When can intelligent design of crops by humans outperform natural selection?

Natural selection operated on the wild ancestors of crop plants for millions of years. Many seemingly intelligent design changes that we could make to enzyme structure or gene expression would duplicate (at least in phenotypic effect) variants already rejected by past natural selection. These variants died out because they decreased individual plant survival or reproduction under preagricultural conditions. Many of the variants rejected by past natural selection would also reduce crop yield or quality today, so it would be a waste of time to duplicate them using molecular methods. For example, most changes to rubisco will decrease photosynthesis (and crop yield) under current conditions, just as they would have decreased photosynthesis (and individual plant fitness) under preagricultural conditions. A few of natural selection’s ‘rejects’, however, would be genuine improvements by human criteria. Can we identify these promising rejects? Opportunities for crop genetic improvement that were missed by past natural selection are likely to fall into three major categories. First, and most important, conflicts of interest among competing plants, or between plants and their microbial symbionts, can cause trade-offs between individual plant fitness (favoured by past natural selection) and the collective performance of the crop community. Therefore, we can sometimes increase yield by reversing the effects of past natural selection for individual competitiveness. Second, changes in climate, soil fertility and pest populations mean that some variants that were less fit in the past will be more fit today. In this case, crop genetic improvement may accelerate changes that are already favoured by ongoing natural selection in an agricultural context. Third, eventually molecular methods may produce genotypes so different from anything that existed in the past that we cannot assume they were tested and rejected by natural selection. C4 photosynthesis has evolved repeatedly, however, so a proposed innovation would have to be more radical than C4 photosynthesis before we can assume it was missed by past natural selection. The relative importance of these three kinds of opportunity is likely to change over the next few decades. Some trade-offs between individual competitiveness and the yield of the crop community have already been exploited, as in dwarf wheat and rice, but other opportunities may remain. Our ability to design radical new enzymes from scratch, or to predict the consequences of major changes in gene expression patterns, may improve over coming decades. Even after most significant opportunities to improve yield potential (yield in the absence of pests and diseases) have been fully exploited, ongoing evolution of pests and pathogens will create a continual need for ‘Red Queen Breeding’, generating a stream of new cultivars to keep up with the latest biotic threat

Inclusive fitness in agriculture

Trade-offs between individual fitness and the collective performance of crop and below-ground symbiont communities are common in agriculture. Plant competitiveness for light and soil resources is key to individual fitness, but higher investments in stems and roots by a plant community to compete for those resources ultimately reduce crop yields. Similarly, rhizobia and mycorrhizal fungi may increase their individual fitness by diverting resources to their own reproduction, even if they could have benefited collectively by providing their shared crop host with more nitrogen and phosphorus, respectively. Past selection for inclusive fitness (benefits to others, weighted by their relatedness) is unlikely to have favoured community performance over individual fitness. The limited evidence for kin recognition in plants and microbes changes this conclusion only slightly. We therefore argue that there is still ample opportunity for human-imposed selection to improve cooperation among crop plants and their symbionts so that they use limited resources more efficiently. This evolutionarily informed approach will require a better understanding of how interactions among crops, and interactions with their symbionts, affected their inclusive fitness in the past and what that implies for current interactions

Controlling the reproductive fate of rhizobia: how universal are legume sanctions?

When a single host plant is infected by more than one strain of rhizobia, they face a tragedy of the commons. Although these rhizobia benefit collectively from nitrogen fixation, which increases host-plant photosynthesis, each strain might nonetheless increase its own reproduction, relative to competing strains, by diverting resources away from nitrogen fixation. Host sanctions can limit the evolutionary success of such rhizobial cheaters (strains that would otherwise benefit by fixing less nitrogen). Host sanctions have been shown in soybean (Glycine max) nodules, where the next generation of symbiotic rhizobia is descended from bacteroids (the differentiated cells that can fix nitrogen). Evidence for sanctions is less clear in legume species that induce rhizobial dimorphism inside their nodules. There, bacteroids are swollen and cannot reproduce regardless of how much nitrogen they fix, but sanctions could reduce reproduction of their undifferentiated clonemates within the same nodule. This rhizobial dimorphism can affect rhizobial evolution, including cheating options, in ways that may affect future generations of legumes. Both the importance of sanctions to hosts and possible physiological mechanisms for sanctions may depend on whether bacteroids are potentially reproductive

Clade-dependent effects of drought on nitrogen fixation and its components - Number, size, and activity of nodules in legumes

