Rice Genetics IV - IRRI books - International Rice Research Institute

used T-DNA-tagging in Arabidopsis to isolate eight male and female gametophyticmutants detected by segregation distortion. This approach led to the isolation ofMEDEA, a polycomb group gene that is involved in maternal control of embryogenesis(Grossniklaus et al 1998). Maternal-effect phenotypes can result from regulationby genomic imprinting, a phenomenon of critical importance for both sexual andapomictic seed development (Grossniklaus et al 2001).IRRI adopted a three-fold strategy for inducing apomixis in rice (Khush et al1994). The three approaches were (1) mutagenesis of diploid germplasm that hadbeen genetically tagged to facilitate detection of putative apomicts, (2) transfer ofcloned apomixis genes from other grasses into rice as they became available, and (3)development of molecular tools for achieving synthetic apomixis in hybrid rice. Thefirst approach has not yet yielded mutants with the desired genotype. The secondapproach has stalled through lack of cloned apomixis genes (see the above section“An overview of apomixis”). The third approach has been pursued collaboratively byIRRI and the Commonwealth Scientific and Industrial Research Organization (CSIRO)of Australia and is the focus of the remainder of this chapter.Collaboration on apomixisIRRI’s project on molecular tools for achieving synthetic apomixis in hybrid rice is acollaboration with the CSIRO Division of Plant Industry in Australia and it comprisesthree different approaches (Bennett at al 1998). One approach being pursued by CSIROis based on the proposal of Peacock (1992) and envisions the induction of autonomousembryo and endosperm development for fis production. The isolation of FISand FIE genes from Arabidopsis (Luo et al 1999, Ohad et al 1999, Luo et al 2000,Vinkenoog et al 2000) has permitted the search for rice homologues. A second approachat CSIRO is the isolation of genes for apomixis from the genus Hieracium(Koltunow et al 1995). An understanding of how such genes function in sexual andapomictic species of Hieracium may permit the isolation and appropriate manipulationof rice homologues.A third approach, pursued by IRRI, envisions a combination of autonomous embryogenesisand normal endosperm formation. Autonomous embryogenesis in thenucellus of hybrid rice would deliver an apomict that remains heterozygous generationafter generation. Normal endosperm formation would deliver the triploid tissue(2m:1p) found in all cereals. By contrast, autonomous endosperm formation wouldproduce a diploid tissue (2m:0p) that may have qualitative and quantitative characteristicsnot acceptable to farmers or consumers. IRRI plans to maintain triploidy in theapomict by not interfering with pollination, but we must therefore address the questionof ablating the sexual (zygotic) embryo to avoid competition with the apomicticembryo. For these reasons, IRRI’s research focuses on two objectives: induction of anucellar embryo and ablation of the sexual embryo. However, before discussing theseobjectives in detail (see the section on “IRRI’s general approach to achieving syntheticapomixis”), we wish to address several important issues related to the projectas a whole.Molecular tools for achieving synthetic apomixis in hybrid rice 383