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We have characterized the tomato (Lycopersicon esculentum Mill.) MADS box gene TM29 that shared a high amino acid sequence homology to the Arabidopsis SEP1, 2, and 3 (SEPALLATA1, 2, and 3) genes. TM29 showed similar expression profiles to SEP1, with accumulation of mRNA in the primordia of all four whorls of floral organs. In addition, TM29 mRNA was detected in inflorescence and vegetative meristems. To understand TM29 function, we produced transgenic tomato plants in which TM29 expression was down-regulated by either cosuppression or antisense techniques. These transgenic plants produced aberrant flowers with morphogenetic alterations in the organs of the inner three whorls. Petals and stamens were green rather than yellow, suggesting a partial conversion to a sepalloid identity. Stamens and ovaries were infertile, with the later developing into parthenocarpic fruit. Ectopic shoots with partially developed leaves and secondary flowers emerged from the fruit. These shoots resembled the primary transgenic flowers and continued to produce parthenocarpic fruit and additional ectopic shoots. Based on the temporal and spatial expression pattern and transgenic phenotypes, we propose that TM29 functions in floral organ development, fruit development, and maintenance of floral meristem identity in tomato.Flower development has been the subject of intensive studies over the last decade, particularly in the model plants Arabidopsis and snapdragon (Antirrhinum majus). These studies led to the formulation of the ABC model of floral organ identity, which explained the activities of three classes of genes in specifying the identity of floral organs (Weigel and Meyerowitz, 1994). This model has been supported by genetic and molecular data in a wide range of angiosperm species.According to the ABC model, expression of a class A gene specifies the formation of sepals (the first whorl organ); in combination with the class B genes expression, specifies petal formation. Expression of class B genes and a class C gene specifies stamen identity, whereas expression of C alone determines a carpel identity (Coen and Meyerowitz, 1991;Weigel and Meyerowitz, 1994). Most of the ABC genes belong to the MADS box family (Yanofsky et al., 1990;Jack et al., 1992;Mandel et al., 1992;Goto and Meyerowitz, 1994).Although the ectopic expressions of the ABC genes are sufficient to determine various floral organ identities within the floral meristem, they are insufficient to convert vegetative leaves to floral organs. This suggested that other regulators, in addition to the ABC genes, are required for floral organ specification. Recently, a group of three related MADS box genes SEP 1, 2, and 3 (SEPALLATA 1, 2, and 3) were shown to be necessary for the activity B and C class genes in the control of floral organ formation. First, the SEP1, SEP2, and SEP3 (formerly AGL2, AGL4, and AGL9) redundantly control the activities of the B and C organ identity genes in Arabidopsis because the triple mutant sep1sep2sep3 flower consists entirely of sepals. The sep1/2/...

Fruit development in higher plants normally requires pollination and fertilization to stimulate cell division of specific floral tissues. In some cases, parthenocarpic fruit development proceeds without either pollination or fertilization. Parthenocarpic fruit without seed has higher commercial value than seeded fruit. Several apple (Malus domestica) mutants (Rae Ime, Spencer Seedless and Wellington Bloomless) are known to produce only apetalous flowers that readily go on to develop into parthenocarpic fruit. Through genetics, a single recessive gene has been identified to control this trait in apple. Flower phenotypes of these apple mutants are strikingly similar to those of the Arabidopsis mutant pistillata (pi), which produces flowers where petals are transformed to sepals and stamens to carpels. In this study, we have cloned the apple PI homolog (MdPI) that shows 64% amino acid sequence identity and closely conserved intron positions and mRNA expression patterns to the Arabidopsis PI. We have identified that in the apetalous mutants MdPI has been mutated by a retrotransposon insertion in intron 4 in the case of Rae Ime and in intron 6 in the case of Spencer Seedless and Wellington Bloomless. The insertion apparently abolishes the normal expression of the MdPI gene. We conclude that the loss of function mutation in the MdPI MADS-box transcription factor confers parthenocarpic fruit development in these apple varieties and demonstrates another function for the MADS- box gene family. The knowledge generated here could be used to produce parthenocarpic fruit cultivars through genetic engineering.

A transformation system was developed for the commercial apple (Malus X domestica Borkh.) cultivar Royal Gala. Leaf pieces from in vitro-grown shoots were cocultivated for 2 days with Agrobacterium tumefaciens strain LBA4404 containing the binary vectors pKIWI105 or pKIWI110. Shoots were produced on a shooting medium containing kanamycin (50 mg·L(-1)). A 2-day incubation period on kanamycin-free medium prior to antibiotic selection enhanced the regeneration of kanamycin-resistant shoots. The majority of the kanamycin-resistant shoots also expressed GUS (β-glucuronidase) activity. The putatively transformed shoots were rooted on a medium containing kanamycin (50 mg·L(-1)). Rooted plants were established in a greenhouse, and plants transformed with pKIWI110, which contains a mutant Arabidopsis acetolactate synthase gene, were shown to be resistant to the herbicide Glean(™). Integration of T-DNA into the apple genome was confirmed by PCR and Southern hybridization analyses.

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