Understanding the Molecular Basis of Differential Growth during Apical Hook Development

Sammanfattning: Plants’ adaptation to their environment often involves change in development, which in many cases involves the establishment of differential growth rates across organs, for instance during phototropic and gravitropic responses. A striking example of differential growth is the formation of the apical hook, a structure that forms to protect the apical meristem as seedlings penetrate through soil. Coordination of differential growth across tissues is a multilayered process involving the combined effect of spatiotemporally controlled events such as gene expression, biosynthesis of proteins and polymers, transport and incorporation of biosynthetic products to their sites of participation, regulation of expansion driven by vacuolar turgor and control of cell mechanical properties via cell wall modifications. This thesis addresses mechanisms that underlie differential growth, using the apical hook as a model. Particularly, this work focuses on the role of two distinct but interrelated processes; transport of components to the cell surface, and regulation of composition of components at the cell surface in apical hook development. This work demonstrates that secretion of different auxin carriers follow distinct routes from the trans-Golgi network (TGN) to the plasma membrane, where delivery of AUX1 but not PIN3 relies the TGN-localized protein ECHIDNA (ECH). Data show that the ECH-dependent secretory pathway is essential for ethylene-mediated differential growth of the apical hook in Arabidopsis. Moreover, this work investigates the mechanism by which ECH operates, and shows that ECH is required for the localization of the GTPase ARF1 and its activator GEFs BIG1-4, which are key components of a vesicle formation machinery at the TGN. ARF1 members and BIG1-4 are, like ECH, required for AUX1 delivery to the PM and for ethylene-mediated hook development. Finally, the thesis explores the role of the cell wall in differential growth, particularly, that of homogalacturonan pectin and its modification by methylesterification. This thesis demonstrates that differential cell elongation during hook development relies on establishing asymmetric cell wall mechanical properties across the hypocotyl via pectin methylesterification modifications in an auxin-dependent manner, and that a mechanochemical component provides feedback to the auxin machinery. Taken together, this thesis demonstrates the multilayered regulation of growth asymmetry which facilitates shape generation.

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