A Nuclearrestorer-of-fertilityMutation Disrupts Accumulation of Mitochondrial ATP Synthase Subunit α in Developing Pollen of S Male-Sterile Maize

Abstract

Mitochondrial biogenesis and function depend upon the interaction of mitochondrial and nuclear genomes. Forward genetic analysis of mitochondrial function presents a challenge in organisms that are obligated to respire. In the S-cytoplasmic male sterility (CMS-S) system of maize, expression of mitochondrial open reading frames (orf355-orf77) conditions collapse of developing haploid pollen. Nuclear restorer-of-fertility mutations that circumvent pollen collapse are often homozygous lethal. These spontaneous mutations potentially result from disruption of nuclear genes required for mitochondrial gene expression, in contrast to homozygous-viable restorer-of-fertility alleles that function to block or compensate for the expression of mitochondrial CMS genes. Consistent with this hypothesis, the homozygous-lethal restoring allele historically designated RfIII was shown to be recessive in diploid pollen produced by tetraploid CMS-S plants. Accordingly, the symbol for this allele has been changed to restorer-of-fertility lethal 1 (rfl1). In haploid rfl1 pollen, orf355-orf77 transcripts and mitochondrial transcripts encoding the α-subunit of the ATP synthase (ATPA) were decreased in abundance. Haploid rfl1 pollen failed to accumulate wild-type levels of ATPA protein, indicating that functional requirements for mitochondrial protein accumulation are relaxed in maize pollen. The CMS-S system and rfl mutations therefore allow for the selection of nuclear mutations disrupting mitochondrial biogenesis in a multicellular eukaryote.

Activation of Mitochondrial orf355 Gene Expression by a Nuclear-Encoded DREB Transcription Factor Causes Cytoplasmic Male Sterility in Maize

“…We speculated that knockout of ZmDREB1.7 would be lethal to the plant. In fact, the homozygous-lethal phenotype of the ZmDREB1.7 knockout line we developed is similar to that previously reported for the restorer of fertility lethal (rfl) mutant in CMS-S maize (Wen et al, 2003;Gabay-Laughnan et al, 2018).…”

“…Restorers in CMS-S maize are reported to arise in real time through genetic mutation. Interestingly, while each spontaneous restorer rescues CMS-S pollen function, many lead to seed-lethal phenotypes (Wen et al, 2003;Gabay-Laughnan et al, 2018). It was hypothesized that these rfl mutations disrupt the expression of CMS genes (orf355) in pollen at the expense of mitochondrial functions (Chase, 2007).…”

Comparative analysis of mitochondrial genomes of maize CMS-S subtypes provides new insights into male sterility stability

“…However, CMS-S maize is a relatively unstable system where revertants frequently arise as a result of genetic mutation [ 12 ]. In the absence of Rf3 , sterility reversions are caused by less effective genetic restorer-of-fertility genes, or by the mtDNA changes that usually arise in de novo [ 8 , 9 , 13 , 14 ]. Although the reversions usually take place in a single floret or a small sector of the tassel, they can potentially reduce the purity of hybrid seeds.…”

“…Although orf355 transcripts can still be detected in these plants, they are not highly expressed and no longer confer sterility [ 13 ]. The restorer-of-fertility lethal 1 ( rfl1 ) mutant disrupts mitochondrial gene expression and the accumulation of the α-subunit of ATP synthase, suggesting that the functional plasticity of mitochondria is linked with the male sterility stability of CMS-S [ 14 ].…”

Comparative analysis of mitochondrial genomes of maize CMS-S subtypes provides new insights into male sterility stability

“…Although orf355 transcripts can still be detected in the plant, they are not highly expressed and no longer initiated the sterility of the plant [13]. Furthermore, the restorer-of-fertility lethal 1 (r 1) mutant disrupts mitochondrial gene expression and the accumulation of α-subunit of the ATP synthase (ATPA), which link the functional plasticity of mitochondria with the spontaneous reversion in CMS-S [14].…”