MANIPULATION OF PLOIDY LEVEL IN CULTURED HAPLOID PETUNIA TISSUE BY PHYTOHORMONE TREATMENTS

We have investigated the extent to which auxin and cytokinin affect ploidy changes in cultured, haploid Petunia << Mitchell >> (1n = 1x = 7) leaf tissue. This was done by exposing leaf tissue explants to different auxin and cytokinin treatments prior to growth on regeneration medium and counting chromosomes in root tip squashes from regenerated plants. On a standard regeneration medium with no pretreatment, only 7% of the regenerated plants were haploid, about 48% were diploid, and most of the rest consisted of mixoploids. Measurement of nuclear DNA content of cultured tissue showed that the leaf cells undergo endomitosis within the first few days on the regeneration medium. Treatment of leaf tissue with 30 to 100-mu-M benzyladenine for 9 to 12 days prior to culture on regeneration medium had a stabilizing effect on the genome such that 80% of the regenerates were haploid. In contrast, pretreatment with 50-mu-M indole-3-acetic acid for two days induced endomitosis and resulted in 90% of the regenerates being diploid. The link between phytohormones and the mitotic cell cycle demonstrated by these investigations suggests that it may be possible to develop protocols that allow for control of at least one aspect of somaclonal variation in plant tissue cultures.