Mitochondrial gene expression in stamens is differentially regulated during male gametogenesis in Citrus unshiu

Abstract

SummaryMitochondrial (mt) gene expression profiles in anthers and their relation to developmental events have been studied in only a few plant species. In the present study, expression of three mt genes atp6, cob and coxII which code for subunit VI of the F1F0-ATP synthase, a subunit of cytochrome b, and subunit II of the cytochrome oxidase complex, respectively, were characterised by northern blotting and in situ hybridisation during stamen development in Citrus unshiu. In Citrus leaves, flower buds at different stages of development, petals and immature fruits, the cob gene was transcribed to produce a single mRNA, while the genes atp6 and coxII each produced two transcripts, respectively. All three mt genes shared similar expression patterns. mRNAs accumulated in flower buds at the microsporocyte meiosis stage, in leaves, and were approximately equally abundant in all other tissues investigated, which indicates organ-specific expression levels. Mitochondrial transcript levels were low in pollen mother cells, increased during meiosis and accumulated to highest abundance during the tetrad stage. The mitochondrial mRNA levels then decreased slightly during the early microspore stage, declined markedly during the mature microspore stage, and finally resulted in the abortion of pollen. mRNA levels in tapetal cells were also temporally regulated during pollen development. Expression levels of all three mt mRNAs were low during the pollen mother cell stage, increased and reached a peak at the tetrad stage, then decreased rapidly during the free microspore stage, finally becoming undetectable in surviving tapetal cells. These results demonstrate that the timing and cell-specificity of mt gene expression are differentially regulated during the development of stamens and pollen, which may reflect the changing respiratory status of mitochondria during male gametophyte development.