Changes in Thylakoid Galactolipids and Proteins during Iron Nutrition-Mediated Chloroplast Development

Abstract

Changes in the amounts of thylakoid plactolipids and proteins were monitored for 96 hours following iron resupply to iron-deficient sugar beet (Beta vulgaris L. cv F58-554H1) plants. During this period of iron nutrition-mediated chloroplast development, the amount of plactolipid per leaf area increased linearly with time. Assuming plactolipids are an index for the amount of thylakoids, then there was a linear synthesis of thylakoid membranes during regreening. Total thylakoid protein synthe-sis, however, lagged behind galactolipid synthesis, suggesting that pro-teins are inserted secondarily into the plactolipid matrix of the thylakoid membrane during development. Iron deficiency caused an increase in the free chlorophyll band under the conditions of gel electrophoresis used. Of the chlorophyll proteins resolved, the chlorophyll protein associated with photosystem I was most diminished in iron-deficient tissue, and appeared to recover most rapidly. Changes in the light-harvesting chlorophyll proteins are also discussed. The number of polypeptides resolved by lithium dodecyl sulfate-polyacrylamide gel electrophoresis was higher in iron-deficient thyla-koids. During regreening, the number of resolved polypeptides decreased. Unlike the membrane lipids of mitochondria and the chloro-plast outer envelope which are high in phospholipids, thylakoid membrane lipids are comprised mainly of galactolipids (for reviews, see 6, 27). These lipids represent approximately 70% of the non-pigment thylakoid lipids (11) and 43% of all chloroplast lipids (15). The role of galactolipids in the structure and function of the thylakoid membrane is not well understood (6, 27). Both mem-brane stability and the correct orientation of pigment molecules for efficient light harvesting may be dependent on the presence of galactolipids (6). Other studies indicate a role for galactolipids in photosynthetic electron transport, e.g. photosynthetic 02 ev-olution, Cyt c photoreduction, and PSI activity (6). When defat-ted BSA is added to thylakoid preparations, however, large amounts of galactolipids may be removed without severely af-fecting electron transport (6). In contrast, 02 evolution and phosphorylation were not as well protected by BSA, but no direct role for galactolipids was suggested (30). Besides lipids, the other major component is protein which constitutes 48% ofthe thylakoid membrane (25). Some thylakoid proteins bind Chl and probably function in orienting Chl mole-cules. Many other proteins, such as Cyt and plastocyanin, play an integral role in photosynthetic electron transport. In bacteria, it is thought that protein and Chl are inserted into preexisting lipid membrane material (16). This view has been supported by studies which have shown that, in synchronously dividing cul-tures of the photosynthetic bacterium, Rhodopseudomonas