Construction and characterization of a soybean yeast artificial chromosome library and identification of clones for the Rps6 region

Physiological and molecular approaches to improve drought resistance in soybean

Drought stress is a major constraint to the production and yield stability of soybean [Glycine max (L.) Merr.]. For developing high yielding varieties under drought conditions, the most widely employed criterion has traditionally been direct selection for yield stability over multiple locations. However, this approach is time consuming and labor intensive, because yield is a highly quantitative trait with low heritability, and influenced by differences arising from soil heterogeneity and environmental factors. The alternative strategy of indirect selection using secondary traits has succeeded only in a few crops, due to problems with repeatability and lack of phenotyping strategies, especially for root-related traits. Considerable efforts have been directed towards identifying traits associated with drought resistance in soybean. With the availability of the whole genome sequence, physical maps, genetics and functional genomics tools, integrated approaches using molecular breeding and genetic engineering offer new opportunities for improving drought resistance in soybean. Genetic engineering for drought resistance with candidate genes has been reported in the major food crops, and efforts for developing drought-resistant soybean lines are in progress. The objective of this review is to consolidate the current knowledge of physiology, molecular breeding and functional genomics which may be influential in integrating breeding and genetic engineering approaches for drought resistance in soybean.

The soybean-Phytophthora resistance locus Rps1-k encompasses coiled coil-nucleotide binding-leucine rich repeat-like genes and repetitive sequences

BACKGROUND

A series of Rps (resistance to Pytophthora sojae) genes have been protecting soybean from the root and stem rot disease caused by the Oomycete pathogen, Phytophthora sojae. Five Rps genes were mapped to the Rps1 locus located near the 28 cM map position on molecular linkage group N of the composite genetic soybean map. Among these five genes, Rps1-k was introgressed from the cultivar, Kingwa. Rps1-k has been providing stable and broad-spectrum Phytophthora resistance in the major soybean-producing regions of the United States. Rps1-k has been mapped and isolated. More than one functional Rps1-k gene was identified from the Rps1-k locus. The clustering feature at the Rps1-k locus might have facilitated the expansion of Rps1-k gene numbers and the generation of new recognition specificities. The Rps1-k region was sequenced to understand the possible evolutionary steps that shaped the generation of Phytophthora resistance genes in soybean.

RESULTS

Here the analyses of sequences of three overlapping BAC clones containing the 184,111 bp Rps1-k region are reported. A shotgun sequencing strategy was applied in sequencing the BAC contig. Sequence analysis predicted a few full-length genes including two Rps1-k genes, Rps1-k-1 and Rps1-k-2. Previously reported Rps1-k-3 from this genomic region 1 was evolved through intramolecular recombination between Rps1-k-1 and Rps1-k-2 in Escherichia coli. The majority of the predicted genes are truncated and therefore most likely they are nonfunctional. A member of a highly abundant retroelement, SIRE1, was identified from the Rps1-k region. The Rps1-k region is primarily composed of repetitive sequences. Sixteen simple repeat and 63 tandem repeat sequences were identified from the locus.

CONCLUSION

These data indicate that the Rps1 locus is located in a gene-poor region. The abundance of repetitive sequences in the Rps1-k region suggested that the location of this locus is in or near a heterochromatic region. Poor recombination frequencies combined with presence of two functional Rps genes at this locus has been providing stable Phytophthora resistance in soybean.