Submitted to: The Plant Genome
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2009
Publication Date: 11/5/2009
Publication URL: http://plantgenome.scijournals.org/content/2/3/247
Citation: Abebe, T., Wise, R.P., Skadsen, R.W. 2009. Comparative Transcriptional Profiling Established the Awn as the Major Photosynthetic Organ of the Barley Spike while the Lemma and the Palea Primarily Protect the Seed. The Plant Genome. 2(3):247-259. Interpretive Summary: Developing seeds of barley are associated with the lemma and palea, organs that enclose the seed and later form the husk. The lemma tip has a long projection called the awn. They are thought to provide nutrients and carbohydrates to the developing seed during grain filling, but their precise roles are unknown. They may also be involved in protecting the seed from microbes and insects. In order to examine these possible roles, the types of genes turned on in these organs were determined by microarray analysis, a relatively new technology that can examine over 22,000 genes simultaneously. The lemma and palea were essentially the same, with very few genes turned on specifically in either organ. However, the awn was very different, having 816 genes specifically expressed. The seed itself was very different from all other organs, having 3,118 specifically expressed genes. A high proportion of genes expressed in the awn were involved with photosynthesis. This suggests that the awn is especially active in supplying the seed with carbohydrates that can be converted into starch. Over 1,000 genes were specifically turned on in the lemma and palea, when compared with the awn. A large number of these were related to defense reactions. Thus, the awn can be assigned a photosynthetic primary function, while the lemma and palea can be assigned a protective primary function. This is the first comprehensive comparison of developmental expression patterns in spike organs of barley, which are critically important during grain filling. Since Triticeae cereal grains are one of our most important food sources, these findings provide new knowledge of broad significance to plant scientists, and will help to produce high quality barley grain with better yields, benefitting farmers and malting and brewing companies.
Technical Abstract: The lemma, palea, and awn of barley are photosynthetic organs and supply the developing seed with carbohydrates. In addition, the lemma and the palea cover the seed and protect it from pathogens and insects. In spite of the important role they play, there is very little information about gene expression in the photosynthetic organs of the spike that justifies their function. In this study, we compared gene expression between lemma, palea, awn, and the developing seed during grain-filling using the Barley1 Genome Array in order to identify genes involved in the primary function of these organs. Hierarchical clustering and mixed model analysis revealed that lemma and palea have closely related gene expression patterns. In addition, the lemma and the palea over-expressed genes for defense response compared with the awn. On the other hand, the awn preferentially expressed genes for photosynthesis, biosynthesis of chlorophyll, and carotenoids and reactive oxygen species scavenging. This suggests lemma and palea are mainly protective organs whereas the awn is primarily a photosynthetic structure. As an actively growing storage organ, the seed was enriched with genes for the biosynthesis of starch and storage proteins, enzyme inhibitors, and cell proliferation.