A wide diversity of isoprenoids is produced in different plant compartments. Most groups of isoprenoids synthesized in plastids, and some produced elsewhere in the plant cell derive from geranylgeranyl diphosphate (GGPP) synthesized by GGPP synthase (GGPPS) enzymes. In Arabidopsis (Arabidopsis thaliana), five genes appear to encode GGPPS isoforms localized in plastids (two), the endoplasmic reticulum (two), and mitochondria (one). However, the loss of function of the plastid-targeted GGPPS11 isoform (referred to as G11) is sufficient to cause lethality. Here, we show that the absence of a strong transcription initiation site in the G11 gene results in the production of transcripts of different lengths. The longer transcripts encode an isoform with a functional plastid import sequence that produces GGPP for the major groups of photosynthesis-related plastidial isoprenoids. However, shorter transcripts are also produced that lack the first translation initiation codon and rely on a second in-frame ATG codon to produce an enzymatically active isoform lacking this N-terminal domain. This short enzyme localizes in the cytosol and is essential for embryo development. Our results confirm that the production of differentially targeted enzyme isoforms from the same gene is a central mechanism to control the biosynthesis of isoprenoid precursors in different plant cell compartments.Plants produce tens of thousands of isoprenoid compounds, including some that are essential for respiration, photosynthesis, and regulation of growth and development. Despite their structural and functional diversity, all isoprenoids derive from the same fivecarbon precursors, the double-bond isomers isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), which can be interconverted by IPP/ DMAPP isomerase (IDI) enzymes. Plants use two unrelated pathways to synthesize these units (Fig. 1). The mevalonic acid (MVA) pathway synthesizes IPP in the cytosol, whereas the methylerythritol 4-phosphate (MEP) pathway supplies both IPP and DMAPP in the plastid (Bouvier et al., 2005;Vranová et al., 2013; Rodriguez-Concepción and Boronat, 2015). IPP and DMAPP units can be exchanged between cell compartments to a certain level. For example, MVA-derived IPP can be imported by mitochondria for the biosynthesis of ubiquinone (Lütke-Brinkhaus et al., 1984;Disch et al., 1998). However, this limited exchange of common isoprenoid precursors is not active enough to rescue a genetic or pharmacological blockage of one of the pathways with IPP/DMAPP produced by the noninhibited pathway (Bouvier et al., 2005;Vranová et al., 2013; Rodriguez-Concepción and Boronat, 2015). Addition of IPP units to DMAPP generates longer prenyl diphosphate molecules, including C10 geranyl diphosphate (GPP), C15 farnesyl diphosphate (FPP), and C20 geranylgeranyl diphosphate (GGPP), which are then used in specific downstream pathways to produce particular isoprenoids (Fig. 1). FPP and GGPP pools represent nodes of the major metabolic branch points in the isoprenoid biosynthesis ...