Thomas E. Carter

RFLP loci associated with soybean seed protein and oil content across populations and locations

Molecular markers provide the opportunity to identify marker-quantitative trait locus (QTL) associations in different environments and populations. Two soybean [Glycine max (L.) Merr.] populations, ‘Young’ x PI 416 937 and PI 97100 x ‘Coker 237’, were evaluated with restriction fragment length polymorphism (RFLP) markers to identify additional QTLs related to seed protein and oil. For the Young x PI 416937 population, 120 F4-derived lines were secored for segregation at 155 RFLP loci. The F4-derived lines and two parents were grown at Plains, G.a., and Windblow and Plymouth, N.C. in 1994, and evaluated for seed protein and oil. For the PI 97100 x Coker 237 population, 111 F2-derived lines were evaluated for segregation at 153 RFLP loci. Phenotypic data for seed protein and oil were obtained in two different locations (Athens, G.a., and Blackville, S.C.) in 1994. Based on single-factor analysis of variance (ANOVA) for the Young x PI 416937 population, five of seven independent markers associated with seed protein, and all four independent markers associated with seed oil in the combined analysis over locations were detected at all three locations. For the PI 97 100 x Coker 237 population, both single-factor ANOVA and interval mapping were used to detect QTLs. Using single-factor ANOVA, three of four independent markers for seed protein and two of three independent markers for seed oil were detected at both locations. In both populations, singlefactor ANOVA, revealed the consistency of QTLs across locations, which might be due to the high heritability and the relatively few QTLs with large effects conditioning these traits. However, interval mapping of the PI 97100 x Coker 237 population indicated that QTLs identified at Athens for seed protein and oil were different from those at Blackville. This might result from the power of QTL mapping being dependent on the level of saturation of the genetic map. Increased seed protein was associated with decreased seed oil in the PI 97100 x Coker 237 population (r = −0.61). There were various common markers (P⩽0.05) on linkage groups (LG) E, G,H,K, and UNK2 identified for both seed protein and oil. One QTL on LG E was associated with seed protein in both populations. The other QTLs for protein and oil were population specific.

Molecular markers associated with seed weight in two soybean populations

Seed weight (SW) is a component of soybean, Glycine max (L.) Merr., seed yield, as well as an important trait for food-type soybeans. Two soybean populations, 120 F4-derived lines of ‘Young’xPI416937 (Pop1) and 111 F2-derived lines of PI97100x‘Coker 237’ (Pop2), were mapped with RFLP makers to identify quantitative trait loci (QTLs) conditioning SW across environments and populations. The genetic map of Pop1 consisted of 155 loci covering 973 cM, whereas Pop2 involved 153 loci and covered 1600 cM of map distance. For Pop1, the phenotypic data were collected from Plains, GA., Windblow, N.C., and Plymouth, N.C., in 1994. For Pop2, data were collected from Athens, GA., in 1994 and 1995, and Blackville, S.C., in 1995. Based on single-factor analysis of variance (ANOVA), seven and nine independent loci were associated with SW in Pop1 and Pop2, respectively. Together the loci explained 73% of the variability in SW in Pop1 and 74% in Pop2. Transgressive segregation occurred among the progeny in both populations. The marker loci associated with SW were highly consistent across environments and years. Two QTLs on linkage group (LG) F and K were located at similar genomic regions in both populations. The high consistency of QTLs across environments indicates that effective marker-assisted selection is feasible for soybean SW.

Aluminum accumulation and associated effects on 15NO3− influx in roots of two soybean genotypes differing in Al tolerance

A study was conducted to examine aluminum (Al) exclusion by roots of two differentially tolerant soybean (Glycine max L. Merr.) lines, Pl-416937 (Al-tolerant) and Essex (Al-sensitive). Following exposure to 80μM Al for up to 2 h, roots were rinsed with a 10 mM potassium citrate solution and rapidly dissected to allow estimation of intracellular Al accumulation in morphologically distinct root regions. Using 10 min exposures to 300μM15NO3− and dissection, accompanying effects on NO3− uptake were measured. With Al exposures of 20 min or 2 h, there was greater Al accumulation in all root regions of Essex than in those of Pl-416937. The genotypic difference in Al accumulation was particularly apparent at the root apex, both in the tip and in the adjacent root cap and mucilage. Exposure of roots to Al inhibited the uptake of 15NO3− to a similar extent in all root regions. The results are consistent with Al exclusion from cells in the root apical region being an important mechanism of Al tolerance.

