First cultivated more than 10,000 years ago, barley belongs to the Triticeae family, which includes wheat and rye, and that together provides around 30 percent of the calories consumed worldwide. It is the world’s fourth most important cereal crop both in terms of area of cultivation and in quantity of grain produced. At 5 billion nucleic acid base pairs (the genetic “letters”), the barley genome is almost twice the size of that of humans, and determining the sequence of its DNA had presented a major challenge. That was mainly because its genome contains a large proportion of closely related sequences, which are difficult to piece together.
The International Barley Sequencing Consortium (IBSC) is comprised of many researchers at many institutions across the world and they say the new resource will facilitate the development of new and better barley varieties able to cope with the demands of diverse climate and in the fight against cereal crop diseases, which cause millions in losses every year. The IBSC was founded in 2006 and along with the US includes scientists from Germany, Japan, Finland, Australia, the United Kingdom and China. The genome sequence and related resources are freely accessible online.
"The barley genome will help us in our quest to help solve the ‘9 billion-people’ question: How to feed two more billion mouths in less than 40 years,” said IBSC member, Rod Wing, professor of plant sciences in the University of Arizona, in a statement. "The larger the genome, the more repetitive its sequence is. If you perform a whole genome shotgun sequencing approach, 90 to 95 percent of the genome sequence you get consists of almost the same sequence over and over, with no order or orientation.”
Wing’s group laid an important piece of the groundwork for the sequencing project by constructing a library containing the genetic sequences of large chunks of the genome. Built in 2000, that library was the largest of its kind at the time. In the current effort, those pieces helped the consortium determine the locations of the genetic sequences obtained by the participating centers so they could construct a map of the barley genome.
Barley has twice the genome of humans - but one third that of wheat. Photo: Alexander von Halem
“With these big genome projects you have different groups that are specialized in different tasks,” Wing said. “This project has been going on for a long time and finally culminated with a high-quality genome sequence.”
The IBSC has succeeded in positioning the DNA sequences of the majority of barley genes into a linear order along each individual chromosome. This is an important milestone toward eventually unravelling a full barley genome sequence.
“Here, we have a physical map and a draft sequence,” Wing said. “We know where most of the genes reside in the barley genome. If the sequence were a text, we’d have almost all the words in the right order, but every 100 words or so, there’d be one that’s missing.”
Their publication provides a detailed overview of the functional portions of the barley genome, revealing the order and structure of most of its 32,000 genes and a detailed analysis of where and when genes are switched on in different tissues and at different stages of development. They describe the location of dynamic regions of the genome that carry genes conferring resistance to diseases.
“Plants carry genes that make them resistant to certain types of bacterial, fungal and viral diseases,” Wing said. “Once we know where all the resistance genes are in the most important crop cereals, we can do comparative genomics – a great tool for plant pathologists to dissect the molecular pathways used by the plants in fighting pathogens. The genome sequence shows you where all the potential disease resistance genes are, and that makes it easier to track them and select the ones you want by conventional breeding approaches.”
According to the IBSC, the barley sequence highlights with unprecedented detail the differences across a range of barley cultivars and provides a springboard for the development of innovative approaches for the use of abundant genetic resources kept in gene banks around the globe. In addition, access to the assembled catalog of gene sequences will streamline efforts to improve barley production through breeding for varieties better able to withstand pests and disease and deal with adverse environmental conditions such as drought and heat stress.
Next up: the wheat genome, the last frontier in the world's most important cereal crops, with a genome about three times larger than barley.
Published in Nature.