Molecular, cellular, and functional aspects of gravity sensing and gravity-oriented tip growth

Abstract

Gravity is the most reliable environmental stimulus that plants use as a guide for the orientation of their organs in order to cope with their environment in a most beneficial way and to optimize exploitation of resources. The adjustment of the growth orientation relative to the direction of gravity is referred to as gravitropism. This mechanism is essential for all plants because it ensures that roots grow into the soil to take up water and nutrients and to anchor the plant, and that shoots grow towards the light to produce energy-rich metabolites via photosynthesis. Two model cell types of the green alga Chara - rhizoids and protonemata - are increasingly used to study specific aspects of gravitropism since these single cells are easily accessible for various experimental approaches and gravitropic signaling pathways are short. By investigating molecular, cellular, and functional aspects of gravity sensing and gravity-oriented tip growth in rhizoids and protonemata, the present study provides new insights into ultrastructural characteristics of polarized plant cells and into mechanisms of gene regulation and receptor activation in gravity sensing. <br /> Differential display analysis of gene expression in gravistimulated and in unstimulated cells provided several partial sequences of genes the expression levels of which were significantly altered upon gravistimulation and which are therefore implicated in gravitropic signaling. One gene that was up-regulated in gravistimulated rhizoids was characterized at full length and identified to encode a glucosyltransferase. This enzyme, which is usually involved in the synthesis of cell wall components, is likely to represent a target of gravitropic signaling in characean rhizoids, and regulation of gene expression may cause changes in cell wall properties during the gravitropic curvature response. <br /> Experiments under microgravity conditions were performed to investigate specific aspects of the early processes of gravity sensing which are closely linked to the sedimentation of intracellular particles, so-called statoliths. Based upon the analysis of statolith distribution in rhizoids and protonemata that were laterally centrifuged in microgravity during the MAXUS-5 sounding rocket flight it was calculated that lateral actomyosin forces in a range of 2 x 10-14 N act on statoliths to keep them in place. These forces represent the threshold value that has to be exceeded by any lateral acceleration stimulus for statolith sedimentation and gravisensing to occur. Experiments during parabolic plane flights provided strong evidence that graviperception in characean rhizoids requires contact of statoliths with membrane-bound receptor molecules rather than pressure or tension exerted by the weight of statoliths. It was therefore ruled out that the gravireceptor molecule in rhizoids is a classical mechanoreceptor.<br /> The protocols for high-pressure freeze fixation and 3D dual-axis electron tomography of characean rhizoids were established in this study as powerful experimental applications to analyze structural principles underlying gravity sensing and tip growth. Clathrin-coated vesicles (CCVs) at the apical plasma membrane were found to be arranged in clusters, which strongly implicates that endocytotic processes are not accomplished over the entire membrane area but confined to distinct 'endocytosis sites'. The close association of a second population of CCVs with the apical aggregate of endoplasmic reticulum (ER) points to a role of CCVs in mediating transport processes proceeding from the ER - a function that has not been assigned to CCVs so far. In the subapical region of rhizoids a prominent compartment was identified by fluorescence labeling and electron microscopy consisting of spherical segments that are interconnected and form an extensive network. This vacuolar reticulum may either mediate long-distance transport of material in the lumen of the compartment or represent an endosomal compartment that could be a target of endocytotic CCVs.<br /> The results presented in this study substantially contribute to the understanding of the mechanisms of endocytosis and intracellular transport in polarized plant cells. The progress that has been made in unraveling early processes of gravity sensing in characean rhizoids and protonemata underlines the significance of single-celled model systems for investigating how plants use gravity as a guide for orientation.