We examined the effects of mechanical load on transcripts of a set of cell wall related genes that are implicated in the formation of supporting tissues, by applying a 50 mg strip of aluminum foil to the inflorescence stem of Arabidopsis thaliana, a weight roughly half the fresh weight of the stem. Transcript levels of 12 of the 15 genes examined were increased by load application, as were the levels of some transcription factors that regulate secondary wall formation. These findings support the involvement of a load-sensing system in regulation of supporting tissue formation via transcriptional regulation of cell wall related genes.
In 2008, the 'Cell Wall' experiment is scheduled to be launched and conducted on the International Space Station with the European Modular Cultivation System (EMCS). The main aim of this in-orbit plant science experiment is to elucidate the effect of gravitational conditions on supporting tissue formation in plants, thereby gaining new insight into the molecular mechanisms by which plants adapted to the land environment. In this first space experiment with in-orbit control experiments, we will specifically aim to elucidate the expression profiles of several candidate genes encoding proteins that are involved in the construction and restructuring of the secondary cell wall in the stem of Arabidopsis thaliana grown both in microgravity and 1G conditions. This review article deals with biological background pertinent to the 'Cell Wall' experiment, the anticipated experimental procedures to be used, together with a perspective of how this space experiment will extend our knowledge in both pure and applied life sciences.
The characteristic growth pattern of vascular p l a n t s l a r g e l y d e p e n d s o n t h e i n t r i n s i c p ro p e r t i e s of t h e ir c e ll wa lls , whic h a r e flexible, but strong enough to support the plant body. The plant body is composed of various tissues each with a specific cell wall type. Different sets of enzymes are required for the construction of these individual cell wall types. The cell wall type-specific enzyme-set hypothesis has been described to explain the mechanisms underlying cell wall construction. This hypothesis suggests that specific sets of transcription factors are required for the construction of each of the cell-wall types. Recent reverse genetic studies investigating secondary wall formation in Arabidopsis thaliana have demonstrated the existence of a hierarchical transcriptional network that governs the regulation of secondary wall formation in cell wall types. The examination of the effects of mechanical stimuli on the expression of genes encoding a particular set of cell wall-related enzymes and transcriptional factors has shown that A. thaliana is able to perceive subtle changes in self-weight of the aerial portions, and use this information as a signal to regulate formation of cell walls in the supporting tissues. However, the mechanisms by which mechanical signals are perceived via sensors presumably located at the cell surface remain unknown. In addition, the pathways through which the signal is transmitted and integrated into the transcriptional network that governs the coordinated actions of cell wallrelated genes are also yet to be described. Current reverse genetic approaches based on comprehensive expression analysis of cell wall-related genes may aid in the elucidation of the regulatory mechanisms underlying supporting tissue formation via mechanical signals. Such information may contribute not only to a further understanding of the molecular basis underlying evolution of the plant vascular system, but may also provide us with the knowledge required for the future development and utilization of plant cell walls as a sustainable resource.
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