Drought affects the growth of legumes directly, and indirectly, by reducing total nitrogen fixation. Here, we compiled published data to compare the sensitivity to water deficit on plant growth and total nitrogen fixation traits, i.e., the number of nodules per plant, average nodule mass, and nitrogen fixation per unit nodule mass. Hierarchies of phenotypic plasticity have been established for seeds and organelles, whereby variation in number associates with conserved size. By analogy, our first hypothesis is that there is a hierarchy of plasticities between nitrogen fixation traits. Our second hypothesis is that determinate nodules are more sensitive to water deficit than their indeterminate counterparts, because the latter can reactivate meristems when water becomes available. In our sample, onset of stress treatment averaged 28 d after sowing; median duration of stress was 12 d; and intensity of stress (ratio of shoot biomass between stressed and control) averaged 0.65. These drought conditions (i) reduced total nitrogen fixation and average nodule mass more severely than plant shoot mass, (ii) elicited a hierarchy of plasticities whereby number of nodules per plant varied substantially, and average nodule mass and nitrogen fixation per unit nodule mass were relatively conserved, and (iii) affected more severely Milletioids (determinate, ureide exporting nodules) than their IRLC counterparts (indeterminate, amide exporting nodules).Nasir Iqbal, Victor O Sadras, R Ford Denison, Yi Zhou, Matthew D Dento

Truncated hemoglobins in actinorhizal nodules of Datisca glomerata

Three types of hemoglobins exist in higher plants, symbiotic, non-symbiotic, and truncated hemoglobins. Symbiotic (class II) hemoglobins play a role in oxygen supply to intracellular nitrogen-fixing symbionts in legume root nodules, and in one case ( Parasponia Sp.), a non-symbiotic (class I) hemoglobin has been recruited for this function. Here we report the induction of a host gene, dgtrHB1, encoding a truncated hemoglobin in Frankia-induced nodules of the actinorhizal plant Datisca glomerata. Induction takes place specifically in cells infected by the microsymbiont, prior to the onset of bacterial nitrogen fixation. A bacterial gene (Frankia trHBO) encoding a truncated hemoglobin with O (2)-binding kinetics suitable for the facilitation of O (2) diffusion ( ) is also expressed in symbiosis. Nodule oximetry confirms the presence of a molecule that binds oxygen reversibly in D. glomerata nodules, but indicates a low overall hemoglobin concentration suggesting a local function. Frankia trHbO is likely to be responsible for this activity. The function of the D. glomerata truncated hemoglobin is unknown; a possible role in nitric oxide detoxification is suggested

Exploration for hot dry rock geothermal resources in the Midcontinent USA. Hot dry rock conceptual models for exploration, HDR test site investigations, and the Illinois Deep Drill Hole Project. Volume 2

Three potential sources of HDR, each covering approximately a 2/sup 0/ x 2/sup 0/ area, were identified and subjected to preliminary evaluation with ad hoc exploration strategies. In the Mississippi Embayment test site, lateral thermal conductivity variations and subcrustal heat sources may be involved in producing abnormally high subsurface temperatures. Studies indicate that enhanced temperatures are associated primarily with basement rift features where vertical displacement of aquifers and faults cause the upward migration of hot waters leading to anomalously high, local, upper crustal temperatures. The Western Nebraska test site is a potential low temperature HDR source also related, at least in part, to groundwater movement. There appear to be much more widespread possibilities for similar HDR sites in the Great Plains area. The Southeast Michigan test site was selected for study because of the possible presence of radiogenic plutons overlain by a thickened sedimentary blanket. There is no direct information on the presence of abnormally high temperatures in this area, but the study does show that a combination of gravity and magnetic anomaly mapping with regional geological information derived from sparse drill holes in the Phanerozoic rocks is useful on a widespread basis for focusing on local areas for detailed evaluation

Human selection and the relaxation of legume defences against ineffective rhizobia

Enforcement mechanisms are thought to be important in maintaining mutualistic cooperation between species. A clear example of an enforcement mechanism is how legumes impose sanctions on rhizobial symbionts that fail to provide sufficient fixed N2. However, with domestication and breeding in high-soil-N environments, humans may have altered these natural legume defences and reduced the agricultural benefits of the symbiosis. Using six genotypes of soya beans, representing 60 years of breeding, we show that, as a group, older cultivars were better able to maintain fitness than newer cultivars (seed production) when infected with a mixture of effective and ineffective rhizobial strains. Additionally, we found small differences among cultivars in the ratio of effective : ineffective rhizobia released from their nodules, an indicator of future rhizobial strain fitness. When infected by symbionts varying in quality, legume defences against poor-quality partners have apparently worsened under decades of artificial selection