Beyond the phloem: Exploring the role of the octopus gene family in plant development
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2023
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Doctoral Thesis
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EMBARGOED UNTIL 2026-05-26
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Abstract
The development of the vascular system was a key evolutionary change that allowed plants to adapt efficiently to growth on land. As sessile organisms, plants depend on the environment surrounding them, and they have to react to changing conditions by prompt adjustments of their growth and physiology. The formation of a proper root system is essential for the uptake of water and nutrients from the soil. Long-distance transport of water and nutrients between below- and above-ground organs is then facilitated by the xylem. In the opposite direction, the phloem transports sugars from the photosynthetic tissues to support the growth of nonphotosynthetic tissues. The vascular system, i.e. phloem and xylem, also represents a communication network; signalling molecules produced in a specific tissue can be delivered to other organs, and coordinate the response to a specific clue.
Correct development of the vascular tissue is thus fundamental. Phloem cells need to undergo a precise differentiation program to become fully functional. Upon cell identity specification, cells elongate and the majority of their organelles is degraded. Phloem differentiation thus leads to the formation of continuous conductive strands with reduced cellular content and reinforced cell walls. In the last decades, an increasing number of factors that regulate the different developmental stages of phloem development have been discovered. However, our understanding of the whole process is still poor. Proper phloem differentiation is the outcome of a well-coordinated cross-talk between the regulatory activities of several hormones and genetic factors. Furthermore, in the root meristem, phloem differentiation occurs earlier than in the other cell files, which are still in the proliferative phase. This requires a tight communication with the neighbouring tissues to allow proper root growth.
The phloem specific OCTOPUS (OPS) gene represents one of the fundamental regulators of phloem differentiation. Absence of OPS function leads to the formation of discontinuous phloem strands in roots, which consequently results in overall poor root growth. The exact mechanism of OPS action is still poorly understood, but it was suggested that it regulates brassinosteroid (BR) signalling as well as the CLE45 peptide signalling pathway to promote proper phloem cell differentiation.
The main goal of this work was to expand our knowledge about the molecular function of OPS and to characterize the role of its homologous proteins OCTOPUS-LIKE (OPL) 1 - 4 in plant development. A detailed analysis of the expression patterns of all OPL genes revealed that they are expressed in distinct and overlapping domains of the root meristem and share their subcellular localization patterns with OPS. Although not specifically expressed in the phloem, all OPLs can rescue the phloem defects of ops to a certain extent when expressed in the OPS expression domain, suggesting that their molecular function was conserved during evolution. A high degree of functional redundancy was also revealed by genetic analyses. Interestingly, I could show that only OPL2 acts redundantly with OPS in phloem formation, but loss-of-function of all OPLs together with OPS leads to an overall decrease of plant growth vigour. Furthermore, results from my experiments suggest that a balanced expression of OPS and the OPL genes in the root meristem is required to promote correct phloem differentiation, meristem expansion and plant growth. I propose that this gene dose balance is in place to tightly coordinate the cellular activities of the different root cell files to ultimately achieve proper phloem differentiation and root elongation.
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Examiner : Zeeman, Samuel C.
Examiner : Truernit, Elisabeth
Examiner : Sánchez-Rodríguez, Clara
Examiner : Geisler, Markus
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ETH Zurich
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03707 - Zeeman, Samuel C. / Zeeman, Samuel C.