Rice Genetics IV - IRRI books - International Rice Research Institute

embryos, several genes characteristic of postgerminative development were expressedat levels typical of wild-type seedlings rather than embryos. West et al (1994) concludedthat postgerminative development begins prematurely and that embryonic andpostgerminative programs operate simultaneously in mutant embryos. The pleiotropiceffects of the mutation indicate that the LEC1 gene plays a fundamental role inregulating late embryogenesis.The LEC1 gene encodes a homologue of the HAP3 subunit of the transcriptionfactor that binds to CCAAT boxes (Lotan et al 1998). LEC1 RNA accumulates onlyduring seed development in embryo cell types and in endosperm tissue. Ectopic postembryonicexpression of the LEC1 gene in vegetative cells induces the expression ofembryo-specific genes and initiates formation of embryo-like structures. The resultssuggested that LEC1 is an important regulator of embryo development and activatesthe transcription of genes required for both embryo morphogenesis and cellular differentiation.LEC1 is not expressed exclusively in embryos. Tsukaya et al (2000), in a study ofheteroblasty in Arabidopsis thaliana, found that phenotypes associated with foliageleaf-specific mutations were also induced ectopically in cotyledons in the presence ofthe lec1 mutation. This implies that LEC1 is also expressed at some stage of cotyledonformation.In Arabidopsis plants carrying the mutation designated as pickle (pkl), the primaryroot meristem retains characteristics of embryonic tissue (Ogas et al 1997). GAsuppressed the expression of this aberrant differentiation state. Analysis of doublemutants of pkl with the loci ga or gai (encoding a GA biosynthesis gene and GAsignalling gene, respectively) suggested that PKL plays a signaling role. Root tissuefrom plants carrying the pkl mutation spontaneously regenerated new embryos andplants without the need for hormones, but root explants from the wild type requiredthe auxin 2,4-D to produce embryogenic calli (Fig. 6). The root formed in the pklmutant expressed LEC1 and other embryonic genes.Expression of embryonic genes(including LEC1)Mutant seed(pkl)Picklerootpklexplant–2,4-DEmbryogeniccallusWild-type seed(PKL)AdultrootPKLexplant+2,4-DEmbryogeniccallusFig. 6. Regulation of gene expression and embryogenic callus formation in wildtypeArabidopsis thaliana and the pkl mutant.392 Bennett et al