Reduced activity of geranylgeranyl reductase leads to loss of chlorophyll and tocopherol and to partially geranylgeranylated chlorophyll in transgenic tobacco plants expressing antisense RNA for geranylgeranyl reductase

The enzyme geranylgeranyl reductase (CHL P) catalyzes the reduction of gera nylgeranyl diphosphate to phytyl diphosphate. We identified a tobacco (Nico tiana tabacum) cDNA sequence encoding a 52-kD precursor protein homologous to the Arabidopsis and bacterial CHL P. The effects of deficient CHL P acti vity on chlorophyll (Chl) and tocopherol contents were studied in transgeni c plants expressing antisense CHL P RNA. Transformants with gradually reduc ed Chi P expression showed a delayed growth rate and a pale or variegated p henotype. Transformants grown in high (500 mu mol m(-2) s(-1); HL) and low (70 mu mol photon m(-2) s(-1); LL) light displayed a similar degree of redu ced tocopherol content during leaf development, although growth of wild-typ e plants in HL conditions led to up to a 2-fold increase in tocopherol cont ent. The total Chl content was more rapidly reduced during HL than LL condi tions. Up to 58% of the Chl content was esterified with geranylgeraniol ins tead of phytol under LL conditions. Our results indicate that CHL P provide s phytol for both tocopherol and Chl synthesis. The transformants are a val uable model with which to investigate the adaptation of plants with modifie d tocopherol levels against deleterious environmental conditions.