The Arabidopsis IspH Homolog Is Involved in the Plastid Nonmevalonate Pathway of Isoprenoid Biosynthesis

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Abstract
Plant isoprenoids are synthesized via two independent pathways, the cytosolic mevalonate (MVA) pathway and the plastid nonmevalonate pathway. The Escherichia coli IspH (LytB) protein is involved in the last step of the nonmevalonate pathway. We have isolated an Arabidopsis (Arabidopsis thaliana) ispH null mutant that has an albino phenotype and have generated Arabidopsis transgenic lines showing various albino patterns caused by IspH transgene-induced gene silencing. The initiation of albino phenotypes rendered by IspH gene silencing can arise independently from multiple sites of the same plant. After a spontaneous initiation, the albino phenotype is systemically spread toward younger tissues along the source-to-sink flow relative to the initiation site. The development of chloroplasts is severely impaired in the IspH-deficient albino tissues. Instead of thylakoids, mutant chloroplasts are filled with vesicles. Immunoblot analysis reveals that Arabidopsis IspH is a chloroplast stromal protein. Expression of Arabidopsis IspH complements the lethal phenotype of an E. coli ispH mutant. In 2-week-old Arabidopsis seedlings, the expression of 1-deoxy-d-xylulose 5-phosphate synthase (DXS), 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), IspD, IspE, IspF, and IspG genes is induced by light, whereas the expression of the IspH gene is constitutive. The addition of 3% sucrose in the media slightly increased levels of DXS, DXR, IspD, IspE, and IspF mRNA in the dark. In a 16-h-light/8-h-dark photoperiod, the accumulation of the IspH transcript oscillates with the highest levels detected in the early light period (2–6 h) and the late dark period (4–6 h). The expression patterns of DXS and IspG are similar to that of IspH, indicating that these genes are coordinately regulated in Arabidopsis when grown in a 16-h-light/8-h-dark photoperiod.