Centennial Honors College
Western Illinois University
Undergraduate Research Day 2014
Poster Presentation
A Mutant of Arabidopsis thaliana that is Defective in Toc159 Receptor
Accumulates Highly Reduced Levels of Pigments
Haley Patterson
Faculty Mentor: Meshack Afitlhile
Biology
The chloroplasts contain an organized system of membranes called thylakoids, which
are the site of the light reactions of photosynthesis. Thylakoids contain chlorophylls,
carotenoids, proteins and lipids, all of which are components of the reaction centers and
are essential in photosynthesis. Enzymes that are required in pigment and lipid
synthesis are nuclear-encoded, synthesized in the cytoplasm and imported into the
plastids, presumably through translocons on the outer/inner envelope membrane
(TOC/TIC). The outer envelope membrane contains Toc159 and Toc132/120, which are
a family of receptors but have evolved to import different sets of proteins. The Toc159
receptor imports light-induced proteins such as enzymes involved in chlorophyll
synthesis. In this study, we measured the accumulation of pigments and total soluble
protein in a mutant that is defective in Toc159 receptor called plastid protein import 2-2,
which is on the Columbia background. The mutant accumulates highly reduced levels of
chlorophylls and carotenoids, suggesting that lack of Toc159 receptor impaired the
import of enzymes that synthesize these pigments. Interestingly, the ppi2-2 mutant has
reduced levels of monogalactosyldiacylglycerol (MGDG), a major thylakoid membrane
lipid. The expression of MGD synthase-1 gene was down-regulated in the mutant,
suggesting that Toc159 receptor is required in the import of this protein. We conclude
that most proteins that are required in chloroplast biogenesis require Toc159 receptor
for their import.