Male-sterility induction in transgenic tobacco plants with an unedited atp9 mitochondrial gene from wheat.

Abstract

Cytoplasmic male sterility in plants is associated with mitochondrial dysfunction. We have proposed that a nuclear-encoded chimeric peptide formed by mitochondrial sequences when imported into the mitochondria may impair organelle function and induce male sterility in plants. A model developed to test this hypothesis is reported here. Assuning that the editing process in higher plant mitochondria reflects a requirement for producing active proteins, we have used edited and unedited coding sequences of wheat ATP synthase subunit 9 (atp9) fused to the coding sequence of a yeast coxlV transit peptide. Transgenic plants containing unedited atp9 exhibited either fertile, semifertile, or male-sterile phenotypes; controls containing edited atp9 or only the selectable marker gave fertile plants. Pollen fertility ranged from 31% to 75% in fertile plants, 10% to 20% in semifertile plants, and <2% in malesterile plants. Genetic and molecular data showed that the chimeric plasmid containing the transgene is inherited as a Mendelian trait. The transgenic protein is imported into the mitochondria. The production and frequency of semifertile or male-sterile transgenic plants conform to the proposed hypothesis.Male sterility in plants leads to pollen abortion. This phenomenon is often observed in alloplasmic plants combining cytoplasmic and nuclear genetic material from two different species (1). Maternally inherited cytoplasmic male sterility (CMS) has been recently reviewed (2). Molecular studies on CMS have revealed the existence of modifications in mitochondrial DNA. In a number of CMS plants, the sterile phenotype is associated with the production of proteins arising from chimeric genes (3, 4); their incorporation into the mitochondrial membrane or into multiprotein enzyme complexes may lead to the impairment of mitochondrial function.The hypothesis developed in this report is that the impairment of mitochondrial function may be obtained by introducing an altered subunit in the ATP synthase complex. The protein ATP9 represents a good candidate because it is involved in the proton channel of the ATP synthase. ATP9 is mitochondrially encoded in plants but nuclear-encoded in Neurospora (5) and mammals (6). A mitochondrially encoded protein can be introduced into nuclei and imported into mitochondria when fused to a targeting sequence (7,8). An approach similar to the one described here has been used to introduce herbicide resistance into tobacco chloroplasts (9).We have shown a discrepancy between the amino acid sequence of the ATP9 protein and the predicted sequence deduced from the gene.

RNA editing in wheat mitochondria proceeds by a deamination mechanism

“…Recent results suggest that mRNA editing plays an important role for the production of functional proteins [6]. RNA editing has been also described in viruses, mammalian, protozoa, fungi and plants.…”

Natural Variation in Arabidopsis Leads to the Identification of REME1, a Pentatricopeptide Repeat-DYW Protein Controlling the Editing of Mitochondrial Transcripts

“…The main purpose of RNA editing has been thought to be the correction of deleterious mutations that would otherwise hamper the proper function of the encoded product. Indeed, an unedited ATP9 protein variant was shown to be not functional in tobacco (Nicotiana tabacum) because its presence disturbed mitochondrial function, resulting in a male sterility phenotype (Hernould et al, 1993).…”

Map-based cloning of the fertility restoration locus Rfm1 in cultivated barley (Hordeum vulgare)

“…This interpretation is further supported by the study of Kurek et al (1997) showing differential levels of RNA editing in transcripts encoding ATP synthase subunits between wheat plants differing in male fertility levels, the sterile background displaying the lowest efficiency of RNA editing. Moreover, the transgenic expression of a non-edited copy of the wheat atp9 gene in tobacco was shown to induce male sterility, further demonstrating the impact of an impropriately edited mitochondrial transcript on male fertility in plants (Hernould et al 1993). Despite the fact that the recognition code of PPR proteins is currently being deciphered (Barkan et al 2012), the target sequence of Rfm1 remains unknown.…”