Guy Golan, Harel Bacher, Elisha Hendel, Yoav Sharaby, Nimrod Schwartz. The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot
In the next half-century, a significant increase in wheat production will be needed in order to meet food demands of the growing human population. This challenge is further exacerbated under the projected climate change scenarios, which predict an aggravation in the intensity and frequency of extreme climatic events. Inadequate water availability, often combined with high temperature stress, is the main environmental factor limiting wheat production worldwide. The development of novel cultivars with more efficient water-use and greater drought resistance capacity is considered a sustainable and economically viable solution to this problem. The implementation of this solution requires wide explorations of potential genetic resources and elucidating the mechanisms underlying plant-resilience to suboptimal field conditions.
Wild progenitors of crop plants harbor a rich allelic repertoire as compared with their descendant crop-plants as consequence of the genetic bottlenecks associated with plant domestication and subsequent selection in man-made agro-ecosystems. Wild emmer wheat is the direct progenitor of durum wheat and bread wheat. It is distributed throughout the Near Eastern 'Fertile Crescent', across a variety of ecological conditions, from hot and dry (with as little as 230 mm annual rainfall) to cool and humid (with over 1500 mm) habitats. The discovery of wild emmer by Aaron Aaronsohn in 1906, opened the possibility to utilize its rich allelic repertoire for improvement of numerous economically important traits, including climate-change resilience. Our research aims to identify, in the wild ancestors of crop plants, valuable "left behind" alleles and introduce them into modern wheat cultivars.
Recovery of two roots after water stress in wild emmer
Optimizing the wheat root system architecture play crucial to achieve future food and nutrition security. The wheat root system is composed of two root types, the seminal roots system which develops upon germination and includes the primary root and two pairs of lateral seminal roots, and adventitious roots which develops later on basal nodes of the shoot. Seminal roots constitute the main route for water and nutrients uptake during the early stages of wheat development. The seminal roots penetrate the soil earlier and deeper than adventitious roots and maintains growth throughout the plant life cycle.
Analyzing photosynthesis of wild wheat in the field (students Harel Bacher and Yoav Sharaby)
Under east Mediterranean climatic conditions, prolonged absence of rain after seed germination results in seedling dehydration. Wild emmer populations are generally more susceptible to desiccation of the seminal root system than field sown modern wheat seedlings because a certain fraction of the seeds germinate on (or very close to) the soil surface. Therefore, wheat evolution over thousands of years resulted in selection of drought-adaptive mechanisms ensuring survival in case of seedling dehydration. Recent study in our lab showed that modern wheat cultivars develops five seminal roots during seedling development, while wild emmer seedlings develop only three roots, preserving the second pair of seminal roots at its primordial state.
Various experiments in the laboratory and in the field showed that under water stress conditions, the seminal roots of modern wheat varieties are dehydrated and the ability of seedlings to recover upon rehydration extremely decreases. However, following rehydration, wild emmer seedlings are capable of activating the second pair of root primordia, by that recovering growth of the root system and the shoot. These results suggest that the maintenance of roots at their primordial state may serve as a seedling protective mechanism against water stress. Genetic dissection of seminal root number showed that it is controlled by few major loci that may be introduced to elite cultivars for breeding resistant cultivars in semi-arid environments.
Collecting wild emmer in the field (Dr. Guy Golan)
Root architecture influences the ability of plants to extract water from the soil, and it reflects plants strategies to survive under limited resources. The increase in the number of seminal roots was found to be associated with a decrease in water movement. Using anatomical, physiological and computational modeling, we examine the changes in root hydraulic properties and revealed that wild wheat has significantly larger metaxylem elements. Accordingly calculated axial conductance of the wild plants was significantly higher as compared with the domesticated seedlings.
An important strategy of plants adaptation to water stress is its ability for genetic plasticity. We developed introgression line with wild emmer wheat chromosome segments on background of elite wheat cultivar. Analysis of dynamic growth showed that under water stress the line exhibited significant shift in root-shoot ratio, while shoot biomass showed reduction the root biomass increased. This shift in resources allocation between root and shoot biomass, contribute to enhance photosynthesis capacity under stress and reshaping the whole plant architecture. This drought adaptive mechanism derived from “left behind” genes, may serve as basis for future wheat breeding programs.
During a long evolutionary history across a range of environmental conditions in the Near Eastern Fertile Crescent wild emmer wheat has accumulated a wealth of genetic diversity and adaptations to environmental stress conditions. Shortly after discovering the wild emmer wheat, A. Aaronsohn envisioned that: "…the cultivation of wheat may be revolutionized by the utilization of these wild forms ..... to produce races better adapted to semiarid regions...". The materialization of this vision, is becoming more feasible with the recently encoding wild emmer genome and new genetic tools being developed. This will promote the identification of novel alleles and/or genes associated drought tolerance of wheat and improve the knowledgebase needed to develop climate-resilient wheat cultivars.