The Activity of a Tobacco Basic Chitinase Promotor in Transgenic White Clover Provides Insights into Plant Development and Symbiosis

Abstract

White clover was transformed with a tobacco basic chitinase promoter:GUS fusion. Basic chitinase promoter activity was detected by histochemical staining. Comparison of the spatial and temporal expression of the chitinase promoter-driven GUS gene in tobacco to that in white clover indicates that transcription from the promoter is induced by similar developmental and environmental response programs in each species. Wound-responsiveness of the white clover transgene was rapid and localised following mechanical and aphid (Family Aphididae) wounding. Developmental expression of the transgene during root morphogenesis reveals strong expression in tap and lateral root meristems but expression in lateral root meristems was observed only after the emergence through the tap root epidermis. No expression of the transgene was detected in the pericycle or the dividing cells of the developing lateral root. The expression of the tobacco basic chitinase promoter:GUS transgene in white clover was then used as a marker to examine the differences between the early developmental pathways leading to lateral root formation and those involoved in nodule formation in response to Rhizobium inoculation. Inoculation of the zone of emerging root hairs with a nodulation-competent Rhizobium strain ANU845(pRI4003), triggered transient transgene expression 2 to 4 h post-inoculation. No transgene expression was detectable after inoculation with purified Nod factor from strain ANU843. Our results suggest that lateral roots and nodules differ both in some of the mechanisms required to initiate cell division, and in their ongoing development after the emergence from the root epidermis.