Molecular cloning, sequence characterization of a novel pepper gene NADP-ICDH and its effect on cytoplasmic male sterility.

NADP-dependent isocitrate dehydrogenase (NADP-ICDH) is an important enzyme involved in energy metabolism. The complete coding sequence of the pepper (Capsicum annuum) NADP-ICDH gene was amplified using a reverse transcriptase PCR based on the conserved sequence information of the tomato and other Solanaceae plants and known highly homologous pepper ESTs. Nucleotide sequence analysis revealed that the pepper NADP-ICDH gene encodes a protein of 415 amino acids that has high homology with the proteins of seven species, Solanum tuberosum (100%), Citrus limon (98%), Daucus carota (98%), Nicotiana tabacum (98%), Vitis vinifera (99%), Arabidopsis thaliana (97%), and Oryza sativa (98%). Tissue expression analysis demonstrated that the pepper NADP-ICDH gene is over expressed in flower, pericarp and seed, moderately in placenta, weakly in stem and leaf, hardly expressed in root. At the abortion stages, activities and expression levels of NADP-ICDH in anthers of a sterile line were strongly reduced, while those in an F(1) hybrid remained normal. Activities and expression levels of NADP-ICDH were too low to maintain balanced energy metabolism in the sterile line, which indicated that stable transcripts of NADP-ICDH are necessary to maintain energy metabolism at a normal level. When the restorer gene was transferred to the cytoplasmic male sterile line, activities and expression level of NADP-ICDH were regulated by the restorer gene and became stable. The restorer gene likely plays an important role in keeping the balance of the energy metabolism within normal levels during microspore development.