Drought Response in the Spikes of Barley: Gene Expression in the Lemma, Palea, Awn, and Seed

Authors: ABEBE, TILAHUN - University Of Northern Iowa; MELMAIEE, KALPALATHA - University Of Northern Iowa; BERG, VIRGINIA - University Of Northern Iowa; Wise, Roger

Submitted to: Functional and Integrative Genomics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2009
Publication Date: 11/20/2009
Citation: Abebe, T., Melmaiee, K., Berg, V., Wise, R.P. 2009. Drought Response in the Spikes of Barley: Gene Expression in the Lemma, Palea, Awn, and Seed. Functional and Integrative Genomics. 10(2):191-205.

Interpretive Summary: Drought is the single most important environmental stress that reduces crop yield. Drought affects all stages of plant development, but the reproductive stage is the most sensitive. Shortage of water during pollination increases sterility and senescence of flowers thereby reducing the quality and quantity of seeds per plant. Hence, understanding how drought affects reproductive development is crucial for improving yield in areas where terminal drought is common. The photosynthetic organs of the barley spike (lemma, palea, and awn) are considered resistant to drought. This is a beneficial trait because they can sustain grain-filling when drought occurs at the reproductive stage. However, there is little information about gene expression in the spike organs under drought conditions. We compared the response of 22,000 genes in the lemma, palea, awn, and seed to drought stress using the Barley1 Genome Array. We showed that the awn expressed many drought-regulated genes, followed by the lemma and the palea. This suggests that expression of several genes in the awn might be a response to the severe damage caused by drought stress. This leads us to propose that the lemma and the palea are more drought resistant than the awn. Thus, under severe drought conditions, the lemma and the palea appear to be the main source of carbohydrate for grain-filling. This is the first comprehensive comparison of developmental expression patterns in spike organs of barley under drought stress, which is the most important abiotic stress that reduces crop yield. 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 photosynthetic organs of the barley spike (lemma, palea, and awn) are considered resistant to drought. This is a beneficial trait because they can sustain grain-filling when drought occurs at the reproductive stage. However, there is little information about gene expression in the spike organs under drought conditions. We compared the response of the transcriptome of the lemma, the palea, the awn, and the seed to drought stress using the Barley1 Genome Array. Barley plants were exposed to drought treatment for four days at the grain-filling stage by withholding water. At the end of the stress, relative water content (RWC) of the lemma, palea, and awn dropped from 85% to 62%. Nevertheless, the water content of the seed only decreased from 89% to 81%. Transcript abundance followed the water status of the spike organs; the awn had more drought-regulated genes followed by lemma and palea, and the seed showed very little change in gene expression. Despite expressing more drought-associated genes, many genes for amino acid, amino acid derivatives, and carbohydrate metabolism as well as photosynthesis, respiration, and stress response were down-regulated in the awn compared with the lemma, palea, and seed. This suggests that the lemma and the palea are more resistant to drought stress compared with the awn.