Abstract
Terminal drought stress during grain filling is the major abiotic factor that limits crop yield in barley. The mother plant acclimates to the environment and perceives signals that result in a change of the physiological state within the grain and therefore affect the seed development and germination of the next generation. Small regulatory RNAs have been described to be involved in plant drought stress response by suppressing the respective target genes. Based on their origin and function, these small RNAs are classified as micro RNAs (miRNA), short interfering RNAs (siRNA) or heterochromatic small interfering RNA (hc-siRNA). In addition, 24mer sized hc-siRNAs are associated with RNA directed DNA methylation (RdDM) and transcriptional gene silencing (TGS). The analysis of hc-siRNA by small RNA sequencing in barley caryopses after imposition of terminal drought stress allowed the identification of stress specific 24mers. Based on the sequence homology of the siRNAs to the promoter region of CYTOKININ-OXIDASE 2.1 (HvCKX2.1), this putative target gene was selected for further investigation. Terminal drought stress leads to an increased level of DNA methylation at the HvCKX2.1 promoter and the seeds derived from drought stressed plants showed faster shoot emergence. Accumulation of cytokinin ribosides, which are the known substrates of cytokinin-oxidase, can explain the observed phenotype of faster shoot emergence from seeds of drought stressed mother plants. Analysis of transgenic plants with modulated levels of abscisic acid (ABA) in the grain confirmed the ABA/drought stress responsive ProHvCKX2.1 methylation and correlation with shoot emergence speed.
Highlights
Among the food crops, barley is the fifth most important crop and ranks fourth among cereal crops in terms of production worldwide after maize, rice and wheat (FAO Statistical Yearbook 2009).The European Union contributes 25% (6.2 c) of the total 152 Mt of barley produced annually worldwide, and Germany ranks next to France in the European region with a total production of 1.2 Mt
Recent studies suggest that abscisic acid (ABA) at its basal level plays an important role during grain filling
Recent studies suggest that ABA at its basal level plays an important role during grain filling events [4]
Summary
Barley is the fifth most important crop and ranks fourth among cereal crops in terms of production worldwide after maize, rice and wheat (FAO Statistical Yearbook 2009). Mediterranean regions represent a significant area of barley production, where it is mostly grown under rain fed conditions These regions are characterized by a long hot and dry summer (coupled with an erratic rain fall pattern), alternated with cold, wet and relatively short winters [1]. Drought imposed after anthesis and fertilization will primarily affect seed size and quality [4,5], and is an important abiotic component influencing barley grain traits. These traits contribute to the malting performance of barley. The second mechanism involves 24-nt heterochromatic small RNAs and addresses gene regulation mediated by RNA directed DNA methylation [16,17]. An increased accumulation of cytokinin (CK) during germination and an earlier shoot appearance was observed in seeds derived from drought stressed mother plants
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