Relationship between sink-induced N remobilisation and seed filling in barley
Final Report Abstract
Sink-induced organ senescence and associated assimilate remobilisation contributes to nitrogen efficiency and crop improvement. During seed filling of barley, carbon and nitrogen compounds are remobilised from ear-near organs, flag leaves, awns and glumes and transported to grains. This project addressed the relationships between seed filling induced remobilisation/senescence in barley organs using gene expression and metabolite profiling. 454-sequencing of the transcriptomes of flag leaves, glumes and grains following combinations of new and known sequences reduced redundancy of the publicly available contig set, increased contig-lengths and identified novel cysteine peptidases and nitrogen transporter genes. Specific AAT-type nitrogen transporters are especially relevant for nitrogen translocation from glumes/flag leaf to grains. The information has been used to assemble a barley AGILENT 60k microarray with new design and improved annotation. This tool was used to analyse temporal changes of transcript abundances together with metabolite profiling in glumes, pericarp and endosperm. For the developing pericarp, transcript studies were combined with magnetic resonance imaging and hormone profiling to identify candidates determining pericarp length extension at the molecular level. Because grain length is best correlated with final grain mass, these candidates might control the important yield-related parameter thousand grain weight. In glumes and endosperm, signals and assimilate transporters were uncovered, which are involved in coordinating metabolism and assimilate translocation between the two tissues. Shifts in glume metabolism and remobilisation were found to be synchronised with grain development. Moreover, glume-specific metabolic pathways could be detected for assimilate remobilisation. These metabolic and transcriptional transitions reveal that barley glumes function as transitory resource buffers during endosperm filling. The project results showed that glumes are photosynthetically active sinks during pre-storage phase and that glumes and endosperm display opposed metabolic shifts at the beginning of grain filling. The glumes undergo transition into remobilising and exporting organs coinciding with the beginning of endosperm storage activity. This metabolic transition occurs in two phases, the first one, occurs around eight days after pollination, consistent with onset of endosperm storage activity, which is probably a consequence of increasing endosperm sink activity, and, the second one, arises 18 DAP, when developmental ageing and senescence denotes later grain filling. Sink to source transition in glumes is accompanied by changed expression of nitrogen transporters and by distinct glume-specific pathways for remobilisation of assimilates and resources. It is suggested that grain filling and filial sink strength coordinate these phase changes in glumes via metabolic, hormonal, and transcriptional control. Amino acid profiles in barley grain vasculature were found to be highly correlated with those in the endosperm, but not in glumes. This indicates that depletion of endosperm amino acid concentrations due to storage protein biosynthesis could be transmitted via the vasculature. Thereby, specific amino acids function as metabolic signals communicating endosperm demand to vegetative organs to induce remobilisation. In summary, the project provided a wealth of transcript and metabolic information and, most important, identified a set of transcription factors, assimilate transporters, metabolic pathways and enzymes, which constitute potential bottlenecks for efficient remobilisation of assimilates from vegetative tissues in barley. A defined set of nitrogen transporters are currently being validated using knock-down approaches in barley.
Publications
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(2012) A putative role for amino acid permeases in sinksource communication of barley tissues uncovered by RNA-seq. BMC Plant Biol 12, 18
Kohl S, Hollmann J, Blattner FR, Radchuk V, Andersch F, Steuernagel B, Schmutzer T, Scholz U, Krupinska K, Weber H, Weschke W
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(2015) Hormone-mediated growth dynamics of the barley pericarp as revealed by magnetic resonance imaging and transcript profiling. J Exp Bot 66, 6927-6943
Pielot R, Kohl S, Manz B, Rutten T, Weier D, Tarkowska D, Rolcik J, Strnad M, Volke F, Weber H, Weschke W
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(2015) Metabolic and transcriptional transitions in barley glumes reveal a role as transitory resource buffers during endosperm filling. J Exp Bot 66, 1397-1411
Kohl S, Hollmann J, Erban A, Kopka J, Riewe D, Weschke W, Weber H