Correlation of shoot and root growth and its role in selecting for aluminum tolerance in soybean

Abstract Aluminum‐tolerant soybean cultivars are needed for deeper rooting and increased drought tolerance in acid subsoils. A major limitation in the development of such cultivars is Al‐screening methodology. Shoot growth is often used to infer root growth, but the genotypic relationships between root and shoot growth of older soybean plants have not been evaluated. Our objectives were (i) to test the hypothesis that shoot growth is a reliable indicator of acid soil (Al) tolerance in soybean, and (ii) to determine the relative Al tolerances of selected soybean genotypes. Nine genotypes were evaluated for Al tolerance by growing them for 37 days in greenhouse pots of unlimed (pH 4.3) and limed (pH 5.3) Tatum subsoil. Aluminum tolerance was determined by root and shoot growth and plant symptoms. Aluminum tolerance was detected using both shoot and root growth, and agreement between these two selection criteria was good. Genotypic correlations between root and shoot growth for unlimed soil, for limed soil, ...

Effects of defoliation on seed protein concentration in normal and high protein lines of soybean

Two high (NC106, NC111) and two normal (NC103, NC107) seed protein concentration lines, derived from two different recurrent selection populations of soybean (Glycine max L. Merr.) were subjected to partial defoliation at beginning seed fill (R5) under outdoor pot culture and field conditions. The aim of this study was to test the hypothesis that capacity to store N in vegetative organs and/or to mobilize that N to reproductive organs is associated with the high seed protein concentration trait. Symbiotic N2 fixation was the sole source of N in the pot experiment and the major source of N (met > 50% of the N requirement) in the low N soil used in the field experiment. Seed protein concentration and seed yield at maturity in both experiments and N accumulation and mobilization between R5 and maturity in the pot experiment were measured. The four genotypes did not differ significantly with respect to the amount of N accumulated before beginning seed fill (R5). Removal of up to two leaflets per trifoliolate leaf at R5 significantly decreased the seed protein concentration of NC107/111 but had no effect on this trait in NC103/106. Defoliation treatments significantly decreased seed yield, whole plant N accumulation (N2-fixation) during reproductive growth and vegetative N mobilization of all genotypes. Differences in harvest indices between the high and low protein lines accounted for approximately 35% of the differences in protein concentration. The two normal protein lines mobilized more vegetative N to the seed (average. 5.26 g plant−1) than the two high protein lines (average. 4.28 g plant−1). The two high seed protein lines (NC106, NC111) exhibited significantly different relative dependencies of reproductive N accumulation on vegetative N mobilization, 45% vs. 29%, in the control treatment. Whereas, NC103 with normal and NC106 with high seed protein concentration exhibited similar relative dependencies of reproductive N accumulation on vegetative N mobilization, (47% vs. 45%). Collectively, these results indicate that N stored in shoot organs before R5 and greater absolute and relative contribution of vegetative N mobilization to the reproductive N requirement are not responsible for the high seed protein concentration trait.

RFLP tagging of QTLs conditioning specific leaf weight and leaf size in soybean.

Selection for high specific leaf weight (SLW) in soybean [Glycine max (L) Merr.] may increase apparent photosynthetic rate per unit leaf area (AP), which in turn may improve seed yield. In general, the SLW and leaf size are negatively correlated in soybean. To maximize total photosynthetic performance, and perhaps the seed yield, of a soybean cultivar, it would be necessary to establish a large leaf area rapidly while maintaining a high SLW. The objective of the present study was to identify quantitative trait loci (QTLs) conditioning SLW and leaf size in soybean. One hundred and twenty F4-derived lines from a ‘Young’×PI416937 population were evaluated using restriction fragment length polymorphism (RFLP) markers. The genetic map consisted of 155 loci on 33 linkage groups (LGs) covering 973 cM of map distance. The phenotypic data were collected from two different environments – a greenhouse at Athens, Ga. and a field site at Windblow, N.C. The SLW and leaf-size measurements were made on leaves from the 8th and 9th node of soybean plants at the V12 stage of development. Combined over environments, six putative independent RFLP markers were associated with SLW, and four of these loci were consistent across environments. Individually, the six markers each explained between 8 and 18% of the phenotypic variation among lines for SLW. The Young alleles contributed to a greater SLW at four of the six independent marker loci, and transgressive segregation occurred among the progeny for SLW. Three putative independent RFLP markers were associated with leaf size, each explaining between 6 to 11% of the phenotypic variation in the trait, and one of these markers was identified in both environments. There was no correlation between SLW and leaf size in this population. Similarly, none of the six QTLs conditioning SLW were linked to any of the three QTLs for leaf size. In this soybean population, it is possible to select for progeny lines with greater SLW than either parent perhaps without affecting the leaf size. It is feasible to pyramid all of the desirable alleles for greater SLW and large leaf size in a single genetic background.

