A novel environment-induced genic male sterile (EGMS) mutant in indica rice
SummaryMale-sterile mutants were isolated from M2 and M3 generations of indica rice variety 26 Zhaizao, dry seeds of which had been exposed to 60Co-γ rays at a dose of 290 Gy. The mutants were planted in early season and ratooned in late season for two successive years for identification of fertility conversion in different growing seasons. One of the mutant lines was further observed in a growth chamber and in the field. Results showed that daily average temperature might be the major factor conditioning the male fertility conversion at a moderate daylength. The critical temperature for the male fertility conversion of the mutant grown under 12.5 h and 14.0 h daylength is about 23°C, below which the plant becomes completely male sterile. Its male fertility conversion character differs from other EGMS lines so far developed. The performance of the hybrids between the mutant and some other indica varieties demonstrated its good combining ability and its potential value in hybrid rice production. The obtained mutant line still sheds KI-stainable pollen grains under male sterilizing conditions. Nevertheless, pollen grains shed from the male sterile plants were much more vulnerable than from normal plants. At sucrose concentration below 1.5 M, the pollen grains from the mutant grown under male sterilizing conditions almost completely broke down, while above 1.5 M they became plasmolysed and shrunken. This is indicative of poor development of the membrane and walls of the pollen grains from the male sterile mutant, causing the pollen grains to be unfunctional. NBT test also clarified the abortion of the pollen grains from the mutant, which were formed in the male sterilizing environment.
Heterosis in crop mutant crosses and production of high yielding lines using doubled haploid systems
Heterosis appearing in crosses between mutants derived from the same parent variety and crosses of mutants with parent varieties has been observed by many authors for many plant species including such important crops as barley, faba bean, maize, pea, pearl millet, rice, sunflower, sesame, sweet clover, triticale and wheat. Mutant heterosis was reported for crosses of spontaneous mutants, mutants obtained after treatment with various mutagens and recently also for somaclonal variants. The heterotic effects are usually related to an increase in some yield components. There is a lack of correlation between mutation of a particular plant character and appearance of heterotic effect. The yielding performance of a mutant was not correlated with its potential to give yield heterosis in crosses with another mutant, often also a very poor one, or with the parent variety. Poor yielding barley mutants from the collection of semi-dwarf forms of the Department of Genetics, Silesian University gave heterosis in crosses with other mutants or parent varieties for such characters as tillering, grain number and weight per plant. In addition to mutants with deleterious mutations of such characters as chlorophyll synthesis or fasciated stem there were also lines mutated for earliness, semi-dwarfness, low glucoside or high protein and lysine content which gave significant heterosis in crosses. One possible explanation of the phenomenon of mutant heterosis is related to the frequency of mutations induced by chemical and physical mutagens. The appearance and the level of heterotic effect of mutated genes will depend on their interaction with other mutated genes or with genes from the parental genotype. High specific combining ability of mutants giving heterotic effect makes hybrid seed production, based on crosses with defined sources of cytoplasmic or genetic male sterility, unfeasible or even impossible. Doubled haploids provide a unique system to attempt the ‘fixing’ of hybrid performance in homozygous lines and to avoid the step of hybrid seed production. The assumption on the ‘fixability’ of hybrid yield in homozygous lines based on reports that large additive genetic variance is responsible for yield heterosis in wheat or barley was proven also for mutant crosses.
Double-stranded RNA and male sterility in rice
SummaryDouble-stranded RNA (dsRNA) was isolated from rice Oryza sativa ssp. japonica, but not from other subspecies. The dsRNA has been found in all of the examined cytoplasmic male-sterile (CMS) lines of BT (Chinsurah Boro II)-type rice, but was not detected in their companionate maintainer lines. It is uniquely and positivley correlated with the CMS trait in BT-type rice. Recently, the dsRNA was also found in a nuclear malesterile (NMS) rice, Nongken 58s, but was not found in its normal Nongken 58. The molecular weight of this dsRNA was estimated to be about 18 kb. Electron microscopic analysis reveals that it is linear snapped. The double strandedness of the RNA molecules was characterized by CF-11 cellulose column chromatography and nuclease treatments. It bound to CF-11 cellulose in the presence of 15% ethanol. It was sensitive to RNase A at low salt concentrations, but insensitive to DNase I, SI nuclease, and RNase A at high salt concentrations. The dsRNA was detected in both mitochondrial and cytoplasmic fractions. Dot-blot hybridization reveals that there is no sequence homology between this dsRNA and mtDNA, but there is homology between this dsRNA and nuclear genomic DNA. We have not been able to transmit this dsRNA to fertile rice.