Genomic Resources of Agronomic Crops

Iron-deficiency chlorosis in crops grown on calcareous soil is a major agricultural problem. In susceptible genotypes, Fe deficiency causes chlorosis and any yellowness in the foliage will lead to losses in seed yield. The purpose of this chapter is to review genomic information available on Fe efficiency in soybean, maize and rice. A summary of model plants is also presented. In soybean, most of the genomic information on Fe acquisition and uptake was collected during the search for quantitative trait loci (QTLs) for Fe efficiency that could be used in marker-assisted selection. Thirty-six QTLs associated with Fe efficiency have been placed in eight linkage groups (LG), with the greatest density of QTLs on LGs B2 and N. As of recently, functional characterization of any of these soybean QTLs has not been reported. Improvement of Fe efficiency in soybean has been mainly accomplished by classical breeding methods. In maize, 41 gene sequences associated with Fe have been identified. Despite the cloning of some of these genes, improvement in Fe efficiency in maize has been done by using tolerant genotypes in hybrid combinations. To date, and of the three species considered in this review, rice has the largest amount of genomic information available both at molecular and biochemical levels. Genetic transformation in rice has been conducted to improve its Fe efficiency using genes cloned primarily from barley; steps are now being taken to use genomic information from rice itself to improve Fe efficiency through genetic engineering. Field evaluations of rice transformants for Fe efficiency will be required to determine effects of the integrated genes on seed yield, disease resistance, and other important agricultural traits. These evaluations will be mandatory before transgenic cultivars may be used commercially. More genomic information is added every day, and it will greatly increase in all major crops, thereby increasing the practicality of plant transformation in the improvement of Fe efficiency. Traditional breeding will fulfill an important role in this effort, as extensive evaluations will be required 449