Characterization of transgenic male sterility in alfalfa

Dependable male sterility would help to make hybrid cultivar development a reality in alfalfa once higher levels of heterosis are attained. Alfalfa plants obtained by genetic transformation with a construct containing the Barnase gene under the control of a tobacco anther tapetum specific promoter were studied. Vacuolization and degeneration of the tapetal cell cytoplasm at a premeiotic stage of development were observed in all five transformed plants (T-0) examined, but the severity of the abnormalities varied greatly among pollen sacs of a genotype. During the meiotic stage, some pollen sacs showed reduction in size, and the tapetum generally appeared thinner when compared to those of the non transgenic plants; tapetal cells showed abnormal vacuolization and signs of cytoplasm degeneration. Despite this, some microspores were formed and some pollen grains were shed in all the T-0 plants, but these were highly variable in size and had very low in vitro germinability. Self-fertility was negligible. The T-0 plants were crossed with one or two unrelated non transgenic male-fertile plants. Mendelian segregation was observed with two exceptions. Instability of the trait in F-1 progenies was noticed, varying for different T-0 parents. F-1 plants exhibiting higher sterility than the primary transformants were observed, indicating that it should be possible to obtain good male sterile plants by backcrossing this trait into different genetic backgrounds. The possible use of this transgenic male sterility in alfalfa breeding is briefly discussed.