The roots of a new green revolution Griet Den Herder 1, 4 , Gert Van Isterdael 2, 3 , Tom Beeckman 2, 3 and Ive De Smet 2, 3 1 Genetics, Faculty of Biology, University of Munich (LMU), D-82152 Martinsried-Mu ¨ nchen, Germany 2 Department of Plant Systems Biology, VIB, Technologiepark 927, 9052 Gent, Belgium 3 Department of Plant Biotechnology and Genetics, Ghent University, Gent, Belgium 4 Current address: Ablynx nv, Technologiepark 21, 9052 Gent, Belgium A significant increase in shoot biomass and seed yield has always been the dream of plant biologists who wish to dedicate their fundamental research to the benefit of mankind; the first green revolution about half a century ago represented a crucial step towards contemporary agriculture and the development of high-yield varieties of cereal grains. Although there has been a steady rise in our food production from then onwards, the currently applied technology and the available crop plants will not be sufficient to feed the rapidly growing world popula- tion. In this opinion article, we highlight several below- ground characteristics of plants such as root architec- ture, nutrient uptake and nitrogen fixation as promising features enabling a very much needed new green revo- lution. Rise of the hidden half In 1798 Thomas Robert Malthus predicted in his An Essay on the Principle of Population that sooner or later a con- tinuously growing world population will be confronted with famine, disease and widespread mortality [1]. About two hundred years later, the world is facing the major chal- lenge of providing food security for an ever growing world population, while the agricultural area is shrinking [2]. In the middle of the previous century, a Green Revolution allowed food production to keep pace with worldwide pop- ulation growth [3]. The International Food Policy Research Institute has launched the 2020 Vision Initiative with the primary goal to reach sustainable food security for all by 2020 and to cut by 50% the number of chronically under- nourished people on the planet by the year 2015 (http:// www.ifpri.org/book-753/ourwork/program/2020-vision- food-agriculture-and-environment). These deadlines are approaching quickly, and we are far from reaching either of these goals. In the immediate future, plant research will again be central in finding alternative crops or methods to cope with the threatening food shortage. In addition, next to finding the ideal food–population balance, improving plant yield will also be vital for exploiting plants further as a renewable energy source. Unfortunately, plant growth and productivity are greatly affected by environmental stresses such as drought, high salinity, nutrient-deficiency and adverse temperatures. Due to climate changes these challenges are currently becoming even more intensified. In the past, improvement of crops and agricultural techniques has mainly focused on increasing shoot biomass and seed yield [4,5], and the relevance of the root system for food production has often been overlooked. Nevertheless, the root system is taking care of indispensable plant func- tions such as uptake of nutrients and water, anchorage in the substrate and interaction with symbiotic organisms. Consequently, root system development is central for the plant to reach optimal growth and is sure to contribute to the levels of yield obtained in crops. Lately the impact of the ‘hidden half’ on plant growth has become apparent not only in Arabidopsis (Arabidopsis thaliana), but also in crops like wheat (Triticum aestivum), rice (Oryza sativa), maize (Zea mays) and legumes, such as soybean (Glycine max), barrel medic (Medicago truncatula), and Lotus japo- nicus [6–12]. Moreover, recent simulations suggested that changes in root architecture can strongly affect yield, which might be sufficient to explain maize yield trends in the USA Corn Belt [13]. This indicates that root growth and development might represent an underestimated and not fully exploited area for strategies to enhance yield. What are the major challenges? Due to climate change, plant roots and their habitats have a high risk of becoming subjected to unfavorable conditions such as water scarceness, increasing ground water salini- ty, decline in soil nutrients and build up of soil pests. In addition the available arable land is becoming more sparse and precious due to erosion of hill-sides, soil degradation, landslides and the increasing demand for biofuels. The contemporary yields obtained by the classical use of water, fertilizers and pesticides have reached a maximum. Attempts to further boost yield by using more fertilizers and/or pesticides are not feasible, not only because of a higher risk for public health and environmental problems, but also because of negative effects on yield [14]. For the root system to live up to the expectations as an important contributor to improved yield, a number of challenges will need to be tackled (see also Box 1). First, the conversion of unsuitable soil to arable land will require agricultural techniques such as the improve- ment of soil conditions through alternative fertilization. For instance, legume crop rotations as green manure have contributed considerably to the improvement of soil quality [15]. Second, and more critical, those soils will require crops which are able to deal with the awkward edaphic and climate conditions. We will need to improve root architecture, nutrient uptake efficiency, nutrient storage and root-to-shoot transport. Furthermore, roots need to fight off pathogens, prevent loss of soil through erosion and be able to resist the increasingly occurring unfavorable conditions such as salt and drought. The past decade Opinion Corresponding author: De Smet, I. (ivsme@psb.ugent.be). 600 1360-1385/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2010.08.009 Trends in Plant Science, November 2010, Vol. 15, No. 11