Turning over a new leaf in plant genomics

Plant species have adapted to most of the planet’s eco­ systems, off ering an amazing range of traits, behaviors and morphologies. Unlocking the underlying mecha­ nisms for this diversity is providing novel insights into the molecular hardwiring of plants and how they interact with their environment, furnishing new opportunities to develop better and more sustainable crops. Access to fast and inexpensive sequencing and advances in compu­ tational methods, coupled with an increasing demand for food and fuel crops, have provided the basis for the current revolution in plant genomics. Th e development of agriculture and the ability to store food laid the foundations of early human civilizations and defi ned modern society very much as we understand it today. Arguably, the domestication of small grain crops over the past 10,000 years ranks as one of the most important events in human history. Th e co­evolution of humans and plants, however, goes back to the origins of the primate lineage. One example is the expansion of hominids, which some authors correlate with the direct adoption in the diet of C 4 plants, or indirectly by preying on animals that consumed C 4 plants [1]. Th e development of salivary amylase, key in the digestion of starch, which emerged as a gene family expansion independently in primates and rodents, provides another example. Th e number of copies of the amylase genes is highly variable in human populations with diff erent starch diets [2]. But it is not only about food: ficrops provided clothes and paper; wood from trees became material for shelters, and until the discovery of fossil fuels, was one of the most important sources of energy. We have also used plant extracts as a source of medicines and other useful bio­ chemicals that in many cases cannot yet be synthesized in the laboratory. Plants also have a direct impact on our environment: as autotrophs they are responsible for fiatmospheric carbon into food and fi ber, with

Plant species have adapted to most of the planet's eco systems, off ering an amazing range of traits, behaviors and morphologies. Unlocking the underlying mecha nisms for this diversity is providing novel insights into the molecular hardwiring of plants and how they interact with their environment, furnishing new opportunities to develop better and more sustainable crops. Access to fast and inexpensive sequencing and advances in compu tational methods, coupled with an increasing demand for food and fuel crops, have provided the basis for the current revolution in plant genomics.
Th e development of agriculture and the ability to store food laid the foundations of early human civilizations and defi ned modern society very much as we understand it today. Arguably, the domestication of small grain crops over the past 10,000 years ranks as one of the most important events in human history. Th e coevolution of humans and plants, however, goes back to the origins of the primate lineage. One example is the expansion of hominids, which some authors correlate with the direct adoption in the diet of C 4 plants, or indirectly by preying on animals that consumed C 4 plants [1]. Th e development of salivary amylase, key in the digestion of starch, which emerged as a gene family expansion independently in primates and rodents, provides another example. Th e number of copies of the amylase genes is highly variable in human populations with diff erent starch diets [2]. But it is not only about food: fi ber crops provided clothes and paper; wood from trees became material for shelters, and until the discovery of fossil fuels, was one of the most important sources of energy. We have also used plant extracts as a source of medicines and other useful bio chemicals that in many cases cannot yet be synthesized in the laboratory. Plants also have a direct impact on our environment: as autotrophs they are responsible for fi xing atmospheric carbon into food and fi ber, with RuBisCO, one of the plant enzymes involved in this process, being probably the most abundant protein on earth.

Rich biology, amazing genomics
Plant species have developed diverse strategies to adapt and thrive in all kinds of climates and terrains, resulting in an exquisite repertoire of natural products. Not having the opportunity to travel to obtain food or run away from adverse conditions, plants have evolved to deal with extreme changes in the environment. Annual plants, for instance, can focus the most resourceintensive activities, such as reproduction, at the most favorable time of the year. Other changes, such as daytonight fl uctuations, are linked to the plant circadian cycle and the syn chro nized expression of specifi c sets of genes at particular times. Plants also need to cope with more sudden changes such as attacks from pathogens and grazing animals. Th ese strategies are supported by rich and complex metabolic networks that enable plants to synthesize a wide range of compounds (for example, alkaloids, terpenoids and phenolics) [3]. Th e constant need for adaptation, the prevalence of polyploid species and past wholegenome duplication events are refl ected in the larger number of genes observed in plants [4] compared with animals. Th is diversity has stimulated many of the scientifi c breakthroughs that have emerged from research in plant sciences, including the discovery of transposable elements in maize by Barbara McClintock back in the 1940s, the characterization of RNA inter ference [5] and, more recently, the use of TALE nucleases to target and edit plant genomes [6]. Th e completion of the Arabidopsis thaliana genome [7] at the turn of the century followed by the rice genome publications [8,9] have also stirred a growing body of research in plant genomics. Th e success of Arabidopsis as a model organ ism is rooted in its accessible genetics tied to a relatively simple genome [1012]. Many plants, especially some of the popular monocot crops and gymnosperm trees, have, however, more complex and larger genomes. Th e Triticeae tribe within the grasses family includes some of the most important cereal crops, such as barley (Hordeum vulgare), with a genome that is 50 times larger than Arabidopsis and almost twice the typical mam malian genome [13]. In the same family, bread wheat (Triticum aestivum) adds another level of complexity with an allohexaploid genome that, over successive rounds of hybridization, has combined three diploid species with genomes the size of barley into a single huge

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*Correspondence: Mario.Caccamo@tgac.ac.uk 1 The Genome Analysis Centre, Norwich Research Park, Norwich NR4 7UH, UK Full list of author information is available at the end of the article genome. Another interesting example is the evolution and domestication of the dicot Brassicaceae family that includes Arabidopsis thaliana and gave rise to a number of economically important vegetable and oil crops such as broccoli, cabbage and oilseed rape. In a relatively short evolutionary time, this family expanded into species with markedly different external structural and anatomy, offer ing a unique model for the study of the underlying genetics of morphogenesis [14]. The complexity of these plant genomes has, however, posed an important challenge to the use of sequencing technologies and the downstream computational analyses [15]. Over the past few years, dramatic advances in cytogenetic techniques, sequencing technologies and more sophisticated sequence analysis methods have impacted research in some of the most complex and challenging crops [1621].

The opportunity of genomics
One of the immediate challenges of plant genomics is to mine more efficiently the vast amount of data that are being generated on a daily basis as more and more plant genomes are being sequenced [22]. There is, however, an opportunity emerging from the integration of the latest technologies with more traditional genetics and the fact that plant genomes are highly plastic. The easy manipu lation of plant genomes is not only a convenient tool for research, but it is also the foundation for modern plant breeding. The possibility of generating doubled haploids to obtain fully homozygous individuals, for instance, can significantly speedup breeding, providing an oppor tunity to go faster 'from the genome to the field' . Genome wide association studies are an example of the application of genomics in both model plant species and crops that have help to identify loci and alleles associated with complex traits [2326]. The development of highdensity molecular markers will be one of the most important tools for informing the design of the breeding programs of the future [27]. However, the most agronomically important traits, such as yield and drought tolerance, involve multiple genes and complex interactions with the environment, requiring more sophisticated breeding strategies such as genomic selection. The application of advanced genomics to improve breeding techniques in grass crops, for instance, will play a key role in securing affordable and nutritious food for an increasing human population.
The reviews, opinions and scientific work presented in this special issue of Genome Biology are a testament to the coming of age of plant genomics research and its applications.