists Schelver (1812) and Henschel (1820), although the sexual functions of the stamens and carpels had been shown conclusively by Camerarius (1694), Linné (1735), Kölreuter (1761-1766), and, finally, Sprengel (1793). One should also keep in mind that Goethes statements were made in the tradition of Platonic idealism and had no phylogenetic implications (Goethe speaks repeatedly of the "inner identity" of flowers and shoots and the "identity of all plant parts").

However, there is no equivalence of the flower and shoot concerning sexual reproductive functions. Kamalay and Goldberg (1980, 1984) showed that "both the anther and the ovary contain approximately 10,000 diverse mRNAs that are not detectable in heterologous organ system mRNA or nuclear RNA populations" (Drews and Goldberg, 1989, p. 259). Several other authors have obtained similar results (Willing and Mascarenhas, 1984; Smith et al., 1989; Koltunow et al., 1990). And, beginning with Sommer et al. (1990) and Yanofsky et al. (1990), several flower morphology genes have been cloned and sequenced that are expressed only in the flower and nowhere else.

Whatever the exact number of special regulatory and target sequences that are involved in and expressed during flower development only, the independent results of several authors that a large number of genes are expressed solely in the flower clearly disproves the simple equivalence of flowers and shoots claimed by Goethe and his followers.

In contrast to the equivalence theory (and bearing in mind that from the functional point of view, all the flower structures finally serve, or are produced for, the aim of sexual reproduction and propagation), one could argue that during flower development most of the typical characters of the vegetative shoot are more or less progressively lost, being replaced by the new genetic programs and corresponding features for sexual reproduction. In this respect, the flower is not equivalent to, but essentially different from, and definitely more than, the shoot. Sexuality is the reason why the meristem structure, organ identity, internode length, phyllotaxy, and determinacy of the flower are all so different from those of the shoot. Let it be emphasized again that entirely different genetic programs are switched on during this stage of development. Or, speaking allegorically, a new chapter with a new topic is opened, a new movement of the developmental symphony has begun.

This does not exclude, of course, the possibility that the introductory phase of flower development (sepals and petals) consists largely of the repetition of already known genetic programs (Drews et al., 1992), transformed by a minority of new regulatory gene functions for special developmental and morphological tasks. The main part of flower development, however, is made up of the organs for sexual reproduction, i.e., the stamens and carpels, so that the large majority of the new programs is expressed here.

Although many researchers have felt that mutants transforming flora organs into leaflike structures seem at first glance to imply that flowers and leaves are in fact equivalent, the following homeotic examples will reveal just the opposite. In the well-known plena mutants (see, for example, Masters, 1869), stamens are replaced by petals, and carpels by sepals, followed by further petaloid structures in the central part of the flower. Because the MADS-box gene PLENA in Antirrhinum (and the homologous gene AGAMOUS in Arabidopsis) is essential for turning on the complex genetic programs for stamen and carpel formation, its loss of function also implies the loss of these sex organs, and the simpler petal and sepal programs are continued into the two inner flower whorls - that is, the field where the PLENA gene is normally expressed (Yanofsky et al., 1990; Bradley et al., 1993).

Now, does this loss of PLENA gene function, with the accompanying loss of the thousands of additional diverse mRNAs for sex organ formation, imply that the genetic programs for leaves, stamens, and carpels are all equivalent? Sattler (1988, p. 1607) gives the answer for the homeotic plena change by the following analogous illustration: "If a botanist in a biology department is replaced by a zoologist (as too often happens), the latter is not a transformed botanist simply because his predecessor was a botanist; he only occupies the same position. From the sameness of position, it does not follow that members occupying it are also essentially the same; they may be similar or very different." And in the case of plena, the members (floral organs) are, in fact, very different: Instead of sex organ formation, we find the substitutional expression of the simpler petal and sepal programs. On the other hand, ectopic expression of PLENA or AGAMOUS in the two outer flower whorls and the corresponding formation of carpelloid sepals and staminoid petals (Mizukami and Ma, 1992, Bradley et al., 1993) means, first and foremost, that the additional diverse mRNAs are expressed earlier in development and does not per se prove that the programs for the vegetative structures are equivalent to those of the sex organs.

Even for such similar nonreproductive organs as leaves, sepals, and petals, the results of molecular genetics are not always as simple as might perhaps be expected. In combining Goethe's ideas on metamorphosis with Darwin's theory, green petal mutants have often been interpreted as atavisms. However, one may raise the question of whether changes in the MADS-box gene DEFICENS (DEF), whose alleles cause different degrees of greenish petals in Antirrhinum (Sommer et al., 1990; Schwarz-Sommer et al., 1992), corroborate this idea. The loss of fully functioning binding sites of a transcription factor that normally upregulates expression of DEF in cooperation with GLOBOSA (Tröbner et al., 1992), as well as all their target genes in petals and stamens, shows only that in the mutants the switch to the petal program is not very successful and that (among other homeotic deviations), the sepal program is now more or less continued into the second flower whorl.

