THE IDENTIFICATION AND CHARACTERIZATION OF REGULATORS OF PLANT EXOCYTOSIS AND THEIR ROLES IN CELLULOSE BIOSYNTHESIS
Open Access
- Author:
- Zhu, Xiaoyu
- Graduate Program:
- Molecular, Cellular and Integrative Biosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 13, 2018
- Committee Members:
- Ying Gu, Dissertation Advisor/Co-Advisor
Ying Gu, Committee Chair/Co-Chair
Yanming Wang, Committee Member
Michael Axtell, Committee Member
Gabriele Brigitte Monshausen, Outside Member - Keywords:
- cellulose synthase complex
exocytosis
exocyst complex
microtubule - Abstract:
- Cellulose, the most abundant biopolymer on the earth, is produced by elegantly regulated biological machineries. In higher plants, cellulose is synthesized exclusively at the plasma membrane by cellulose synthase complexes (CSCs). Although the CSCs are considered to be assembled in Golgi apparatus, and then transported via secretory vesicles to the plasma membrane, little is known about the molecular mechanisms of their assembly, modification, and delivery. In this dissertation, essential mediators for the secretion of CSCs, the exocyst complex, as well as a plant-specific protein PATROL1 (PTL1), were identified via parallel proteomic screens. The exocyst and PTL1 are functionally correlated, and they both play significant roles in the delivery of CSCs. Moreover, the cellulose synthase interactive protein, CSI1, is required for directing the secretion sites of CSCs, primarily via its association with cortical microtubules. Real-time live cell imaging revealed a timeline for the functioning of CSI1, PTL1 and exocyst in the delivery process of CSCs to the plasma membrane. As CSCs represent unique cargo proteins that are distinct from yeast or mammalian systems, the discovery of exocyst and PTL1 not only opens up the door for the characterization of exocytosis in plants, but also provides insights into the evolution of exocytosis in eukaryotes. Aside from proteins involved in the secretion of CSCs, a protein named PASTICCINO2 (PAS2) was identified as a CSC interactive partner in both yeast two- hybrid (Y2H) screens and proteomic screens. Directly interacting with CSCs in Golgi, the PAS2 is essential for the activity of CSCs. Loss of PAS2’s function lead to barely mobile CSCs on the plasma membrane and significant reduction of cellulose deposition. PAS2 is therefore a promising candidate for dissecting the CSC assembly or modification in the Golgi. The discovery of both the secretory regulators and PAS2 shed light on the study of cellulose biosynthesis regulation. Hopefully, a combination of genetic and proteomic screens would help to unveil more key players within the cellulose synthesis machinery, and the functional characterization of which would bring the jigsaw puzzle pieces together to make a complete picture of cellulose biosynthesis in plants.