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Achieving new levels of crop yield and new uses for crops
- From Crop Domestication to Super-domestication

The future course of domestication will continue to rely onchanges to the architecture, metabolism and physiology of crops.To reach new levels of crop yield and new uses for crops, acombination of applied and also theoretical approaches involvingcomputer or systems' biology models will be required. To copewith new challenges to crop production, efficient approachesto screen germplasm for genes to both biotic and abiotic stressesin wild and cultivated germplasm are needed. Crops primarilyused for food are now also being used to meet demands for sustainablefuel supply, while the use of plants for construction timbersand fibres in paper, textiles and composite products continuesto increase. To provide value-added crops, new products fromcrops are being developed. Specialized plant products for processing(such as starches, oils, even plastics) for food, pharmaceutical,cosmetic and industrial uses are required for growing markets(Heslop-Harrison, 2002).

As information accumulates on domestication-related traits andtheir genome distribution, new avenues to attain higher yieldand to tailor-make crops are opened up. Analysis of yield andplant height in a cross between two Japanese rice varieties,‘Koshihikari’ and ‘Habataki’, revealedseveral QTLs for each trait. One QTL, Gn1a, increased grainproductivity and acts by altering the production of the enzymecytokinin oxidase/dehydrogenase that degrades the phytohormonecytokinin. By reducing the expression of Gn1a, cytokinin accumulatesin inflorescence meristems, resulting in an increased numberof grains and, hence, a plant with the potential for increasedyield (Ashikari et al., 2005). By accumulating a variety ofyield-related QTLs for increasing both source to produce photosynthateand sink to accept photosynthate, new levels of yield may beachieved.

Throughout history, plants have been subjected to changing climate,and farmers have adopted new species and varieties to meet thechallenges; indeed, post-glacial climate changes may have beenone of the factors leading to the origin of agriculture andplant domestication. Climate change is affecting agriculturein the 21st century; some changes will be met within existingadaptations of plants, but other factors such as increased UVand carbon dioxide levels require new selections based on understandingof plant responses. Hidema and Kumagai (2006) reported considerablevariation in UVB sensitivity of rice cultivars, which was causedby differences of one or two bases in the CPD (cyclobutane pyrimidinedimer) photolyase, altering the activity of the enzyme. Theysuggest that the resistance of rice to UVB radiation can thereforebe increased by selective breeding or bioengineering of thegenes encoding CPD photolyase. Although carbon dioxide enhancementis regularly used to improve glasshouse production, it is notclear how field crops will respond to changes in atmosphericcarbon dioxide concentrations, involving complex interactionsof phytosythesis with light and dark respiration (Bunce, 2005).

There is no naturally occurring waxy wheat variety but in breadwheat there are waxy loci in each of its three different genomes,A, B and D. The waxy locus encodes starch granule protein 1(SGP-1). Different isoforms are encoded by genes in each genome(sgpA1, sgpB1 and sgpD1). In the germplasm collection of breadwheat, cultivars lacking one of the three isoforms were found,two cultivars from Korea lacked SGP-A1, one from Japan lackedSPG-B1 and one from Turkey lacked SPG-D1. By making appropriatecrosses, these genes were combined in a single plant, resultingin the first waxy wheat, which had a null for all three isoformsof SGP-1 (Yamamori et al., 2000), an example of using markersfor identification of alleles and then marker-assisted selectionto find the desired allele combination. The effort to producewaxy wheat in Japan has led to its use as an ingredient forimproved Japanese-style noodles. These examples show the valueof screening germplasm collections with diverse material foruseful genetic variation, but also emphasize that it is notalways necessary to search in exotic material or employ radicaltechniques to make innovative progress in plant breeding.

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