Author

Yuan SuFollow

Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Biology, Molecular and Cellular Biology

Date of Defense

4-27-2018

Graduate Advisor

Xuemin Wang

Committee

Xuemin Wang

Bethany Zolman

Wendy Olivas

Lon Chubiz

Abstract

Phosphate (Pi) is one of three macronutrients for plants, which is vital for plant growth and development. Understanding the mechanism by which plants respond and adapt to Pi deficiency not only unveils functions of genes and pathways involved, but also provides potential tools to manipulate crops to better stand Pi stress in low Pi-containing lands. One of the significant metabolic changes in plants under Pi starvation is the membrane lipid remodeling that converts Pi-containing lipids such as phospholipids to Pi-free lipids, such as glycolipids. To elucidate the metabolism and regulation of lipid remodeling, this dissertation characterizes the role of two phospholipases, phospholipase D zeta2 (PLDζ2) and nonspecific phospholipase C4 (NPC4) in the lipid remodeling process. I generated double knockout pldζ2npc4Arabidopsis and performed comprehensive growth and membrane lipid analysis in single and double knockout under Pi deprivation. NPC4 contributed DGDG accumulation at an early stage of Pi deprivation in roots while PLDζ2 displayed a dominant effect on lipid remodeling at a later stage of Pi deprivation in leaves. In addition, NPC4 facilitated root hair elongation, but had no effect on root hair density. By comparison, PLDζ2 constrained both root hair elongation and density. The results indicate that PLDζ2 and NPC4 mediate the Pi deprivation-induced lipid remodeling in a tissue- and time-specific manner, and that PLDζ2 negatively modulates root hair density and length whereas NPC4 promotes root hair elongation in response to Pi deprivation.

To probe the regulation of membrane lipid remodeling, I found that S6K, a ribosomal protein kinase in the TOR (Target of Rapamycin) signaling pathway, binds to phosphatidic acid (PA), a central lipid intermediate. Knockout of S6K2but not S6K1in Arabidopsis impeded membrane lipid remodeling and root growth in response to Pi deficiency. In addition, comparative lipidomic profiling and growth analysis of s6k2, phr1(PHOSPHATE RESPONSE 1) and s6k2phr1indicate that S6K2participates in the same pathway as PHR1because similar growth phenotypes and membrane lipid composition were observed between single mutants and double mutants. The results indicate that S6K2 is part of the regulatory pathway that controls lipid remodeling and growth adaption to Pi limitation.

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