Intercellular signaling in metaphloem development
Final Report Abstract
The development of a transport system which allows the exchange of water and photosynthetic metabolites as well as signaling molecules within their organism has allowed vascular plants to become the dominant autotrophs in most terrestrial ecosystems. Within the vasculature, the sieve elements of the phloem are transporting photo assimilates and signaling molecules in a directed manner between source and sink tissues. Within the phloem, two different types of sieve elements can be discerned. The metaphloem sieve elements, important for the transport between mature tissues and the protophloem, facilitating the transport in developing tissues and towards the meristems. The protophloem sieve elements undergo differentiation early and are the first functional conductive tissue in the root. In the recent past many factors controlling the differentiation of stem cell initials into conductive protophloem cells have been identified. However, only little is known about the differentiation process of the metaphloem and the factors controlling its development. In the frame of this project I studied the development of the metaphloem in the primary root of Arabidopsis thaliana. The analysis of this process in the wildtype shows that the metaphloem undergoes differentiation only relatively late in the root, not in the transition zone but later in the differentiation zone after the protoxylem has already formed and root hairs emerged. The analysis of this process in mutants disturbed in their protophloem differentiation indicates, that the neighboring protophloem does not directly influence metaphloem development. By comparing published and non-published cell specific gene expression datasets I identified and tested several genes that showed expression in developing sieve elements but are only weakly active in the protophloem as suitable markers for tracking sieve element development in the metaphloem. Of these eight lines worked well as markers for metaphloem development and they were introduced in different mutant backgrounds. During the course of the project, the role of OPS as a major regulator of protophloem development became further established when it was shown that a hyperactive version of OPS is able to rescue most of the known protophloem defects. I used this version of OPS in a third side suppressor screen to identify new factors controlling sieve element differentiation. The screen resulted in several interesting mutants which have been sequenced and candidate genes responsible for the mutant phenotypes are currently tested. I became further involved into the investigation of the function of OPS in the context of protophloem differentiation by analyzing the interaction of OPS with the CRN-CLV2 complex. We could show that OPS interferes with the interaction between CRN and the CLE45 receptor BAM3 and in this way can reduces CLE45 signaling. The presence of OPS in the developing protophloem insulates this tissue against the outputs of the CLE45-BAM3 signaling network and so promotes its differentiation. While required experiments and crosses to verify the candidates of the suppressor screen will take some additional time, the final experiments of the study of metaphloem development are currently conducted. The results show that, in contrast to the literature, the metaphloem is developing only relatively late in the root and its fate is rather controlled by positional cues than by the cell linage. New clearing and staining techniques together with the developed markers will allow further and more detailed studies of the development of sieve elements and help us to better understand the differentiation processes happening outside of the root meristem.
Publications
- A Cellular Insulator against CLE45 Peptide Signaling. Current Biology, Volume 29, Issue 15, 2019, Pages 2501-2508.e3
Alice S. Breda, Ora Hazak, Patrick Schultz, Pauline Anne, Moritz Graeff, Rüdiger Simon, Christian S. Hardtke
(See online at https://doi.org/10.1016/j.cub.2019.06.037)
