THE ACTIVITY OF A TOBACCO BASIC CHITINASE PROMOTOR IN TRANSGENIC WHITE CLOVER PROVIDES INSIGHTS INTO PLANT DEVELOPMENT AND SYMBIOSIS

White clover was transformed with a tobacco basic chitinase promoter:G US fusion. Basic chitinase promoter activity was detected by histochem ical staining. Comparison of the spatial and temporal expression of th e chitinase promoter-driven GUS gene in tobacco to that in white clove r indicates that transcription from the promoter is induced by similar developmental and environmental response programs in each species. Wo und-responsiveness of the white clover transgene was rapid and localis ed following mechanical and aphid (Family Aphididae) wounding. Develop mental expression of the transgene during root morphogenesis reveals s trong expression in tap and lateral root meristems but expression in l ateral root meristems was observed only after the emergence through th e tap root epidermis. No expression of the transgene was detected in t he pericycle or the dividing cells of the developing lateral root. The expression of the tobacco basic chitinase promoter:GUS transgene in w hite clover was then used as a marker to examine the differences betwe en the early developmental pathways leading to lateral root formation and those involved in nodule formation in response to Rhizobium inocul ation. Inoculation of the zone of emerging root hairs with a nodulatio n-competent Rhizobium strain ANU845(pRI4003), triggered transient tran sgene expression 2 to 4 h post-inoculation. No transgene expression wa s detectable after inoculation with purified Nod factor from strain AN U843. Our results suggest that lateral roots and nodules differ both i n some of the mechanisms required to initiate cell division, and in th eir ongoing development after the emergence from the root epidermis.