Analysis of the agravitropic mutant of Arabidopsis thaliana, eir1 and the EIR1 gene, suggests that it is the pump predicted by the polar transport model. The EIR1 protein, expressed only in the root of the plant has similarities to some bacterial efflux pumps. Experiments to provide direct evidence for the function of EIR1 in polar transport will be done. Mutants of the EIR1 gene that enhance and inhibit efflux will be generated in yeast, a system which provides a unique tool to probe this pathway in plants. Transport experiments will measure the efflux of indolic compounds directly in yeast. The role of EIR1 in asymmetric transport of indol acetic acid (IAA) from the root tip to the elongation zone will be tested by determining whether there is asymmetric localization of EIR1 in the plasma membrane of root cells. Genetic screens for suppressors and enhancers of eir1 should identify the upstream and downstream genes that transduce the signal for gravitropism. Molecular analysis of the other family members of EIR paralogs will determine whether these are the IAA pumps responsible for the effects of auxin on tropisms in other tissues. These genes will be cloned and transgenic plants containing dominant negative mutations in these EIR1 paralogs (validated in a yeast assay), will be used to construct transgenic plants, which will then be analyzed for tropism defects. A genetic pathway for cell expansion will be constructed using mutants obtained from a screen, which has been developed. Other experiments designed to delineate the pathway of lateral root formation involve cloning the gene encoding ALF4. af1 mutants block lateral root formation both by endogenous and exogenous IAA. In addition, a novel approach is described to engineer transgenic plants resistant to high salt.
The goal of this work is to identify the signal transduction pathway(s) by which the plant hormone auxin (or IAA) promote plant tropisms. In gravitropism, the root bends towards the center of the earth. The polar transport hypothesis posits an efflux transporter that pumps IAA up from the root tip to the side of the root closest to the center of the earth. There the IAA inhibits the elongation of the cells, whereas those on the opposite side elongate, with the result that the root bends down and grows in that direction.
