Metabolic Regulation in Developing Barley Seeds – Novel Insights from Transcriptome Analyses

The barley (Hordeum vulgare) seed, termed caryopsis, is of high importance for feed and food industry. In addition, it provides an excellent model to study the development and physiology of starch-storing cereal seeds. Gene expression in caryopses, like all plant organs, is to a large extent regulated by the cellular sugar status. As part of the sugar signalling cascade, we have previously identified the sucrose-inducible barley WRKY transcription factor SUSIBA2 which activates an isoamylase gene involved in starch biosynthesis. Subsequently, the aims of this thesis work were i) to clarify the phylogenetic relationship of SUSIBA2 with other WRKY genes and ii) to investigate the effects of diurnal changes and heat stress on metabolite levels and global gene expression in order to identify new potentially sugar status-dependent processes in developing barley caryopses. A total number of 45 barley WRKY gene family members were identified. Based on comparative phylogenetic analyses with genes from Arabidopsis (A. thaliana) and rice (Oryza sativa), barley WRKY genes putatively involved in development and stress response were assigned. In an attempt to non-invasively study the effect of altered sugar levels, we applied the 22K Barley1 GeneChip microarray to monitor the diurnal changes of global gene expression in barley caryopses. Among 2,091 diurnally regulated genes, almost 900 were highly correlated or anti-correlated to sugar levels, and, thus, potentially sugar status-regulated. Investigations of the shortterm (0.5h, 3h and 6h) exposure to heat identified stress response mechanisms and revealed a global repression of carbon metabolism-related genes after as short as 3h of heat stress. Summarized, this thesis provides the barley research community with a repository of genes involved in metabolic regulation of barley caryopsis development during its adaptation to diurnal changes and heat stress. Candidate genes were identified which merit further investigation of their functions in order to understand and, thereon, modify sugar signalling and carbon metabolism in the barley caryopsis as a model for cereal seeds.