The MADS-box sequences are conserved from yeast to humans. As expected, losses of functions in MADS-box genes affect the different biological systems very differently, and there is no trend to classify the deviations as atavisms (for a review, see Davies and Schwarz-Sommer, 1994). Moreover; hardly anyone would assert that the different (promoter; MADS-box, and K-box) alleles of DEF causing different degrees of greenish petals (including the temperature-sensitive allele defA-101) are the atavistic gene sequences of flowering plants. The same can probably be said of the alleles of the homologous APETALA3 gene in Arabidopsis (Jack et al., 1992). The substitution of a late developmental program by an earlier one does not necessarily prove the equivalence of both. Although the bulk of diverse mRNAs is equivalent in leaves and petals, the different governing regulatory systems and the corresponding qualitative and quantitative mRNA differences of these distinct organ classes are obviously not. Because of organ-specific gene expression at the RNA and/or protein levels (Jack et al. 1994), targeted gene tagging can often concentrate on special flower (or other) characteristics (see, for example, Lönnig and Huijser, 1994).

Furthermore, the null alleles of DEF and APETALA3 display not only sepals instead of petals but also carpels in place of stamens. As far as I am aware, it has never been proposed from this that stamens are "derived" from carpels.

In the often quoted case of Goethe's "example of a rose grown through" (Goethe, 1790; Weberling, 1989) - a malformation in which flower development is stopped shortly before, or at the beginning of, anther development and shoot growth is resumed - the flower programs are switched off at this developmental stage and the stem programs are turned on again. Asserting the equivalence of both programs for this reason would be simply illogical.

In short, the expression of special shoot features in abnormal flowers shows only that such shoot programs (or parts of them) were turned on at the wrong time and at the wrong place. They do not prove the equivalence of the genetic programs for stems and flowers any more than the accidental mixing of two different computer text programs on biology and poetry proves their equivalence, although the same letters, words, and basic grammar may be involved (the words may be identical, but the message is not).

Beginning with systematic research by Payer (1857), many other authors have basically come to the same conclusion (involving also the telome theory, in which both stamens and integuments are thought to derive not from leaves but from telomes, as well as the sui generis theories of flower structures and many further ideas and arguments that cannot be discussed here). Abridged histories of the different views are given by Meeuse (1987) and Leroy (1993).

Molecular genetics has definitely disproved Goethe's idea of the equivalence of flowers and shoots by showing that anthers and ovaries express thousands of diverse mRNAs that are not detectable in heterologous organ systems and that certain specific cloned genes are expressed only in the flower and not in the shoot (or vice versa). Although "simply different modifications of a common growth plan" may be envisioned to be involved in some of the changes between the stem and the flower, the basic cause for all the differences in meristem structure, flora axis anatomy, organ identity, internode length, phyllotaxy, and determinacy is sexual reproduction having no equivalence in the stem.

Because misconceptions on this topic are very widespread, I think that publication and discussion of this letter would help plant science to clarify these points.

Bradley, D., Carpenter, R., Sornmer, H., Hartley, N., and Coen, E. (1993). Complementary floral homeotic phenotypes result from opposite orientations of a transposon at the plena locus of Antirrhinum. CeIl 72, 85 - 95.

Camerarius, R.J. (1694). Epistola ad M.B. Valentini de sexu plantarum. In Ostwald's Klassiker der exakten Naturwissenschaften, No. 105, 1899 (Leipzig: Verlag von Wilhelm Engelmann).

Coen, E., and Carpenter, R. (1993). The metamorphosis of flowers. Plant Cell 5, 1175 - 1181.

Davies, D., and Schwarz-Sommer, Z. (1994). Control of floral organ identity by homeotic MADS-box transcription fadons. In Plant Promotens and transoription factors, L. Nover, ed (Berlin: Springer-Verlag), pp. 235 - 258.

Drews, G.N., and Goldberg, R.B. (1989). Genetic control of flower development. Tnends Genet. 5, 256 - 261.

Drews, G.N., Beals, T.B., Bui, A.Q., and Goldberg, R.B. (1992). Regional and cell-specific gene expression patterns during petal development. Plant Cell 4, 1383 - 1404.

Goethe, J.W. (1790). Versuch die Metamorphose der Pflanzen zu erklären. Gotha: Carl Wilhelm Ettinger, 1984 facsimile (Weinheim: Acta humaniora der Verlag Chemie GmbH.)

