Functional Characterization of the Arabidopsis thaliana ARG1 Gene Involved in Gravity Signal Transduction

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Plant organs use the gravity vector as an environmental cue to direct their growth. Gravitropism allows roots to grow downward into the soil where they take up the water and mineral ions required for plant growth and development. It also forces shoots to grow upward toward light, allowing them to photosynthesize. This important plant growth response to its environment also allows plant shoots to resume upward growth after having been prostrated by the action of wind and rain, and roots to resume downward growth after having overcome an obstacle placed in their growth path.A change in the orientation of a specific plant organ within the gravity field is perceived mainly by the sedimentation of dense amyloplasts in the cytoplasm of specialized cells named statocytes. Amyloplast sedimentation activates a signal transduction pathway that result in the production of a physiological signal, believed to be an auxin and/or apoplastic Ca2+ gradient across the gravistimulated organ. That signal is then transmitted to the site of response where it promotes a differential cellular growth responsible for the development of curvature. In roots, gravity sensing is thought to occur in the columella cells of the root cap, while the differential growth response occurs in the distal and central elongation zones.Very little is known about the molecular mechanisms allowing plant organs to convert the physical information derived from amyloplast sedimentation into a physiological signal. A molecular genetic approach using Arabidopsis thaliana now allows investigations that should help to better understand that process. Mutations in the ARG1 gene result in a specific defect in root and hypocotyl gravitropism. Mutant organs show wild-type growth rates, growth sensitivities to phytohormones and develop wild-type kinetics of phototropic response, strongly suggesting a specific defect in the early phases of gravity signal transduction. ARG1 encodes a novel dnaJ-like protein containing a putative coiled coil domain with amino acid similarity with coiled coils found in a number of proteins known to interact with the cytoskeleton. Interestingly, the cytoskeleton has been proposed to be implicated in the sensing phase, but not in the curvature response phase, of gravitropism. Based on these data, it is hypothesized that the ARG1 protein interacts with the cytoskeleton in the statocytes, facilitating the transduction of physical forces derived from amyloplast sedimentation into a physiological signal. To test this model the pattern of ARG1 expression will be analyzed in Arabidopsis thaliana seedlings, minimal spatial ARG1 expression requirements will be defined for complete restoration of gravitropism in transgenic arg1 plants, and the regions of ARG1 that are required for its function in gravitropism will be investigated. In situ immunofluorescence strategies will be used to localize the putative ARG1 protein within the columella cells of the root cap and in hypocotyl cells, and to compare its localization with the distribution of microtubules and microfilaments in statocytes. Proteins that interact with the various domains of ARG1 will be identified, and the genes encoding them will be cloned and characterized. Finally, ARG1 paralogs will be identified and functionally characterized. This combination of genetic, physiological and immunocytological studies will help define the molecular function(s) of ARG1 and ARG1 paralogs. Interestingly, a highly conserved ARG1 ortholog was found in Coenorhabditis elegans. This observation suggests an important role for the corresponding protein in the regulation of plant and animal growth, development and/or responses to the environment. Thus, it is expected that research will provide the foundation for a better understanding of the functions played by this novel type of dnaJ-like protein in higher eukaryotes.