Rice Genetics IV - IRRI books - International Rice Research Institute

1996), and a putative hydroxyproline-rich protein of the extensin family (barley, Sturaroet al 1998).All of the above proteins are expressed abundantly 0–10 d after flowering, whenthe nucellus is undergoing programmed cell death to supply nutrients and space forthe developing grain and embryo. We are more interested in identifying genes that areexpressed earlier in nucellar formation, prior to the initiation of programmed celldeath. Schiefthaler et al (1999) have identified a gene, NOZZLE (NZZ), that is involvedin pattern formation and early sporogenesis in Arabidopsis thaliana. In severalnzz mutants, the nucellus and the pollen sac fail to form, indicating that NZZplays an early and central role in the development of both types of sporangia. NZZmay also have an early function during male and female sporogenesis. It is speculatedthat NZZ is a nuclear protein and possibly a transcription factor. If rice contains ahomologue of NZZ, its expression would mark the earliest limit for nucellar geneexpression. Genes that operate downstream from NZZ might be useful candidates forearly induction of nucellar embryogenesis.Another gene that is likely to be expressed in the nucellus is ASG-1 (apomixisspecificgene), discovered by Chen et al 1999. They sampled different developmentalstages of embryo sac formation in obligate sexual and facultatively apomictic genotypesof guinea grass (Panicum maximum). From a cDNA library, they isolated astage-specific cDNA clone of a gene that they designated ASG-1. The gene was expressedin flower buds of the apomictic but not of the sexual accession. The gene wasexpressed only during a phase characterized by the appearance of aposporous initialcells in the nucellus. The appearance of these cells is strictly limited to apomicticgenotypes. The gene codes for a 34.2-kDa protein related in sequence to various knownproteins, including RD22, a seed-specific and drought-induced gene of Arabidopsisthaliana, and Polyg1, a polygalacturonase 1 beta chain precursor of Lycopersiconesculentum. A rice homologue of RD22 has already been reported by the Rice GenomeProject of Japan.Genes for induction of apomictic embryosTwo mutations (lec1 and pkl) in Arabidopsis have implicated the corresponding proteinsin the control of ectopic embryogenesis. We have isolated rice homologues ofthese genes to test their embryogenic potential in rice.Leafy cotyledon 1 (lec1) is an embryo-defective mutation that affects cotyledonidentity in Arabidopsis (West et al 1994). Mutant cotyledons possess a cellular organizationthat is intermediate between that of cotyledons and leaves from wild-typeplants; for example, they possess trichomes, which are normally restricted to leaves.Several lines of evidence suggest that the control of late embryogenesis is compromisedby the mutation. First, although mutant embryos were desiccation-intolerant,they could be rescued before they dried and yielded homozygous recessive plants thatproduced defective embryos exclusively. Second, although many genes normally expressedduring embryonic development were active in the mutant, at least one maturationphase-specific gene (12S storage proteins) was not activated. Third, the shootapical meristem was activated precociously in mutant embryos. Fourth, in mutantMolecular tools for achieving synthetic apomixis in hybrid rice 391