Botanical fruits derive from ovaries and their most important function is to favor seed dispersal. Fleshy fruits do so by attracting frugivorous animals that disperse seeds together with their own excrements (endozoochory). Gymnosperms make seeds but have no ovaries to be transformed into fruits. Many species surround their seeds with fleshy structures and use endozoochory to disperse them. Such structures are functionally fruits and can derive from different anatomical parts. Ginkgo biloba and Taxus baccata fruit-like structures differ in their anatomical origin since the outer seed integument becomes fleshy in Ginkgo, whereas in Taxus, the fleshy aril is formed de novo. The ripening characteristics are different, with Ginkgo more rudimentary and Taxus more similar to angiosperm fruits. MADS-box genes are known to be necessary for the formation of flowers and fruits in Angiosperms but also for making both male and female reproductive structures in Gymnosperms. Here, a series of different MADS-box genes have been shown for the first time to be involved also in the formation of gymnosperm fruit-like structures. Apparently, the same gene types have been recruited in phylogenetically distant species to make fleshy structures that also have different anatomical origins. This finding indicates that the main molecular networks operating in the development of fleshy fruits have independently appeared in distantly related Gymnosperm taxa. Hence, the appearance of the seed habit and the accompanying necessity of seed dispersal has led to the invention of the fruit habit that thus seems to have appeared independently of the presence of flowers.

This study suggests that B-sister genes have a main function in ovule/seed development and a subsidiary role in the formation of fleshy fruit-like structures when the latter have an ovular origin, as occurs in Ginkgo. Thus, the 'fruit function' of B-sister genes is quite old, already being present in Gymnosperms as ancient as Ginkgoales, and is also present in Angiosperms where a B-sister gene has been shown to be involved in the formation of the Arabidopsis fruit.

BackgroundThe identity of flower organs is specified by various MIKC MADS-box transcription factors which act in a combinatorial manner. TM8 is a MADS-box gene that was isolated from the floral meristem of a tomato mutant more than twenty years ago, but is still poorly known from a functional point of view in spite of being present in both Angiosperms and Gymnosperms, with some species harbouring more than one copy of the gene. This study reports a characterization of TM8 that was carried out in transgenic tomato plants with altered expression of the gene.ResultsTomato plants over-expressing either TM8 or a chimeric repressor form of the gene (TM8:SRDX) were prepared. In the TM8 up-regulated plants it was possible to observe anomalous stamens with poorly viable pollen and altered expression of several floral identity genes, among them B-, C- and E-function ones, while no apparent morphological modifications were visible in the other whorls. Oblong ovaries and fruits, that were also parthenocarpic, were obtained in the plants expressing the TM8:SRDX repressor gene. Such ovaries showed modified expression of various carpel-related genes. No apparent modifications could be seen in the other flower whorls. The latter plants had also epinastic leaves and malformed flower abscission zones. By using yeast two hybrid assays it was possible to show that TM8 was able to interact in yeast with MACROCALIX.ConclusionsThe impact of the ectopically altered TM8 expression on the reproductive structures suggests that this gene plays some role in the development of the tomato flower. MACROCALYX, a putative A-function MADS-box gene, was expressed in all the four whorls of fully developed flowers, and showed quantitative variations that were opposite to those of TM8 in the anomalous stamens and ovaries. Since the TM8 protein interacted in vitro only with the A-function MADS-box protein MACROCALYX, it seems that for the correct differentiation of the tomato reproductive structures possible interactions between TM8 and MACROCALYX proteins might be important.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0319-y) contains supplementary material, which is available to authorized users.

One successful mechanism of seed dispersal in plants involves production of edible fleshy structures which attract frugivorous animals and transfer this task to them. Not only Angiosperms but also Gymnosperms may use the fleshy fruit habit for seed dispersal, and a similar suite of MADS-box genes may be expressed as these structures form. Magnolia grandiflora produces dry follicles which, at maturity, open to reveal brightly colored fleshy seeds. This species thus also employs endozoochory for seed dispersal, although it produces dry fruits. Molecular analysis reveals that genes involved in softening and color changes are expressed at late stages of seed development, when the fleshy seed sarcotesta softens and accumulates carotenoids. Several MADS-box genes have also been studied and results highlight the existence of a basic genetic toolkit which may be common to all fleshy fruit-like structures, independently of their anatomic origin. According to their expression patterns, one of two AGAMOUS genes and the three SEPALLATA genes known so far in Magnolia are of particular interest. Duplication of AGAMOUS already occurs in both Nymphaeales and Magnoliids, although the lack of functional gene analysis prevents comparisons with known duplications in the AGAMOUS lineage of core Eudicots.

BackgroundThe involvement of MADS-box genes of the AGAMOUS lineage in the formation of both flowers and fruits has been studied in detail in Angiosperms. AGAMOUS genes are expressed also in the reproductive structures of Gymnosperms, yet the demonstration of their role has been problematic because Gymnosperms are woody plants difficult to manipulate for physiological and genetic studies. Recently, it was shown that in the gymnosperm Ginkgo biloba an AGAMOUS gene was expressed throughout development and ripening of the fleshy fruit-like structures produced by this species around its seeds. Such fleshy structures are evolutionarily very important because they favor the dispersal of seeds through endozoochory. In this work a characterization of the Ginkgo gene was carried out by over-expressing it in tomato.ResultsIn tomato plants ectopically expressing the Ginkgo AGAMOUS gene a macroscopic anomaly was observed only in the flower sepals. While the wild type sepals had a leaf-like appearance, the transgenic ones appeared connately adjoined at their proximal extremity and, concomitant with the development and ripening of the fruit, they became thicker and acquired a yellowish-orange color, thus indicating that they had undergone a homeotic transformation into carpel-like structures. Molecular analyses of several genes associated with either the control of ripening or the ripening syndrome in tomato fruits confirmed that the transgenic sepals behaved like ectopic fruits that could undergo some ripening, although the red color typical of the ripe tomato fruit was never achieved.ConclusionsThe ectopic expression of the Ginkgo AGAMOUS gene in tomato caused the homeotic transformation of the transgenic sepals into carpel-like structures, and this showed that the gymnosperm gene has a genuine C function. In parallel with the ripening of fruits the related transgenic sepals became fleshy fruit-like structures that also underwent some ripening and such a result indicates that this C function gene might be involved, together with other gens, also in the development of the Ginkgo fruit-like structures. It seems thus strengthened the hypothesis that AGAMOUS MADS-box genes were recruited already in Gymnosperms for the development of the fleshy fruit habit which is evolutionarily so important for the dispersal of seeds.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0418-x) contains supplementary material, which is available to authorized users.

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