As water demand for agriculture exceeds water availability, cropping systems need to become more efficient in water usage, such as deployment of cultivars that sustain yield under drought conditions. Soybean cultivars differ in how quickly they wilt during water-deficit stress, and this trait may lead to yield improvement during drought. The objective of this study was to determine the genetic mechanism of canopy wilting in soybean using a mapping population of recombinant inbred lines (RILs) derived from a cross between KS4895 and Jackson. Canopy wilting was rated in three environments using a rating scale of 0 (no wilting) to 100 (severe wilting and plant death). Transgressive segregation was observed for the RIL population with the parents expressing intermediate wilting scores. Using multiple-loci analysis, four quantitative trait loci (QTLs) on molecular linkage groups (MLGs) A2, B2, D2, and F were detected (P

Background: Iron is one of fourteen mineral elements required for proper plant growth and development of soybean (Glycine max L. Merr.). Soybeans grown on calcareous soils, which are prevalent in the upper Midwest of the United States, often exhibit symptoms indicative of iron deficiency chlorosis (IDC). Yield loss has a positive linear correlation with increasing severity of chlorotic symptoms. As soybean is an important agronomic crop, it is essential to understand the genetics and physiology of traits affecting plant yield. Soybean cultivars vary greatly in their ability to respond successfully to iron deficiency stress. Microarray analyses permit the identification of genes and physiological processes involved in soybean's response to iron stress.

Soybean is a major crop cultivated in Brazil, and acetolactate synthase (ALS)-inhibiting herbicides are widely used to control weeds in this crop. The continuous use of these ALS-inhibiting herbicides has led to the evolution of herbicide-resistant weeds worldwide. Greater beggarticks is a polyploid species and one of the most troublesome weeds in soybean production since the discovery of ALS-resistant biotypes in 1996. To confirm and characterize the resistance of greater beggarticks to ALS inhibitors, whole-plant bioassays and enzyme experiments were conducted. To investigate the molecular basis of resistance in greater beggarticks theALSgene was sequenced and compared between susceptible and resistant biotypes. Our results confirmed that greater beggarticks is resistant to ALS inhibitors and also indicated it possesses at least three isoforms of theALSgene. Analysis of the nucleotide and deduced amino acid sequences among the isoforms and between the biotypes indicated that a single point mutation, G–T, in oneALSisoform from the resistant biotype resulted in an amino acid substitution, Trp574Leu. Two additional substitutions were observed, Phe116Leu and Phe149Ser, in a second isoform of the resistant biotype, which were not yet reported in any other herbicide-resistantALSgene; thus, their role in conferring herbicide resistance is not yet ascertained. This is the first report ofALSmutations in an important, herbicide-resistant weed species from Brazil.

IDC in soybean has been improved through conventional breeding approaches (Cianzio, 1991;Cianzio and Soybean [Glycine max. (L.) Merr.] breeders have improved resis- Voss, 1994;Hintz et al., 1987), IDC-resistant lines may tance to iron-deficiency chlorosis (IDC) using conventional breeding have lower yield potential than IDC-susceptible cultiapproaches; however, many IDC-resistant cultivars have lower yields vars (Fehr, 1982vars (Fehr, , 1983). compared to IDC-susceptible cultivars. The importance of environment on IDC-resistance expression hinders progress in breeding for Selection for IDC resistance has been based on foliarresistance. An environment-independent selection strategy, such as chlorosis symptoms observed in plantings on calcareous marker-assisted selection (MAS), may increase breeding efficiency. Our soils (Cianzio et al., 1979). Complex polygenic inheriobjective was to determine whether simple sequence repeat (SSR) tance of IDC resistance and genotype ϫ environment markers located in previously reported quantitative trait loci (QTL) interactions may result in inaccurate assessment of resisfor IDC resistance would be associated with IDC resistance in a tance, which reduces the value of phenotypic rating and breeding population. One-hundred and eight SSR markers genetically decreases efficiency of breeding to improve the trait linked to eight QTLs on eight molecular linkage groups (MLGs) pre-(Lin et al., 2000b). Therefore, if an environment-indeviously identified for IDC were tested in a breeding population evalpendent approach could be devised, there might be the uated for IDC resistance on calcareous soils in Iowa. The breeding potential to improve breeding efficiency for IDC. population was developed from a cross between Pioneer 9254 and A97-770012. The F 2 lines were genotyped with markers and the F 2 -Quantitative trait loci for IDC resistance have been derived lines (F 2:4 and F 2:5 ) were evaluated for IDC resistance. Three identified using restriction fragment length polymormarkers were associated with IDC resistance: Satt211, Satt481, and phism (RFLP) markers in two soybean populations inSat_104. However, of the three markers, only Satt481 was associated field and hydroponic evaluations . to IDC resistance across environments. Although Satt481 accountedLack of common RFLP markers in the two populations for only 12% of the total phenotypic variation, molecular analysis of prompted Lin et al. (2000b) to conclude that the examthe eleven-most resistant lines in the population indicated that 73% ined molecular markers would be inefficient in MAS. Population Development Ames.A breeding population was developed from a cross between missouri.edu). a high-yielding cultivar, Pioneer 9254 (P9254), and an advanced Published in Crop Sci. 45:2394-2399(2005.

Drought is a limiting factor for N2 fixation in soybean [Glycine max (L.) Merr.] thereby resulting in reduced biomass accumulation and yield. Drought-sensitive genotypes accumulate ureides, a product of N2 fixation, during drought stress; however, drought-tolerant genotypes have lower shoot ureide concentrations, which appear to alleviate drought stress on N2 fixation. A key enzyme involved in ureide breakdown in shoots is allantoate amidohydrolase (AAH). It is hypothesized that AAH gene expression in soybean determines shoot ureide concentrations during water-deficit stress and is responsible for the differential sensitivities of the N2-fixation response to drought among soybean genotypes. The objectives were to examine the relationship between AAH transcript levels and shoot ureide concentration and drought tolerance. Drought-tolerant (Jackson) and drought-sensitive (Williams) genotypes were subjected to three water-availability treatments: well-watered control, moderate water-deficit stress, and severe water-deficit stress. Shoot ureide concentrations were examined, in addition to gene expression of AAH and DREB2, a gene expressed during water-deficit stress. As expected, DREB2 expression was detected only during severe water-deficit stress, and shoot ureide concentrations were greatest in the drought-sensitive genotype relative to the drought-tolerant genotype during water-deficit stress. However, expression of AAH transcripts was similar among water treatments and genotypes, indicating that AAH mRNA was not closely associated with drought tolerance. Ureide concentrations in shoots were weakly associated with AAH mRNA levels. These results indicate that AAH expression is probably not associated with the increased ureide catabolism observed in drought-tolerant genotypes, such as Jackson. Further study of AAH at the post-translational and enzymatic levels is warranted in order to dissect the potential role of this gene in drought tolerance.