TISSUE-SPECIFIC INHIBITION OF TRANSIENT GENE-EXPRESSION IN CASSAVA (MANIHOT-ESCULENTA CRANTZ)

We have surveyed a variety of promoter-reporter gene constructs and transient transformation protocols to identify strong root specific promoters in the tropical crop plant cassava (Manihot esculenta, Crantz). Using a cauliflower mosaic virus 35S promoter, we observed several hundred localized regions which expressed beta-glucuronidase activity in cassava leaves but virtually none in roots. Similarly, a low frequency of transient gene expression was observed in roots when bombarded with a soybean root-specific glutamine synthetase promoter-beta-glucuronidase reporter gene. Another reporter gene (luciferase) fused to the CaMV 35S promoter gave similar results indicating that the low frequency of transient gene expression in roots was not a consequence of the particular reporter gene used. In our attempts to resolve the basis for the low frequency of transient gene expression in loots, we determined that root tissues have high levels of DNase activity. It is postulated that the high levels of DNase activity compromises the integrity of the transforming DNA resulting in reduced transformation efficiency.