Gravity-sensing processes and gravity-dependent gene expression in plants

Abstract

The present study focused on the cellular and molecular gravisensing-related processes in Arabidopsis and in the unicellular rhizoids of the green alga Chara. Experiments during sounding-rocket and parabolic plane flights were conducted in order to assess the effect of altered gravity conditions on gravitropism-related processes in plant cells. The cellular findings on statolith-mediated gravisusception and graviperception processes in characean rhizoids and higher plant root statocytes as well as the obtained data on gene-expression regulation during altered gravitational conditions considerably extend our knowledge about gravity-related mechanisms in plants. <br /> During the sounding rocket experiment of MAXUS-8, determination of the minimum acceleration that is required to induce lateral displacement of statoliths revealed a low threshold value (= 0.08<i>g</i>) for the gravisensitivity in Chara rhizoids. This high level of gravisensitivity ensures a gravisensing mechanism that is highly efficient so that even small deviations from the genetically determined set-point angle are rapidly corrected by the cell. The positioning of sedimented statoliths in the statocytes of horizontally stimulated Arabidopsis root seedlings during the different acceleration phases of parabolic plane flights have been analyzed. The findings revealed the missing evidence that graviperception in higher plants is accomplished by contact-interactions between statoliths and gravireceptors rather than by mechanical pressure. Once the statoliths are sedimented onto the gravireceptors, no pressure mediated by the weight of the statoliths is required for activation of the gravireceptors. In this respect, higher plant root statocytes and Chara rhizoids apparently share the same mechanism of graviperception. <br /> The findings about cellular graviperception mechanisms in plant cells were complemented by new results on the gravity-related gene-expression in Arabidopsis. At the level of gene expression, plants are not only sensitive to changes in orientation at 1<i>g</i> conditions but also to the altered conditions provided by parabolic plane flight. It was demonstrated that, in principle, plants are highly sensitive to even transient changes in gravity conditions. The rapidly changing acceleration conditions of parabolic plane flights (1<i>g</i>, 2<i>g</i> and µ<i>g</i>) significantly affected the gene regulation of thousands of genes including gravitropism-related genes, which were actually involved in the plant response to 90° reorientation on ground. Furthermore, it was demonstrated that gravitational conditions differing from 1<i>g</i> conditions on Earth not only induced general stress responses but also more specific modifications, like e.g. the rearrangement of cell-wall components durring hyper-g conditions. Findings on gene-expression level confirmed hypergravity-induced processes which are independent of gravitropism-related mechanisms in plants, and, therefore, support the assumption that gravireceptor activation is independent of pressure or tension forces. <br /> In summary, findings on cellular and molecular level, presented in this study, demonstrated that a high gravisensitivity and a great efficiency provide the fundamental basis of plant gravisensing in order to allow for a most beneficial gravitropic plant response.