Goethe, J.W. (1820). Verstäubung, Verdunstung, Vertropfung. In Goethe, Sämtliche Werke, Vol. 12, 1989 (München: Carl-Hanser-Verlag), pp. 212 - 224.

Henschel, A.W. (1820). Von der Sexualltät der Pflanzen (Breslau: Wilhelm Gottlieb Korn).

Jack, T., Brockmann, L.L., and Meyerowitz, E.M. (1992). The homeotic gene APETALA3 of Arabidopsis thaliana encodes a MADS box and is expressed in petals and stamens, Cell 68, 683 - 697.

Jack, T., Fox, G.L., and Meyerowitz, E.M. (1994). Arabidopsis homeotic gene APETALA3 ectopic expression: Transcriptional and posttranscriptional regulation determine flora organ identity. Cell 76, 703 - 716.

Kamalay, J.C., and Goldberg, R.B. (1980). Regulation of structural gene expression in tobacco. Cell 19, 935 - 946.

Kamalay, J.C., and Goldberg, R.B. (1984). Organ-specific nuclear RNAs in tobacco. Proc. Natl. Acad. Sci. USA 81, 2804 - 2804.

Kölreuter, J.G. (1761-1766). Vorläufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden Versuche und Beobachtungen, nebst Fortsetzungen 1, 2, u. 3. In Ostwald's Klassiker der exakten Wissenschaften, No. 41,1893 (Leipzig: Verlag von Wilhelm Engelmann).

Koltunow, A.M., Truettner, J., Cox, K.H., Wallroth, M., and Goldberg, R.B. (1990). Different temporal and spatial gene expression patterns occur during anther development. Plant Cell 2, 1201 - 1224.

Leroy, J.-F. (1993). Origine et évolution des Plantes à fleurs (Paris: Masson).

Linné, C. (1735). Systema Naturae (Leiden: Theodorus Haak).

Lönnig, W.-E., and Huijser, P. (1994). Gene tagging by endogenous transposons. In Plant Molecular Biology Manual, S.B. Gelvin, R.A. Schilperoort, and D.P.S. Verma, eds (Dordrecht: Kluwer Academic Publishers). in press.

Masters, M.T. (1889). Vegetable Teratology (London: Ray Sociely).

Meeuse, A.D.J. (1987). All about Angiosperms (Delft: Eburon).

Mizukami, Y., and Ma, H. (1992). Ectopic expression of the floral homeotic gene AGAMOUS in transgenic Arabidopsis plants alters floral organ identity. Cell 71, 119 - 131.

Payer, J.-B. (1857). Traité d'Organogénie Comparée de la Fleur, reprint, 1986 (Lehre: J. Cramer).

Sattler, R. (1988). Homeosis in plants. Am. J. Bot. 74, 1606 - 1817.

Schelver, F.J. (1812). Kritik der Lehre von den Geschlechtern der Pflanze (Heidelberg: Gottlieb Braun).

Schwarz-Sommer, Z., Hui, I., Huijser, P., Flor, P.-J., Hansen, R., Tetens, F., Lönnig, W.-E., Saedler, H., and Sommer, H. (1992). Characterization of the Antirrhinum floral homeotic MADS-box gene deficiens: Evidence for DNA binding and autoregulation of its persistent expression throughout flower development. EMBO J. 11, 251 - 261.

Smith, A.G., Gasser, C.S., Budelier-Sachs, K.A., Hinchee, M.A., McCormick, S., Horsch, R.B., and Fraley, R.T. (1989). Structure and regulation of organ- and tissue-specific genes: Regulated expression of flower-specific genes. in Cell Culture and Somatic Cell Genetics, J. Schell, and I.K. Vasil, eds (San Diego: Academic Press), pp. 197 - 214.

Sommer, H., Beltran, J.-P., Hujiser, P., Pape, H., Lönnig, W.-E., Saedler, H., and Schwarz-Sommer, Z. (1990). Deficiens, a homeotic gene involved in the control of flower morphogenesis in Antirrhinum majus: The protein shows homology to transcription factors. EMBO J. 9, 805 - 618.

Sprengel, C.K. (1793). Das entdeckte Geheimnis der Natur im Bau und in der Befruchtung der Blumen. In Ostwald's Klassiker der exakten Naturwissenschaften, Nos. 48 - 51, 1894 (Leipzig: Verlag von Wilhelm Engelmann).

Tröbner, W., Ramirez, L., Motte, P., Hue, I., Huijser, P., Lönnig, W.-E., Saedler, H., Sommer, H., and Schwarz-Sommer, Z. (1992). GLOBOSA: A homeotic gene which interacts which DEFICIENS in the control of Antirrhinum floral organogenesis. EMBO J. 11, 4693 - 4704.

Weberling, F. (1989). Morphology of Flowers and Inflorescences (Cambridge: Cambridge University Press).

