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THE LAZY-2 REVERSED GRAVITROPIC RESPONSE GENE OF TOMATO: HIGH RESOLUTION MAPPING USING GENETIC AND MOLECULAR APPROACHES. J. Well, M.G. Ivanchenko, K. Oh, V.L. Slater, T.J. White and T.L. Lomax. Dept. of Botany and Plant Pathology, Oregon State University, Corvallis OR 97331-2902

     When grown under red light, shoots of the lazy-2 tomato mutant (lz-2) exhibit a reversed gravitropic response, mediated by the red-light absorbing photoreceptor phytochrome (Gaiser, J.C. and Lomax, T.L. (1993) Plant Physiol. 102:339-344). The lz-2 phenotype suggests that the Lz-2 gene product is a regulator of differential growth shared by both gravity and phytochrome signaling pathways. Therefore, isolation of the Lz-2 gene would provide unique means to investigate the interaction between these two essential plant regulative pathways. Using a map-based genetic approach we have localized the Lz-2 gene to the centromeric region of the tomato chromosome 5 (Behringer, F.J. and T.L. Lomax (1999) J. Heredity 90: 489-493). Proximity to the centromere complicates mapping due to dramatically suppressed recombination. To increase the frequency of genetic recombination, we have generated a mapping population of plants by crossing the lz-2 mutant (in the domestic tomato, L. esculentum) with the relatively closely related wild tomato species L. pimpinellifolium (Well, J. et al. (2000) Gravitational and Space Biology Bulletin 14: 13). We now report an expanded map of the Lz-2 region on chromosome 5 based on this new mapping population. Our cloning strategy involves a combination of comparative microsynteny analysis between tomato and Arabidopsis, the construction of a physical map spanning the genetic interval and genome walks upstream and downstream of identified molecular marker sequences. These approaches have proven useful for increasing marker density in the centromeric regions and for overcoming the potential difficulty in genotyping due to the lack of polymorphisms between L. .esculentum. and L. pimpinellifolium.

     (Supported by NASA NAG2-1341 and Howard Hughes Medical Institute Summer Research Fellowship to J.W.)