Genetic analysis of nuclear control of T-urf13/orf221 transcription in T cytoplasm maize

Cytoplasmic male sterility in plants: molecular evidence and the nucleocytoplasmic conflict

A much-debated issue in plant evolutionary biology concerns the maintenance of a high frequency of male sterility in natural populations. For the past decade, a theoretical framework has been provided by the concept of nucleocytoplasmic conflict. Recent molecular studies on cytoplasmic male sterility indicate that novel chimeric genes, resulting from duplications and rearrangements of mitochondrial DNA sequences, are involved In its control. Thus, male sterility, which is phenotypically the loss of the male function, is encoded by a new mitochondrial function at the molecular level. Molecular data are in agreement with theoretical models that consider cytoplasmic male sterility as a stage in the coevolution between nucleus and mitochondria, and not simply as a deleterious mitochondrial mutation.

An unedited 1.1 kb mitochondrial orfB gene transcript in the wild abortive cytoplasmic male sterility (WA-CMS) system of Oryza sativa L. subsp. indica

BACKGROUND

The application of hybrid rice technology has significantly increased global rice production during the last three decades. Approximately 90% of the commercially cultivated rice hybrids have been derived through three-line breeding involving the use of WA-CMS lines. It is believed that during the 21st century, hybrid rice technology will make significant contributions to ensure global food security. This study examined the poorly understood molecular basis of the WA-CMS system in rice.

RESULTS

RFLPs were detected for atp6 and orfB genes in sterile and fertile rice lines, with one copy of each in the mt-genome. The RNA profile was identical in both lines for atp6, but an additional longer orfB transcript was identified in sterile lines. 5' RACE analysis of the long orfB transcript revealed it was 370 bp longer than the normal transcript, with no indication it was chimeric when compared to the genomic DNA sequence. cDNA clones of the longer orfB transcript in sterile lines were sequenced and the transcript was determined unedited. Sterile lines were crossed with the restorer and maintainer lines, and fertile and sterile F1 hybrids were respectively generated. Both hybrids contained two types of orfB transcripts. However, the long transcript underwent editing in the fertile F1 hybrids and remained unedited in the sterile lines. Additionally, the editing of the 1.1 kb orfB transcript co-segregated with fertility restoring alleles in a segregating population of F2 progeny; and the presence of unedited long orfB transcripts was detected in the sterile plants from the F2 segregating population.

CONCLUSION

This study helped to assign plausible operative factors responsible for male-sterility in the WA cytoplasm of rice. A new point of departure to dissect the mechanisms governing the CMS-WA system in rice has been identified, which can be applied to further harness the opportunities afforded by hybrid vigor in rice.

NUCLEAR CONTROL OF PLASTID AND MITOCHONDRIAL DEVELOPMENT IN HIGHER PLANTS

The nucleus must coordinate organelle biogenesis and function on a cell and tissue-specific basis throughout plant development. The vast majority of plastid and mitochondrial proteins and components involved in organelle biogenesis are encoded by nuclear genes. Molecular characterization of nuclear mutants has illuminated chloroplast development and function. Fewer mutants exist that affect mitochondria, but molecular and biochemical approaches have contributed to a greater understanding of this organelle. Similarities between organelles and prokaryotic regulatory molecules have been found, supporting the prokaryotic origin of chloroplasts and mitochondria. A striking characteristic for both mitochondria and chloroplast is that most regulation is posttranscriptional.

The mitochondrial atpA/atp9 co-transcript in wheat and triticale: RNA processing depends on the nuclear genotype

