Biotechnology: Engineered male sterility in plant hybrid breeding

Abstract

The exploitation of hybrid crop varieties in agriculture has enabled enormous increases in food productivity because of increased uniformity and hybrid vigour. Because of the hybrid vigour or heterosis, hybrids are characterized by increased resistance to diseases and enhanced performance in different environments compared with the heterozygous hybrid progeny (called F1 hybrids) over the homozygous parents (Lefort-Buson et al. 1987). Heterotic hybrid varieties in major crops such as wheat, cotton and rice show more than 20% yield advantage over the conventional ones under the same cultivation conditions. The increased vigour, uniformity and yield of F1 hybrids have been exploited in most crops where the pollination system allows for economical and convenient cross hybridization (Basra 2000). In hybrid seed production, one line is designated as the female parent and the other as the male parent. The production of hybrid seeds requires a pollination control system in order to prevent unwanted self-pollination of the female line. Especially for those crop species with hermaphrodite flowers, this can be a great challenge. Many methods exist to prevent selfpollination of the female line during hybrid seed production: application of male-specific gametocides, such as mitomycin and streptomycin (Jan and Rutger 1988), some interand intraspecific crosses (Hanson and Conde 1985), mechanical removal of male flowers or anthers, or use of genetic cytoplasmic (CMS) or nuclear-encoded male sterility. Naturally occurring genetically male sterile plants generally maintain fully normal female functions. The phenotypic characteristics of male sterility are very diverse from the complete absence of male organs, the abortion of pollen at any step of its development, the failure to develop normal sporogenous tissues, the absence of stamens dehiscence or the inability of mature pollen to germinate on compatible stigma. The generation of, mainly nuclear-encoded, male sterility is the basis of new reliable and cost-effective pollination control systems for genetic engi-