Ectopic expression of CsMADS24, an AGAMOUS ortholog from cucumber, causes homeotic conversion of sepals into carpels in transgenic arabidopsis plants

The floral homeotic C-function MADS gene AGAMOUS (AG) in Arabidopsis plays crucial roles in specifying stamen and carpel identities as well as determining floral meristem. However, there have been only a few studies of floral homeotic C-function genes in cucumber thus far. In the present study, CsMADS24, a putative AG ortholog from cucumber, was isolated and characterized. Sequence analysis and protein sequence alignment revealed that the deduced CsMADS24 protein contained the typical MIKC structure and the N-terminal extension, as well as two highly conserved AG motifs (I and II). Phylogenetic analysis showed that CsMADS24 fell into the clade of core eudicots, while being distant from the AG orthologs of basal eudicots, monocots and gymnosperms. Expression analysis by RT-PCR showed that CsMADS24 was exclusively expressed in female flower buds. In situ hybridization revealed that CsMADS24 expression was only detected in the carpels. Functional analyses indicated that the sepals were partly converted into carpelloid-like structures in 35S::35S::CsMADS24 transgenic plants. In addition, earlier flowering and delayed floral organ abscission during the development of siliques were also observed in transgenic Arabidopsis. Our findings demonstrate that the AG ortholog plays an exclusive role in carpel specification of cucumber, providing a basis for revealing the mechanisms of reproductive development in cucumber.

Cell-to-cell movement of mitochondria in plants

We report cell-to-cell movement of mitochondria through a graft junction. Mitochondrial movement was discovered in an experiment designed to select for chloroplast transfer from Nicotiana sylvestris into Nicotiana tabacum cells. The alloplasmic N. tabacum line we used carries Nicotiana undulata cytoplasmic genomes, and its flowers are male sterile due to the foreign mitochondrial genome. Thus, rare mitochondrial DNA transfer from N. sylvestris to N. tabacum could be recognized by restoration of fertile flower anatomy. Analyses of the mitochondrial genomes revealed extensive recombination, tentatively linking male sterility to orf293, a mitochondrial gene causing homeotic conversion of anthers into petals. Demonstrating cell-to-cell movement of mitochondria reconstructs the evolutionary process of horizontal mitochondrial DNA transfer and enables modification of the mitochondrial genome by DNA transmitted from a sexually incompatible species. Conversion of anthers into petals is a visual marker that can be useful for mitochondrial transformation.

Isolation and characterization of novel mutants of Arabidopsis thaliana defective in flower development

We have isolated a number of mutants of Arabidopsis thaliana, a member of the mustard family, that have defects in flower development and morphogenesis. Of these, five mutants have been extensively characterized. Two mutants (Fl-40, Fl- 48) lacking petals show homeotic conversion of sepals to carpels. One mutant (Fl-54) displays highly variable phenotypes, including several types of homeotic variations, loss or distorted positions of the floral organs as well as abnormal structures on the inflorescence. Two other mutants (Fl-82, Fl-89) show aberrant structures in the pistils. Genetic analyses have revealed that these mutations are single and recessive, except for one mutant whose mutational loci still remain to be determined. These mutants may prove useful for the analysis of the genetic control of flower development and morphogenesis in the higher plant.