Seed Development group
Head: Dr. Jozefus Schippers
The Seed Development Group investigates regulatory mechanisms of cell specification, tissue differentiation, organ growth and storage product accumulation in developing crop seeds. Aim is to understand components of yield formation and its interactions. Molecular and metabolic / hormonal regulation and functional genomics are integrated towards the identification of key genes and regulators of seed shape and size. Special emphasis is laid on assimilate transfer and maternal-filial interactions to identify bottlenecks for grain formation. Further focal points are sink-source relationships and cellular disintegration processes during seed development. Final aim of the group work is to improve yield potential and quality of crop seeds, particularly in the cereals barley and wheat.
Source-sink relationships and remobilisation of assimilates
Seed development depends on available assimilates and nutrients delivered by vegetative organs. Thereby, demand of the growing seed (sink) is communicated to vegetative organs (source). Identification of components mediating sink-source communications and finally, nutrient acquisition by the seed is a prerequisite for crop yield improvement. During grain filling of barley, carbon and nitrogen compounds are remobilised from flag leaf, awn and glumes and transported to grains. Transcript profiling of relevant nitrogen transporters in glumes and endosperm elucidated mechanisms of assimilate translocation, which are probably coordinated by changing sink strength of grains via metabolic, hormonal and transcriptional control (Kohl et al. 2015). Specific N transporters are currently validated by knockdown approaches in barley. Preliminary results confirm a role in remobilisation and N transport during grain filling (DFG proposal in preparation). RNAi-mediated repression of the vacuolar sucrose transporter 2 (HvSUT2) resulted also in down-regulation of the plasma lemma-bound HvSUT1 and led to sucrose accumulation along the sucrose delivery route in NP, ERCs, and the starchy endosperm, although grains become apparently sucrose-limited. The results suggest that both HvSUT2 and HvSUT1 control sucrose homeostasis during grain filling. Their deficiency affects maturation and certain biosynthetic pathways and initiates sugar starvation-induced salvage of resources (Radchuk et al. 2017). In an ongoing project, we are performing tissue-specific RNA-sequencing of phloem cells and transfer tissues from source and reproductive sink organs of wheat (cooperation with BASF). Global transcriptome profiling of phloem cells from source organs, including the primary leaf, flag leaf, stem and peduncle, and sink tissues, spike rachis and developing grains, will provide a holistic view on source-sink relationships at the whole plant level. By following the flow of assimilate transfer from source to specified transfer tissues in grains (vascular bundle, nucellar projection and endosperm transfer cells) we want to understand the interaction of transfer tissues in providing resources for final storage product accumulation in the endosperm. Data will gain novel insights into molecular mechanisms determining phloem/transfer tissue development, coordinated activity of transporter genes and associated metabolic pathways. This will permit to draw novel conclusions about source-sink relationships in wheat.
Signal transduction and hormonal regulation
Developing cereal grains are complex structures consisting of maternal and filial tissues. Temporal and spatial sequences of cell proliferation, differentiation, maturation and disintegration occur in different grain tissues. Cellular disintegration of maternal seed tissues and growing of the fertilization products embryo and endosperm determine morphogenesis of the growing seed (Sreenivasulu et al. 2010) and is tightly correlated to the expression of vacuolar processing enzymes (Radchuk et al. 2011; 2017). Laser-assisted microdissection was used for high-resolution transcriptome and metabolite analysis of the supplying maternal tissue (NP) but also of the filial endosperm transfer cells (ETCs) responsible for assimilate uptake into the endosperm. Differentiation and function of the two tissues underlie distinct hormonal regulation to ensure coordination between assimilate supply and changing demand of the developing grain (Thiel et al. 2008, 2009). Tissue-specific mRNA-sequencing of developing ETCs identified Two-Component-Signalling (TCS) phosphorelays as major signal transduction pathway in differentiating barley ETCs. Co-expression modules of TCS-elements and confirmed protein interactions by BiFC identified signalling elements participating in distinct spatiotemporally activated phosphorelays. Hormone-dependent transcriptional activation and correlative data revealed a crosstalk of ABA/ethylene and TCS elements during ETC differentiation (Thiel, 2014). Histidine kinase 1 (HvHK1) was identified as a receptor component with a unique expression in the syncytial endosperm domain adjacent to NP. Knockdown of HvHK1 by RNAi impairs specification in the central ETC region, inhibiting cell wall ingrowths and produces smaller grains with reduced starch. Comparative transcriptome profiling by RNA-seq of altered cells and wildtype ETCs confirmed loss of ETC identity as revealed by a strongly reduced cell wall metabolism/synthesis and diminished assimilate transporter activity. A gene regulatory network of HvHK1 identified genes involved in cell/tissue differentiation, auxin signalling, TCS signalling and protein degradation as potential targets of HvHK1. TALEN-mediated gene modification of type-C response regulator HvRR15 and/or gene copies induced strong endosperm-specific phenotypes with disrupted central endosperm and abnormal aleurone differentiation in the lobe region. New models for modification of further TCS genes with potential functions in grain and spike development have been established using the CRISPR/Cas9-technology (DFG project). Phenotypes of mutated plants are characterised by genetic and histological analysis.
Application/Evaluation of new wheat lines with improved sucrose transport
Wheat represents one of the most important food sources in the world accounting for 26.4% of the total world cereal market (FAO STAT 2016). However, in the last decade wheat production reached a plateau and in combination with the demand of an increasing word population represents a threat for food security worldwide. Increasing the yield potential by knowledge-based improvement is an unavoidable challenge. Our research addresses this demand by trying to enhance the sugar transport machinery in cereals. Winter wheat lines ectopically expressing a barley sucrose transporter (HOSUT) controlled by Hordein B1 promoter (HOSUT) are characterized by an increased sucrose uptake capacity and partitioning (Weichert et al. 2010). In-depth analysis of these lines under field-like conditions shows that grain size and yield are increased together with higher iron and zinc concentrations compared to the non-transformed control (Saalbach et al. 2014; Weichert et al. 2017). This makes these lines interesting models for improved assimilate supply to grains. In cooperation with AGROSCOPE in Zürich we carried out field trials that allow us to analyse the yield advantages and characteristics of the HOSUT-lines under field conditions.
WheatScan - Evaluation of wheat intolerances
Causes for wheat intolerances are evaluated using expression analysis of gluten and amylase trypsin inhibitor gene families in grains of wheat varieties from the last 100 years together with sequencing of regulatory and coding sequences. The work is accompanied by analysis of protein fractions and studies on relevant patients (WheatScan Projekt, Leibniz-Wettbewerbs 2016, cooperation with Uniklinikum Mainz and Erlangen, Leibniz-Institut für Lebensmittel-Systembiologie, TU München Helmholtz Zentrum München).