Rice Genetics IV - IRRI books - International Rice Research Institute

Embryo-inducing geneInactive ablating genecre recombinaseManual pollination by breedersMP × FPHybridMolecular switchto apomixisApomictEmbryo-inducing geneActive ablating genecre recombinaseFig. 4. The allocation of genes between the two parental lines delays ablationof the sexual embryo until after hybridization. FP = female parent, MP= male parent.tinct tasks. The induction of the adventitious embryo includes three tasks: (1) isolationof genes capable of inducing ectopic embryogenesis, (2) isolation of promotersexpressed specifically in the nucellus, and (3) development of assays to detect successfulfunction of the embryo-inducing transgene in the nucellus. The ablation of thezygotic embryo involves another three tasks: (1) isolation of genes capable of ablatingthe sexual embryo, (2) isolation of promoters expressed specifically in the zygoticembryo but not in other postmeiotic tissues, and (3) isolation of meiosis-specific promoters.In the next section and in “Ablation of the sexual embryo,” we report progressto date.Induction of an adventitious embryo in the rice nucellusIsolation of nucellus-specific promotersIn addition to being nucellus-specific, the desired promoters should be expressedbefore the nucellus begins the programmed cell death that supplies nutrients to thedeveloping embryo and endosperm. Several genes are known to be specifically orpreferentially expressed in the nucellus. The first such gene to be identified encodedan aspartate proteinase known as nucellin (Chen and Foolad 1997). Transcriptionalactivity of the nucellin gene in barley begins 1 d before flowering and reaches a peaka few days after flowering. The function of the putative proteinase activity of nucellinis not known but the protein contains a C-terminal tripeptide serine-arginine-leucine(SRL) that may target the enzyme to microbodies. Chen and Foolad (1997) suggestthat nucellin is required for the programmed cell death that occurs in the nucellus,thus helping to provide the growing endosperm and embryo with amino acids andpeptides.388 Bennett et al