Identification and Characterization of Plant Vascular Regulators

Plants colonized the land some 430 million years ago. The successful transition from aquatic to terrestrial habitats, forced plants to develop mechanisms for long-distance transport of assimilates, water and nutrients, and for mechanical support. The vascular system fulfills these roles, and thus, represents a major determinant of the growth dynamics of land plants. Typically, the vasculature is composed of two types of conducting tissues: the xylem, which is the main conduit for water and nutrients, and the phloem, through which assimilates and signaling molecules are transported. These specialized cell types originate from either procambium or cambium cells, giving rise to all differentiated cell types of the vasculature. Given its essential role for plant growth, biomass and wood production, and as a model to study cell fate regulation in plants, detailed knowledge about the molecular mechanisms regulating the establishment of vascular identity is highly desirable and can be expected to contribute tremendously to our understanding of the general concepts of plant growth and development. In this project, we propose to characterize transcriptome remodeling in cells establishing procambium identity in Arabidopsis thaliana embryos and compare it with the transcriptome remodeling during cambium formation in Arabidopsis shoots that we have identified previously. Genes identified in these data sets represent a unique collection of candidate genes commonly involved in the establishment and organization of vascular tissues. Functional studies on a selection of these genes, and on a selection of known candidates for common vascular regulators, will be undertaken to identify essential mechanisms during the establishment of vascular cell fate, thereby broadening our understanding of embryogenesis, vascular development, and the regulation of plant cell fate in general. In parallel, the general role of MP/ARF5, a transcription factor known to be involved in early vascular development, will be further characterized to formulate common concepts of establishing vascular identity. The project addresses a significant problem that is otherwise difficult to tackle without having high-potential candidate genes, as is the case for the selection of already available genes and as it will be for potential regulators identified by the planned transcriptome comparisons. Collectively, we expect that the project will generate specific insights into fundamental processes of plant development and cell fate determination in multicellular organisms and, in the long-term, will be highly beneficial for wood production, long-distance transport capacities and standing qualities of major crop species.

In this project, the establishment of the vascular system in plants was analyzed. Plants colonized the land some 430 million years ago. The successful transition from aquatic to terrestrial habitats forced plants to develop mechanisms for long-distance transport of assimilates, water and nutrients, and for mechanical support. The vascular system fulfills these roles, and thus, represents a major determinant of the growth dynamics of land plants. Typically, the vasculature is composed of two types of conducting tissues: the xylem, which is the main conduit for water and nutrients; and the phloem, through which assimilates and signaling molecules are transported. These specialized cell types originate from either procambium or cambium cells, giving rise to all differentiated cell types of the vasculature. Given its essential role for plant growth, and as an important model to study cell fate regulation in plants, detailed knowledge about the molecular mechanisms regulating the establishment of vascular identity is highly desirable and can be expected to contribute tremendously to our understanding of the general concepts of plant growth and development.In the course of the project, we identified genes being active in cells forming the vascular system during critical phases of plant development. These phases were the first phase of plant life, the embryogenesis, and the formation of the vascular cambium, a stem cell niche responsible for wood formation. We furthermore characterized the mechanisms by which the phytohormone auxin influences the activity of the cambium. Auxin stimulates the cambium-based formation of vascular tissues but the exact molecular mechanism in this context was unknown. We found that a concerted action of transcription factors from the ARF family is important for this effect. Different members of this family act in cambium cells and have positive or negative effects on cambium activity. By identifying target genes of those factors, we provided mechanisms of how auxin signaling is integrated into the regulatory program important for cambium activity. We also discovered a link between the action of ARF transcription factors and strigolactone signaling, another hormonal signaling pathway. Here, we found that strigolactone signaling regulates the transport of sugars produced in leaves to growing organs and tissues like the cambium. By discovering a connection between a signaling pathway influencing plant growth processes and primary energy metabolism, we were able to add one essential piece to the puzzle of our understanding of plant growth regulation.Especially by providing a mechanism of how sugar distribution is regulated, we opened up exciting opportunities of modulating resource allocation in plants, an aspect which is essential for the optimization of crop production and for a sustainable agriculture.