The arabidopsis IspH homolog is involved in the plastid nonmevalonate pathway of isoprenoid biosynthesis

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.