Transgenic plants expressing the Thf1 gene in antisense orientation are stunted with altered thylakoid organization, especially in young seedlings. Genetic complementation of a Thf1 knockout line confirms the role of this gene in chloroplast and leaf development. Green sectors of Thf1-disrupted leaves contain some chloroplasts that form organized thylakoid membranes, indicating that an inefficient compensatory mechanism supports thylakoid formation in the absence of Thf1. Nongreen sectors of variegated leaves lacking Thf1 expression contain plastids that accumulate membrane vesicles on the interior and lack organized thylakoid structures. Disruption of the Thf1 gene via T-DNA insertion results in plants that are severely stunted with variegated leaf patterns. Transcript levels for Thf1 are induced in the light and decrease under dark conditions, paralleling profiles of light-regulated nuclear genes involved in chloroplast function. This gene is conserved in oxygenic photoautotrophs ranging from cyanobacteria to flowering land plants. The Arabidopsis Thf1 gene encodes an imported chloroplast protein, as shown by in vitro import and localization of a Thf1-green fluorescent protein fusion product in transgenic plants. The Arabidopsis (Arabidopsis thaliana) Thylakoid formation1 (Thf1) gene product is shown here to control an important step required for the normal organization of these vesicles into mature thylakoid stacks and ultimately for leaf development. Development of thylakoid membranes depends upon the transport of membrane vesicles from the chloroplast inner envelope and subsequent fusion of vesicles within the interior of the plastid.
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