The gene region coding for subunits alpha and 9 of the mitochondrial ATP synthase exhibit an identical DNA sequence in wheat, rye, and the intergeneric hybrid triticale (xTriticosecale Wittmack). However, co-transcripts containing both genes show different sizes depending on the nuclear genotype. To investigate nuclear-mitochondrial interactions leading to this variation, we performed a comparative transcript analysis with various lines carrying defined nuclear and cytoplasmic genotypes. Northern analyses showed that all wheat lines investigated possess a single atpA/atp9 mRNA of 2.6kb, whereas in rye and five independent triticale lines an additional transcript of 2.35kb appeared. Primer-extension and RNase-protection analyses indicate that the co-transcripts of this gene have staggered 5' termini in some lines, whereas the 3' termini seem to be similar in wheat, rye, and triticale. Transcription is initiated at position -338/-339 upstream of the atpA gene in all lines investigated, giving rise to a 2.6-kb mRNA. In rye and triticale, staggered 5' termini were observed closer to the translational start. The DNA sequences upstream of these termini exhibit homology to plant mitochondrial-processing sites, therefore the proximal 5' ends are most probably generated by RNA processing. As the processing event occurs more frequently in triticale carrying the Triticum timopheevi cytoplasm, trans-acting factors from rye are likely to interact with other cytoplasmic factors resulting in the observed RNA modification. Most interestingly, the T. timopheevi cytoplasm inducing male sterility in alloplasmic wheat, fails to generate the CMS phenotype in triticale. The data support our hypothesis that nuclear factors affect mitochondrial gene expression and thus control sexual fertility in wheat and triticale.

Interaction of nuclear and mitochondrial genomes in the alteration of maize mitochondrial orf221 transcripts

The interaction of nuclear and mitochondrial genomes in the alteration of maize (Zea mays L.) mitochondrial orf221 transcript patterns was examined. Northern analyses involving specific maize nuclear genotypes associated with N, C or S cytoplasms revealed considerable orf221 transcript heterogeneity. F1 progenies were developed from maize inbred-cytoplasm combinations that differed for orf221 transcript patterns. Northern analyses revealed that the presence or level of abundance of certain orf221 transcripts was dependent on nuclear genotype. The maize inbred B37(C) exhibits orf221 transcripts of 3500, 3200, 2800, and 1300 nt whereas the F1 of B37(C) x Ky21(N) does not exhibit a 2800-nt transcript but does give transcripts of 2100 and 1250 nt in addition to 3500-, 3200- and 1300-nt transcripts. Northern analyses also suggested that the size or the presence of certain orf221 transcripts was related to the mitochondrial genome configuration. Maize inbred A619 exhibits a 2300-nt orf221 transcript when associated with N cytoplasm and a 2100-nt orf221 transcript when associated with C and S cytoplasms. As a result of deletion of the gene T-urf13, the A188(T7) mitochondrial mutant exhibits only a 3100-nt orf221 transcript and not the very complex T-urf13/orf221 transcript pattern associated with A188(T). The genetic stock A188(T7) x W64A(N)(2) gives a highly abundant 2100-nt orf221 transcript not detected in A188(T7). Deletion of T-urf13 has enabled the nuclear genotype of W64A(N) to alter orf221 transcript patterns in a manner not detected in T cytoplasm. This observation suggests that alteration of the mitochondrial genomic configuration adjacent to orf221 results in a different response to nuclear gene products from that observed when or2f21 is present in the T mitochondrial genome configuration.

Mapping complementary genes in maize: positioning the rf1 and rf2 nuclear-fertility restorer loci of Texas (T) cytoplasm relative to RFLP and visible markers

There are three major groups of cytoplasmic male-sterile cytoplasms in maize; C (Charrua), S (USDA), and T (Texas). These cytoplasms can be classified by the unique nuclear genes that suppress the male-sterility effects of these cytoplasms and restore pollen fertility. Typically, plants that carry Texas (T) cytoplasm are male fertile only if they carry dominant alleles at two unlinked nuclear restorer loci,rf1 andrf2. To facilitate analysis of T-cytoplasm-mediated male sterility and fertility restoration, we have mappedrf1 andrf2 relative to closely-linked RFLP markers using five populations. Therf1 locus and/or linked visible markers were mapped in four populations; therf2 locus was mapped in two of the populations. Data from the individual populations were joined with the aid of JoinMap software. The resulting consensus maps placerf1 between umc97 and umc92 on chromosome 3 andrf2 between umc153 andsus1 on chromosome 9. Markers that flank therf1 andrf2 loci have been used to identify alleles atrf1 andrf2 in segregating populations. These analyses demonstrate the possibility of tracking separate fertility restorer loci that contribute to a single phenotype.