Willing, R.R, and Mascarenhas, J.P. (1984). Analysis of the complexity and diversity of mRNAs from pollen and shoots of Tradescantia. Plant Physiol. 75, 885 - 868.

Yanofsky, M.F., Ma, H., Bowman, J.L., Drews, G.N., Feldmann, K.A., and Meyerowitz, E.M. (1990). The protein encoded by the Arabidopsis homeotic gene agamous resembles transcription factons. Nature 346, 35 - 39.

Our review (Coen and Carpenter, 1993) describes how both the similarities and differences between flowers and shoots can be explained by the action of a set of genes modifying a common growth plan. This does not imply that flowers and shoots are directly derived from one another or that they serve the same function or that they express all of the same genes.

We would also like to point out that it is verymisleading to claim that Goethe denied sexuality in plants because it did not fit with his theory. He only came to seriously question sexuality in plants 14 years after publication of his theory of metamorphosis after a conversation with F.J. Schelver. On hearing Schelver's doubts about the theory of sexuality, Goethe (1820) was surprised: "In my nature studies I had religiously accepted the dogma of sexuality in plants and was, therefore, taken aback now to near a concept directly opposed to my own."

It is clear that Goethe, quite correctly, did not feel that sexuality in plants was incompatible with his theory of metamorphosis even though he was later attracted by Schelver's new scheme. "Accustomed as I had always been to preserve complete flexibility in my application of metamorphosis, I likewise found this [Schelver's] viewpoint not uncomfortable, although at the same time I could not immediately relinquish the other" (Goethe, 1820).

Coen, E., and Carpenter, R. (1993). The metamorphosis of flowers. Plant Cell 5, 1175 - 1181.

Goethe, J.W. (l820). Natura Science in General; Morphology in Particular, VOl. 1, No. 3. In Goethe's Botanical Writings, trans. B. Mueller (Honolulu: University of Hawaii Press).

Ich hatte das Dogma der Sexualität bei meinen Naturstudien gläubig angenommen und war deshalb jetzt betroffen gerade das meiner Ansicht Entgegengesetzte zu vernehmen; doch konnt' ich die neue Lehre nicht für ganz ketzerisch halten, da aus des geistreichen Mannes Darstellung hervorging: die Verstäubungslelnre sei eine natürliche Folge der mir so werten Metamorphose.

Nun traten mir die, gegen das Geschlechtssystem von Zeit zu Zeit erregten Zweifel sogleich vor die Seele und was ich selbst über diese Angelegenheit gedacht hatte ward wieder lebendig; manche Anschauung der Natur, die mir nun heiterer und folgereicher entgegen trat, begünstigte die neue Vorstellungsart, und da ich ohnehin die Anwendung der Metamorphose nach allen Seiten beweglich zu erhalten gewohnt war, so fand ich auch diese Denkweise nicht unbequem, ob ich gleich jene nicht alsobald verlassen konnte.

...Nun hat durch Henschels gewichtiges Werk die geistige Lehre einen Körpergewonnen; sie verlangt ernstlich ihren Platz in der Wissenschaft, obgleich nicht abzusehen ist, wie man ihr denselben einräumen könne. Indessen ist die Gunst für sie schonbelebt; Rezensenten, anstatt, von ihrem früheren Standpunkte her, widersprechend zu schelten, gestehen sich bekehrt, und min muß nun abwarten was sich weiter hervortun wird.

...Schelver geht aus von dem eigentlichsten Begriff der gesunden und geregelten Metamorphose, welcher enthält, dass das Pflanzenleben, in den Boden gewurzelt, gegen Luft und Licht strebend, sich immer auf sich selbst erhöhe und, in stufenweiser Entwicklung, den letzten abgesonderten Samen aus eigner Macht und Gewalt umher streue; das Sexual-System dagegen verlangt zu diesem endlichen Hauptabschluß ein Äußeres, welches mit und neben der Blüte, oder auch abgesondert von ihr als dem Innersten entgegengesetzt wahrgenommen und einwirkend gedacht wird.

...Diese neue Verstäubungslehre wäre nun beim Vortrag gegen junge Personen und Frauen höchst willkommen und schicklich: denn der persönlich Lehrende war bisher durchaus in großer Verlegenheit. Wenn sodann auch solche unschuldige Seelen, um durch eigenes Studium weiter zu kommen, botanische Lehrbücher in die Hand nahmen, so konnten sie nicht verbergen daß ihr sittliches Gefühl beleidigt sei, die ewigen Hochzeiten, die man nicht los wird, wobei die Monogamie, auf welche Sitte, Gesetz und Religion gegründet sind, ganz in eine vage Lüsternheit sich auflöst, bleiben dem reinen Menschensinne völlig unerträglich.