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The bacterial gene cod Aencodes cytosine deaminase (CD), which converts 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU), a compound that is toxic to cell growth. Consequently, CD can potentially be used as a conditional negative selection marker when 5-FC is applied. Rice seedlings and calli are sensitive to low concentrations of 5-FU, while the substrate of CD, 5-FC, is not toxic. To test whether the cod Agene can be used as a conditional negative selection marker in rice, we developed transgenic rice plants to contain the cod Acoding sequence fused with the phloem-specific promoter of rice tungro bacilliform virus. The transgenic rice plants containing the cod Agene can be killed by the addition of 5-FC to the medium. The first generation of rice transgenic lines with the cod Agene showed a 3:1 segregation pattern of 5-FC sensitivity. We therefore conclude that the cod Agene from E. colican be used as a conditional negative selection gene in rice.

The complete sequence of 8159 nucleotides of the double stranded DNA genome of cassava vein mosaic virus (CsVMV) was determined (# U59751) and revealed a significant difference in genome organization when compared with a previous report (# U20341). When transferred to cassava plants by microbombardment, the full length CsVMV clone was infectious, confirming the genome organization here described. Sequence comparisons between CsVMV and members of the genera Caulimovirusand Badnavirusrevealed high homologies between consensus sequences of several proteins that are indispensable for virus replication, including a potential transactivator factor not reported previously. The presence of a sequence complementary to a plant Met tRNA confirms that CsVMV is a plant pararetrovirus and is most closely related to members of the genus Caulimovirusas previously assessed. However, differences in genome organization, number and size of the ORFs, in addition to sequence comparisons with other plant pararetroviruses, shows that either the genetic variability of caulimoviruses is much greater than previously thought, or that CsVMV is the unique representative of a new genus within the Caulimoviridaefamily. On the basis of this study, it is proposed to upgrade the floating genus Caulimovirusto the family level and to divide the Caulimoviridaefamily into at least three genera with CsVMV being the type member of a new genus.

Methods for improving the efficiency of plant recovery from somatic embryos of cassava (Manihot esculenta Crantz) were investigated by optimizing the maturation regime and incorporating a desiccation stage prior to inducing germination. Somatic embryos were induced from young leaf lobes of in vitro grown shoots of cassava on Murashige and Skoog medium with 2,4-dichlorophenoxy acetic acid. After 15 to 20 days of culture on induction medium, the somatic embryos were transferred to a hormone free medium supplemented with activated charcoal. In another 18 days mature somatic embryos became distinctly bipolar and easily separable as individual units and were cultured on half MS medium for further development. Subsequent desiccation of bipolar somatic embryos resulted in 92% germination and 83% complete plant regeneration. The plants were characterized by synchronized development of shoot and root axes. Of the non-desiccated somatic embryos, only 10% germinated and 2% regenerated plants. Starting from leaf lobes, transplantable plantlets were derived from primary somatic embryos within 70 to 80 days.