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A Man Who Makes Chromosomes Genetic Engineering A man who makes chromosomes A new method of genetic engineering – constructing small, whole artificial chromosomes in plant cells – may avoid many of the drawbacks of existing methods and generate high-yielding, pest-resistant agricultural crops. Professor James A Birchler, of the University of Missouri, USA, is at the forefront of research bringing this technology into practice. n a world with a rising population and Prof Birchler’s pioneering new technique ever-accelerating climatic change, avoids these dangers by generating agriculture is constantly playing catch- entire, artificial chromosomes in plant cells up. Degraded soils, resistant pests, alongside their existing genetic material, unpredictable rainfall and extreme giving scientists much more control of the temperatures can all wreak havoc with genetic engineering process and its results, Icrop yields, so new crop varieties that can and providing the potential to change withstand these pressures are now needed multiple characteristics of a crop plant in one more than ever to meet the growing fell swoop. demands of the human population. CONSTRUCTING CHROMOSOMES Traditional plant breeding methods, Chromosomes are like the pages in the by deliberate crossing, rely on chance instruction book for building an organism. mutations and may take many years to Each cell of an animal or plant contains develop crop varieties with the desired several pairs of these strings of DNA, each traits. More recently, science has turned to comprising multiple genes and associated orthodox genetic engineering for better- coded instructions for switching them on targeted and faster results. This usually and off. Each chromosome also contains a involves adding genes from other species specific structure used to organise it during (‘transgenes’) to a crop’s existing genome. cell division (the ‘centromere’) and protective caps at the ends of each string known as However, transgenes are usually inserted ‘telomeres’. randomly into the genome, which can cause their activity to be unpredictable. Even Although artificial chromosomes have been worse, they can land in the middle of an produced successfully in microorganisms existing gene, disrupting its precise code such as yeasts for many years, plants have with potentially deleterious consequences proved much more complex. However, in for plant survival. 2007 Prof Birchler and his lab successfully A minichromosome … is independent of the plant’s natural genetic material and can be stably transmitted to the next generation: a blank slate for designing bespoke crops 49 Genetic Engineering © Nat Graham Detail RESEARCH OBJECTIVES Dr Birchler’s research focuses on using Can you explain why it was so chromosomes are separate from the artificial chromosome technology in difficult initially to generate artificial other chromosomes, which allows their plants, and the benefits it can have on chromosomes in plants compared to inheritance without linkage to them. More improving agricultural practices. yeast? importantly, gene editing and insertion The first artificial chromosomes were technology is limited in the size of the FUNDING produced in yeast, which served as an insertion; artificial chromosomes can likely National Science Foundation (NSF) inspiration for attempts in plants. However, accept tens of kilobases (up to ~100 kb) with the centromeres in yeast are determined by continued ability for further additions – BIO DNA sequence but functional centromeres all as an independent entity. James A. Birchler is Curators' in plants are epigenetically determined by Distinguished Professor perpetuation of chromatin state. Until this What are the remaining barriers of Biological Sciences at latter realisation became known, attempts to using minichromosomes for the University of Missouri, at producing artificial chromosomes breeding new varieties of crops? Columbia. After obtaining a in plants were destined to fail (with Small chromosomes behave differently BS degree from Eastern Illinois the exception that de novo epigenetic than normal sized chromosomes. However, University in Botany and Zoology in centromeres might form on introduced one can envision straightforward ways 1972, he attended graduate school at DNA). in which normal transmission from one Indiana University majoring in Genetics generation to the next can be made to with a minor in Biochemistry. Why did you choose maize as a model be faithful. A separate issue, but one species to test the artificial chromosome that impacts both artificial chromosome CONTACT technology? technology and gene editing, is the need James Birchler, PhD Our lab had worked on maize as a for more efficient crop plant transformation. University of Missouri genetic model for several years and we Division of Biological Sciences were experienced with working with its What would be your prediction for the 105 Tucker Hall chromosomes. first crop species and/or traits to be Columbia, MO 65211-7400 successfully engineered and brought USA constructed an artificial ‘minichromosome’ DESIGNER CROPS Engineered minichromosome. A root tip metaphase Can you outline the advantages of into agricultural production using karyotype of a plant carrying an engineered in maize. The success of the method The next step envisaged by Prof Birchler minichromosome in maize is shown. The arrow depicts artificial chromosomes over existing minichromosome technology? T: +1 573 882 4905 was demonstrated by proving that the and his team is to start ‘stacking’ a truncated supernumerary B chromosome that carries methods of genetic engineering? One never knows from whence the next E: [email protected] minichromosome was both genetically active multiple beneficial genes upon a single a transgene at the terminus. The green signals are Artificial chromosomes are independent new idea will emerge. The beauty of ipg.missouri.edu/faculty/birchler.cfm probes to various repetitive sequence clusters that W: within its host cell, and successfully passed minichromosome, a process that would allow chromosome identification. The red signal is of the normal set and allow multiple genes science is that it builds on the contributions W: birchler.biology.missouri.edu/maize- on to the next generation of maize plants. be almost impossible to manage through the B chromosome centromeric specific sequence. to be added to them. Thus, the artificial of many individuals going forward. minichromosomes/ traditional modification of the plant’s existing The B chromosome is a nonvital chromosome that is essentially inert and can serve as a foundational The new minichromosome was derived from chromosomes. Incorporating multiple genes platform for engineered minichromosomes. The Find out more: a small, otherwise inactive chromosome in this way could introduce whole new engineered minichromosome was produced by You can find out more about Prof transforming a line with B chromosomes with a in maize known as a ‘B’ chromosome. The biochemical pathways into the plant, more truncating construct containing genes and a telomere Birchler’s work at: https://birchler. use of B chromosomes minimises the risk and more of which could be added as the repeat at one end a minichromosome will vary depending on greatly expand the options available to plant biology.missouri.edu/maize- that the minichromosome will interact needs of producers and consumers develop. the surrounding genetic environment of the breeders. minichromosomes/ detrimentally with the function of the host cell. plant’s other, natural ‘A’ chromosomes. In Ultimately, Prof Birchler believes we could Prof Birchler’s current project, funded by the Artificial chromosomes may be particularly essence, the minichromosome contains produce crops that are resistant to multiple US National Science Foundation, explores ACROSS THE PLANT KINGDOM useful in clonally-propagated crops, both only a centromere, with the telomeres pests including bacteria, viruses, fungi how these ideas can become a reality, by Artificial chromosomes have now been because it is in these species that the and transgenes coding for the new traits and insect herbivores. At the same time, testing methods that allow large numbers developed in several plant species including minichromosomes will be most stable, and desired. It is independent of the plant’s they could be higher-yielding, resistant to of genes to be added to a minichromosome rice and barley, and Prof Birchler sees no also because these are the most difficult to original genetic material and can be stably herbicides, and tolerant to stresses such as in a single step. The methods are still under reason why this method of manufacturing improve by traditional breeding. Clonally transmitted to the next generation: a blank salinity, drought, or extremes of temperature. development, but emerging technologies, minichromosomes – termed ‘telomere propagated crops include some of the slate for designing bespoke crops. It should even be possible to introduce novel such as enzyme-based gene editing and truncation’ – should not be transferrable to most important staple foods of developing traits such as the production of therapeutic haploid breeding, look likely to facilitate many others. This is because the process countries, such as cassava, sweet potato drugs, vaccines, antibodies or micronutrients. the process. Prof Birchler says it will also be uses a chromosome structure – the telomere and banana. Although, as Prof Birchler puts important to explore the wide realm of non- – that is ubiquitous throughout
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