One approach for multiplying male-sterile plants is to produce plants that are conditionally fertile. During female parent multiplication, male-sterile plants are treated with a fertility-restoring chemical and can self-fertilize. For the production of hybrid seeds, chemical application is not required and the plants remain sterile. This system has some advantages over the selection of male-sterile plants by herbicide application. For example, the chemical has to be used during female parent multiplication and not during hybrid seed production and can be applied to a smaller acreage.
Based on conditional male fertility, several pollination control systems have been described. An example of the regulation of male fertility is that the manipulation of hormones in male reproductive tissues (Huang et al. 2003) induced malesterile plants through tissue-specific expression of the CKXl genes and gai, which are involved in oxidative cytokinin degradation and gibberellin signal transduction. In this dominant male-sterility system, the male-sterile phenotype is achieved in transgenic plants that are homozygous for the transgene, and it is reversible by exogenous hormone application.
Alternatively, fertility can be induced by environmental conditions. In thermo-sensitive genetic male-sterile (TGMS) and photoperiod-sensitive genetic malesterile (PGMS) mutants of rice, male sterility is influenced by temperature and photoperiod length (He et al. 1999; Dong et al. 2000). The temperature just after panicle initiation is the most critical in the expression of fertility and sterility. Most rice TGMS lines are male fertile at temperatures less than 25°C and sterile at higher temperatures (Sun et al. 1989). The seeds from TGMS lines are multiplied by selfing when exposed to the right temperature at the critical growth stage. The PGMS lines are fertile under the conditions of a natural short day and are male-sterile under long-day conditions. In this system, the male-sterile female line can be propagated by growing it under the environmental conditions that restore fertility. This approach requires no restorer lines and no chemical treatment. However, controlled environmental conditions are needed to avoid the plants being constantly challenged by unfavourable fluctuations in their environment. Other conditional male fertility systems are based on repressing the male sterility gene or the inducible expression of a fertility restorer gene that complements the defect (Cigan and Albertsen 2000). Recently, a combination of reversible male sterility and doubled haploid production by targeted inactivation of cytoplasmic glutamine synthetase in developing anthers and pollen was established (Ribarits et al. 2007).
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