The plant root system is an important yet underscored agronomic trait. Plant roots perform many essential functions including water and nutrient uptake, storage of reserves, anchorage to the soil and establishment of biotic interactions in the rhizosphere. The size and architecture of the root system determine the plant’s ability to fulfil these functions. Water and nutrient shortages, in particular, limit growth in many agricultural ecosystems and pose increasing problems, for example, the phosphate concentration is too low in 80% of agricultural soils. Similarly, access to nitrogen in the form of ammonium and nitrate is vital during vegetative growth but is often limited. These substances are added through fertilization. Fertilization is expensive, energy consuming (~50% of the energy used in agriculture is for fertilization) and often causes environmental problems such as eutrophication. In addition, worldwide agriculture uses ~75% of the freshwater resources. Root length and/or root density are positively correlated with mineral uptake, in particular for those with limited diffusion. Root architecture determines also access to water in many instances. Thus the negative impact of the aforementioned factors may be reduced by optimizing root system architecture (RSA).

Root enhancement is difficult to assess with classical breeding strategies. RSA cannot yet be determined in the field. In laboratory experiments, root systems can be visualized, but proper characterization at high throughput has not been achieved yet and the relevance of lab results for the prediction of crop traits in field plots is questionable. We present an alternative approach in order to overcome these limitations. Recent work has revealed that the status of the plant hormone cytokinin and certain evolutionary conserved miRNAs are factors controlling RSA. Importantly, these factors can be manipulated in a targeted fashion and proof has been obtained in model plants that genetic engineering generates an enhanced root system, improved nutrient accumulation in the aerial organs and improved drought resistance. A central aim of this project is, because of their high agricultural relevance, to transfer these novel approaches to a cereal plant.

We selected barley (Hordeum vulgare L.) as an important European cereal crop plant. Spain, Germany and France are the leading barley growing nations in Europe with a total surface of >6 Mill ha. Together, these three nations produce >20% of the world´s total barley yield (c. 135 Mio t p.a., FAO report). Global climate changes pose increasing drought problems, in particular during early summer and during seed filling. In addition, barley shares the common agricultural problem associated with fertilization. Barley is a diploid plant and used as a genetic model, it can be transformed and increasing genomic resources are becoming available. Importantly, barley can serve as a reference for wheat, the most relevant European crop but currently less amenable for experimental work.    

In addition to the generation of lines with enhanced root systems, the interplay of different factors in determining root architecture, focusing on nitrogen and phosphate, will be studied, novel methods to visualize and study RSA will be developed. These studies will involve work in Arabidopsis thaliana as it is an advantageous system to study crosstalk between factors that modulate root architecture and to identify novel genes. Specific activities in the project will be as follows:

• the generation of transgenic barley plants with an enhanced root system based on knowledge recently gained in model plants;
• to understand the regulation of RSA by different intrinsic regulatory pathways (hormones, miRNA) and identify their interactive mode of action with exogenous cues (nitrogen, phosphate); and
• to refine and apply optical imaging and tomographic methods to study RSA and to generate model-based understanding of the interaction of root growth and the environment.

In order to achieve the research objectives, this proposal brings together a consortium with a complementary set of expertises in genomics and molecular biology, nutrient physiology and phenotyping methods. The project exploits the most recent technologies and will provide valuable training to participating young scientists. The results have a high commercial potential as breeding lines with improved performance under limiting conditions, i.e. a reduced need for water and fertilization are an important breeding aim. More generally, the project will provide important insights into the function of several parameters regulating RSA and their interaction.

Features of the project which correspond to the PLANT-KBBE program include genomic approaches to identify genes that are relevant to control root growth, an important parameter to determine yield under abiotic stress (e.g. drought) and in low input systems (e.g. low mineral content of the soil). An improved root system will increase yield stability compared to near-isogenic lines lacking this trait and reduce the use of fertilizers thus contributing to better and healthier food production. We will not only work in a model plant (Arabidopsis) but also transfer recent knowledge in which partners of this project are specialists into a cereal, barley. Potential application beyond barley is an expected outcome. Thus this proposal supports the overall objectives of the PLANT-KBBE program.