Improved root penetration of soil hard layers by a selected genotype

Abstract Crops can be effectively grown on hardpan soils and water effectively used from deep in the profile if hard layers in soils can be penetrated or if they are broken up by tillage. Addition of gypsum to the soil or exploitation of genetic differences in root penetrability may help improve root penetration through hard layers with less need to depend on the energy requirements of deep tillage. To test this theory, a single‐grained Ap horizon of Norfolk loamy sand soil was compacted into soil columns to compare root penetrability of soybean [Glycine max (L.) Merr.] genotypes Essex and PI 416937 in the presence and absence of gypsum and at two soil compaction levels (columns with uniform compaction at 1.4 g cm‐1 and columns with increasing compaction with depth from 1.4 to 1.75 g cm‐1). Compaction treatments were imposed by constructing soil columns composed of 2.5‐cm‐deep, 7.5‐cm‐diameter cylindrical cores compacted to predetermined bulk densities (1.40,1.55,1.65,and 1.75 g cm.3). Soil penetration resistances were measured on duplicate cores using a 3‐mm‐diameter cone‐tipped penetrometer. Columns were not watered during the study; soybean genotypes were grown in the columns until they died. Both genotypes lived one day longer in columns with lower bulk density and penetration resistance. Although root growth was more abundant for Essex than for PI 416937, root growth of PI 416937 was not decreased by compaction as much as it was for Essex. These results suggest that PI 416937 may possess the genetic capability to produce more root growth in soils with high penetration resistance. This study suggests that genetic improvement for root growth in soils with hard or acidic layers may potentially reduce our dependence on tillage. Gypsum did not affect root growth in this study.

Effect of soil pH on the pathogenesis of Heterodera glycines and Meloidogyne incognita on Glycine max genotypes

The effect of soil pH 4.3, 4.6, and 5.9 on the pathogenicity of Heterodera glycines and Meloidogyne incognita on acid soiladapted soybean genotypes (Davis and PI 416937) was investigated in three glasshouse experiments over 28 days after inoculation with 0 or 1000 (Experiments 1 and 2) and 0 or 5000 (Experiment 3) second-stage juveniles. Although nematodes of both species infected both genotypes at all of the soil pH, the numbers decreased with decreasing soil pH. Both genotypes seem to be better hosts for M. incognita than for H. glycines, Davis more so than PI 416937. Both nematodes decreased shoot weight at high inoculum levels, indicating that H. glycines may be more pathogenic than M. incognita. Nematode development after infection of roots was not affected by soil pH or by genotypes. Overall, the results suggest that adaptation of these nematodes should be considered in breeding programmes to develop low pH tolerant soybean cultivars.

Genetic Diversity in Crop Improvement: The Soybean Experience

SUMMARY The use of genetic diversity to form modern crops is one of the most remarkable accomplishments of agriculture. Even in the age of genomics, genetic diversity remains the cornerstone of crop improvement. There is extensive diversity in soybean and its ancestors. Much of this diversity has been collected though opportunities to extend collections remain. Genetic diversity has been used extensively in Asian breeding but utilization of exotic germplasm has been limited in North America. Capturing the value of diversity is easy for some traits but quite difficult for other traits, such as yield. New procedures and technology may greatly facilitate understanding and effective use of these exotic yield alleles. For the foreseeable future though, progress in capturing yield value from exotic germplasm through traditional breeding will continue to outstrip our scientific understanding of the alleles themselves. Public institutions will need to facilitate access to germplasm to increase utilization of diversity so it favorably impacts humanity.

The effects of drought and herbivory on plant–herbivore interactions across 16 soybean genotypes in a field experiment

As the Earths climate continues to change, drought and insect population outbreaks are predicted to increase in many parts of the world. It is therefore important to understand how changes in such abiotic and biotic stressors might impact agroecosystems. The plant stress hypothesis predicts that, owing to physiological and biochemical changes, plants experiencing drought will be more susceptible to insect herbivory, which could have synergistic negative effects on plant performance. By contrast, the plant vigor hypothesis predicts that insects will preferentially feed on fast‐growing vigorous plants. These hypotheses were tested in a field experiment using 16 soybean (Glycine max (L.) Merr.) genotypes to determine: (i) the combined effects of drought and herbivory on plant performance; (ii) the impact of drought on soybean resistance to herbivores; and (iii) how genetically variable phenotypic traits in soybean correlate with these responses. It was found that drought had a greater effect on soybean performance than herbivory, and drought and herbivory did not interact to impact on any measure of plant performance. Drought caused decreased insect herbivory on average, suggesting that the plant vigor hypothesis is consistent with the effects of drought stress on soybean resistance to leaf‐chewing insect herbivores. This conclusion is further supported by genotypic correlations which show that plant growth rate is positively correlated with the amount of herbivory plants received. These results suggest that, although the effects of climate‐associated changes in drought and herbivory will have negative effects on soybean, these potential effects are quantifiable with simple experiments and can be mitigated through continued breeding of varieties that are tolerant and resistant to these abiotic and biotic stressors.