High-resolution mapping and genetic characterization of the Lazy-2 gravitropic mutant of tomato

Mutation of the Lazy-P (Lz-2) gene in tomato (Lycopersicon esculentum Mill. ) produces a phytochrome-dependent reversal of shoot gravitropism, providin g a unique genetic resource for investigating how signals from light modula te gravitropism, We mapped the Lz-2 gene using RFLPs and a PCR-based techni que to assess the feasibility of positional cloning. Analysis of a 1338 pla nt backcross population between L. esculentum and L, pennellii placed Lz-2 within a 1.2 cM interval on chromosome 5, 0.4 cM from TG504, and 0.8 cM fro m CT201A. We did not observe crossovers between several other markers in th e TG504-CT201A interval. The inability to resolve these markers indicates t hat Lz-2 resides in a centromeric region in which recombination is highly s uppressed. Lazy-2 is tightly linked to but does not encode the gene for ACC 4, an enzyme involved in ethylene biosynthesis. We also observed that lz-2 is partially dominant under certain conditions and stages of development.