LYS3 encodes a prolamin-box-binding transcription factor that controls embryo growth in barley and wheat

Mutations at the LYS3 locus in barley have multiple effects on grain development, including an increase in embryo size and a decrease in endosperm starch content. The gene underlying LYS3 was identified by genetic mapping and mutations in this gene were identified in all four barley lys3 alleles. LYS3 encodes a transcription factor called Prolamin Binding Factor (PBF). Its role in controlling embryo size was confirmed using wheat TILLING mutants. To understand how PBF controls embryo development, we studied its spatial and temporal patterns of expression in developing grains. The PBF gene is expressed in both the endosperm and the embryos, but the timing of expression in these organs differs. PBF expression in wild-type embryos precedes the onset of embryo enlargement in lys3 mutants, suggesting that PBF suppresses embryo growth. We predicted the down-stream target genes of PBF in wheat and found them to be involved in a wide range of biological processes, including organ development and starch metabolism. Our work suggests that PBF may influence embryo size and endosperm starch synthesis via separate gene control networks.

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LYS3 encodes a transcription factor called Prolamin Binding Factor (PBF) that is expressed in grains only.

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Wheat and barley LYS3/PBF mutants have enlarged embryos suggesting that this gene suppresses embryo growth.

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The predicted targets of wheat PBF are involved in a wide range of biological processes, including organ development.

Identification and Characterization of LARGE EMBRYO, a New Gene Controlling Embryo Size in Rice (Oryza sativa L.)

BACKGROUND

Although embryo accounts for only 2-3% of the total weight of a rice grain, it is a good source of various nutrients for human health. Because enlarged embryo size causes increase of the amount of nutrients and bioactive compounds stored within rice grain, giant embryo mutants of rice (Oryza sativa L.) are excellent genetic resources for improving the nutritional value of rice grains.

RESULTS

Three giant embryo mutants, including large embryo (le), giant embryo (ge) and super-giant embryo (ge^s), with variable embryo size were used in this study. We investigated whether genes controlling embryo size in these mutants (le, ge and ge^s) were allelic to each other. Although ge and ge^s was allelic to GIANT EMBRY (GE), le was not allelic to ge and ge^s in allelism test. The GE gene carried a unique nucleotide substitution in each of the two mutants (ge and ge^s), resulting in non-synonymous mutations in exon 2 of GE in both mutants. However, the GE gene of the le mutant did not carry any mutation, suggesting that the enlarged embryo phenotype of le was governed by another gene. Using map-based cloning, we mapped the LE gene to the short arm of chromosome 3. The le mutant showed mild enlargement in embryo size, which resulted from an increase in the size of scutellar parenchyma cells. The LE encodes a C3HC4-type RING finger protein and was expressed to relatively high levels in seeds at a late developmental stage. Knockdown of LE expression using RNA interference increased the embryo size of rice grains, confirming the role of LE in determining the embryo size.

CONCLUSION

Overall, we identified a new gene controlling embryo size in rice. Phenotypic and molecular characterization results suggest that the le mutant will serve as a valuable resource for developing new rice cultivars with large embryos and nutrient-dense grains.