Expression and functional analysis of the barley Nud gene using transgenic rice

Most cereal crops have hulless grains (naked caryopses) with a free-threshing trait, whereas the majority of barley cultivars show hulled (covered) caryopses. The naked caryopsis in barley is genetically controlled by a single locus, nod. The Nud gene (the covered caryopsis allele) encodes an ethylene response factor (ERF) family transcription factor that regulates a lipid biosynthetic pathway. For functional analysis of the barley Nud gene, we produced transgenic rice expressing Nod in the developing caryopses. All transgenic lines had caryopses that were easily dehulled at maturity, indicating that the naked caryopsis phenotype remained in spite of expression of the Nod transgene. Histochemical and lipid analyses of the transgenic rice caryopses did not show increased lipid accumulation on the surface of developing caryopses, suggesting that the Nud-mediated lipid pathway may not function in rice caryopses. The predicted rice ortholog of Nod, Os06ERF was expressed specifically in the developing caryopses. However, expression of Os06ERF ceased at an earlier developmental stage than that of the native Nod gene in barley caryopses, which was also the case for expression of the Nod transgene. This raises the alternative hypothesis that the timing of Nod expression may be critical for activating the pathway for hull-caryopsis adhesion.