GENETIC-EVIDENCE THAT THE ENDODERMIS IS ESSENTIAL FOR SHOOT GRAVITROPISM IN ARABIDOPSIS-THALIANA

Shoots of higher plants exhibit negative gravitropism. However, little is known about the mechanism or site of gravity perception in shoots. We have identified two loci that are essential for normal shoot gravi tropism in Arabidopsis thaliana. Genetic analysis demonstrated that th e shoot gravitropism mutants sgr1 and sgr7 are allelic to the radial p attern mutants, scr and shr, respectively. Characterization of the aer ial phenotype of these mutants revealed that the primary defect is the absence of a normal endodermis in hypocotyls and inflorescence stems. This indicates that the endodermis is essential for shoot gravitropis m and strongly suggests that this cell layer functions as the gravity- sensing cell layer in dicotyledonous plant shoots. These results also demonstrate that, in addition to their previously characterized role i n root radial patterning, SCR and SHR regulate the radial organization of the shoot axial organs in Arabidopsis.