Cytoplasmic diversity in alloplasmic common wheats with cytoplasms of Triticum and Aegilops revealed by photosynthetic and respiratory characteristics

Characterization of two non-homoeologous nuclear genes encoding mitochondrial alternative oxidase in common wheat

Mitochondrial alternative oxidase (AOX) is the terminal oxidase responsible for cyanide-insensitive and salicylhydroxamic acid-sensitive respiration. We have isolated two non-homoeologous genes (Waox1a and Waox1c) encoding AOX proteins from common wheat (Triticum aestivum L.). These two genes were orthologous to rice AOX1a and AOX1c, and their exon/intron structure was conserved, as it is in most other plant AOX genes. Southern blot analysis indicated that both Waox1a and Waox1c were located in at least three homoeologous loci and that additional AOX genes with lower homology were present in the genome of common wheat. The Waox1a and Waox1c loci were respectively assigned to the homoeologous group 2 and 6 chromosomes. The steady-state level of Waox1a and Waox1c transcripts increased under cold stress, while only that of Waox1a was increased by cyanide treatment.

Overexpression of mitochondrial genes in alloplasmic common wheat with a cytoplasm of wheatgrass (Agropyron trichophorum) showing depressed vigor and male sterility

An alloplasmic hybrid (nucleus-cytoplasm hybrid) of common wheat (Triticum aestivum) with a cytoplasm of wheatgrass (Agropyron trichophorum) shows highly depressed vigor and complete male sterility. The presence of one short-arm telocentric homeologous group 1 chromosome (telosome) of the cytoplasm donor, however, restores normal vigor and male fertility of the hybrid. To study role(s) of the telosome on vigor/fertility restoration, mitochondrial genome organization and gene expression were compared among seedlings of the alloplasmic line showing depressed vigor, the corresponding restored line having a pair of the telosomes, and a euplasmic nuclear donor as control. No differences were detected in the mitochondrial genome structure between the depressed line and the restored line. Northern blot analysis using ten mitochondrial genes as probes showed no differences in transcript size and number between the depressed and restored lines, although clear differences were found in size of the major transcripts of two genes (cob and orf25) between the alloplasmic lines and the euplasmic control. Steady-state transcript levels were higher in the depressed line than in the other lines for all the mitochondrial genes analyzed including rrn18&5 when the same amount of mitochondrial RNA was loaded. The amount of rrn18&5 transcript in the total cellular RNA, however, did not differ among the lines. Run-on transcription analysis demonstrated markedly elevated transcriptional activities of all the mitochondrial genes analyzed in the depressed line based on unit amount of mitochondrial DNA, RNA and protein. The presence of Agropyron telosomes apparently normalized the level of mitochondrial transcription. These observations suggest either direct or indirect association of the observed mitochondrial gene overexpression with the depressed vigor and male sterility of the alloplasmic hybrid.

Variations in chloroplast proteins and nucleotide sequences of three chloroplast genes in Triticum and Aegilops

Two alloplasmic wheat lines having the same common wheat nucleus but the cytoplasms of Aegilops crassa and Ae. columnaris together with the corresponding normal line (control) were used in the two-dimensional gel electrophoresis of soluble and thylakoid membrane proteins of the chloroplast. Three chloroplast polypeptides: the Rubisco large subunit, the beta subunit of ATP synthase, and an unidentified 31 kDa protein, differed in the common wheat and two Aegilops cytoplasms. Three chloroplast genes, atpB, atpE and trnM, that respectively encode the beta and epsilon subunits of ATP synthase and tRNA(met), were sequenced. The atpB gene differed by two synonymous base substitutions, whereas the other two genes were identical in the two Aegilops cytoplasms. From the predicted amino acid sequences, the beta subunits of the ATP synthase in the Aegilops cytoplasms were assumed to have three amino acid substitutions: Ala by Val, Asp- by Ala, and Gln by Lys+, in contrast to the cytoplasm of common wheat. This accounts for the difference in pI values found for the common wheat and Aegilops cytoplasms. The two base substitutions for the atpE genes of common wheat and the Aegilops cytoplasms were synonymous. The differences detected in the genes encoding the two subunits of ATP synthase do not appear to be ascribable to the differences in phenotypic effects for the common wheat and Aegilops cytoplasms. The base substitution rate of the atpB-atpE-trnM gene cluster was similar to that of the rbcL gene. From the rate for the atpB gene alone, evolutionary divergence of the wheat-Aegilops complex is assumed to have begun ca. 3.0 x 10(6) years ago, as compared to ca. 8.0 x 10(6) years ago for the divergence of the wheat-Aegilops complex and barley.