The auxin-resistant diageotropica mutant of tomato responds to gravity viaan auxin-mediated pathway

Hypocotyls of the diageotropica (dgt) mutant of tomato (Lycopersicon escule ntum Mill.) do not elongate in response to exogenous auxin, but can respond to gravity. This appears paradoxical in light of the Cholodny-Went hypothe sis. which states that shoot gravicurvature results from asymmetric stimula tion of elongation by auxin. While light-grown dgt seedlings can achieve co rrect gravitropic reorientation, the response is slow compared to wild-type seedlings. The sensitivity of dgt seedlings to inhibition of gravicurvatur e by immersion in auxin or auxin-transport inhibitors is similar to that of wild-type plants, indicating that both an auxin gradient and auxin transpo rt are required for the gravitropic response and that auxin uptake, efflux. and at least one auxin receptor are functional in dgt. Furthermore, dgr gr avicurvature is the result of asymmetrically increased elongation as would be expected for an auxin-mediated response. Our results suggest differences between elongation in response to exogenous auxin (absent in dgt) and elon gation in response to gravistimulation (present but attenuated in dgt) and confirm the presence of two phases during the gravitropic response, both of which are dependent on functional auxin transport.