Abstract
The caryopses of barley (Hordeum vulgare), as of all cereals, are complex sink organs optimized for starch accumulation and embryo development. While their early to late development has been studied in great detail, processes underlying the caryopses' diurnal adaptation to changes in light, temperature, and the fluctuations in phloem-supplied carbon and nitrogen have remained unknown. In an attempt to identify diurnally affected processes in developing caryopses at the early maturation phase, we monitored global changes of both gene expression and metabolite levels. We applied the 22 K Barley1 GeneChip microarray and identified 2,091 differentially expressed (DE) genes that were assigned to six major diurnal expression clusters. Principal component analysis and other global analyses demonstrated that the variability within the data set relates to genes involved in circadian regulation, storage compound accumulation, embryo development, response to abiotic stress, and photosynthesis. The correlation of amino acid and sugar profiles with expression trajectories led to the identification of several hundred potentially metabolite-regulated DE genes. A comparative analysis of our data set and publicly available microarray data disclosed suborgan-specific expression of almost all diurnal DE genes, with more than 350 genes specifically expressed in the pericarp, endosperm, or embryo tissues. Our data reveal a tight linkage between day/night cycles, changes in light, and the supply of carbon and nitrogen. We present a model that suggests several phases of diurnal gene expression in developing barley caryopses, summarized as starvation and priming, energy collection and carbon fixation, light protection and chaperone activity, storage and growth, and embryo development.
Highlights
The caryopses of barley (Hordeum vulgare), as of all cereals, are complex sink organs optimized for starch accumulation and embryo development
Contained within the 2,091 differentially expressed (DE) genes, we identified several of the key genes involved in Suc transport and metabolism, such as the cell wall invertase gene 1 (HvCWINV1) and genes for a Suc transporter (HvSUT2), hexose transporter (HvHSTP1), and Suc: fructan 6-fructosyltransferase (Hv6-SFT)
We expected that relatively few DE genes in a sink organ such as the cereal caryopsis should be responsive to light signals, as only genes in the pericarp and the husk would be directly influenced by photocycles
Summary
The caryopses of barley (Hordeum vulgare), as of all cereals, are complex sink organs optimized for starch accumulation and embryo development. Metabolic processes in plants and most other organisms operate in concert with day/night cycles This coordination is accomplished by diurnal oscillations in transcriptional and posttranscriptional activities integrated by light, temperature, carbon status, and circadian signaling The filial endosperm, which constitutes the largest part of the caryopsis during middle to late stages of development, accumulates storage starch and nourishes the embryo during seed germination In addition to these compartments, thin layers of specialized cells that connect the different suborgans and/or function in transporting metabolites from vascular tissues into filial tissues add to the complexity of the caryopsis (Patrick and Offler, 2001; Weschke et al, 2003; Abebe et al, 2004; Thiel et al, 2008; Sabelli and Larkins, 2009)
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