The major protein import receptor of plastids is essential for chloroplast biogenesis

Light triggers the developmental programme in plants that leads to the production of photosynthetically active chloroplasts from non-photosynthetic proplastids¹. During this chloroplast biogenesis, the photosynthetic apparatus is rapidly assembled, mostly from nuclear-encoded imported proteins^{2, 3, 4}, which are synthesized in the cytosol as precursors with cleavable amino-terminal targeting sequences called transit sequences. Protein translocon complexes at the outer (Toc complex)^{5, 6, 7} and inner (Tic complex)^{6, 8, 9} envelope membranes recognize these transit sequences, leading to the precursors being imported. The Toc complex in the pea consists of three major components, Toc75, Toc34 and Toc159 (formerly termed Toc86)^{6, 7, 10, 11}. Toc159, which is an integral membrane GTPase¹², functions as a transit-sequence receptor</sup>5, 6, 7, 13</sup>. Here we show that <i>Arabidopsis thaliana</i> Toc159 (atToc159) is essential for the biogenesis of chloroplasts. In an <i>Arabidopsis</i> mutant (<i>ppi2</i>) that lacks atToc159, photosynthetic proteins that are normally abundant are transcriptionally repressed, and are found in much smaller amounts in the plastids, although <i>ppi2</i> does not affect either the expression or the import of less abundant non-photosynthetic plastid proteins. These findings indicate that atToc159 is required for the quantitative import of photosynthetic proteins. Two proteins that are related to atToc159 (atToc120 and atToc132) probably help to maintain basal protein import in <i>ppi2</i>, and so constitute components of alternative, atToc159-independent import pathways.