Method for selection of soybeans tolerant to seed cracking under chilling temperatures.

In Hokkaido, northern Japan, soybean [Glycine max (L.) Merr.] crops are damaged by cold weather. Chilling temperatures result in the appearance of cracking seeds (CS) in soybean crops, especially those grown in eastern and northern Hokkaido. Seed coats of CS are severely split on the dorsal side, and the cotyledons are exposed and frequently separated. CS occurrence causes unstable production because these seeds have no commodity value. However, little is known about the CS phenomenon. The aims of this study were to identify the cold-sensitive stage associated with CS occurrence and to develop a method to select CS-tolerant lines. First, we examined the relationship between chilling temperatures after flowering and CS occurrence in field tests. The average temperature 14 to 21 days after flowering was negatively correlated with the rate of CS. Second, we evaluated differences in CS tolerance among soybean cultivars and breeding lines in field tests. 'Toyohomare' and 'Toiku-238' were more CS-tolerant than 'Yukihomare' and 'Toyomusume'. Third, we developed a selection method in which plants were subjected to 21-day chilling-temperature treatment from 10 days after flowering in a phytotron. This enabled comparisons of CS tolerance among cultivars. This selection method will be useful for breeding CS-tolerant soybeans.

Quantitative trait loci associated with lodging tolerance in soybean cultivar 'Toyoharuka'.

Lodging tolerance (LT) is an important trait for high yield and combine-harvesting efficiency in soybean [Glycine max (L.) Merr.]. Many previous studies have investigated quantitative trait loci (QTLs) for lodging score (LS) in soybean. Most of the investigated QTLs were located in the proximal region of maturity or growth habit loci. The aim of this study was to identify genetic factors for LT not associated with maturity or growth habit. QTL analysis was performed using a recombinant inbred line (RIL) population derived from a cross between 'Toyoharuka' (TH), a lodging-tolerant cultivar, and 'Toyomusume' (TM). The genotypes of TH and TM were estimated as both e1e2E3E4 and dt1. The average LS over 4 years was used for QTL analysis, identifying a major and stable QTL, qLS19-1, on chromosome 19. The LS of the near-isogenic line (NIL) with the TH allele at Sat_099, the nearest marker to qLS19-1, was significantly lower than the NIL with the TM allele at that position. The TH allele at Sat_099 rarely had a negative influence on seed yield or other agronomic traits in both NILs and the TM-backcrossed lines. Our results suggest that marker-assisted selection for qLS19-1 is effective for improving LT in breeding programs.

Variation of GmIRCHS (Glycine max inverted-repeat CHS pseudogene) is related to tolerance of low temperature-induced seed coat discoloration in yellow soybean.

In yellow soybean, seed coat pigmentation is inhibited by post-transcriptional gene silencing (PTGS) of chalcone synthase (CHS) genes. A CHS cluster named GmIRCHS (Glycine max inverted-repeat CHS pseudogene) is suggested to cause PTGS in yellow-hilum cultivars. Cold-induced seed coat discoloration (CD), a commercially serious deterioration of seed appearance, is caused by an inhibition of this PTGS upon exposure to low temperatures. In the highly CD-tolerant cultivar Toyoharuka, the GmIRCHS structure differs from that of other cultivars. The aim of this study was to determine whether the variation of GmIRCHS structure among cultivars is related to variations in CD tolerance. Using two sets of recombinant inbred lines between Toyoharuka and CD-susceptible cultivars, we compared the GmIRCHS genotype and CD tolerance phenotype during low temperature treatment. The GmIRCHS genotype was related to the phenotype of CD tolerance. A QTL analysis around GmIRCHS showed that GmIRCHS itself or a region located very close to it was responsible for CD tolerance. The variation in GmIRCHS can serve as a useful DNA marker for marker-assisted selection for breeding CD tolerance. In addition, QTL analysis of the whole genome revealed a minor QTL that also affected CD tolerance.

A novel major quantitative trait locus controlling seed development at low temperature in soybean (Glycine max).

Low temperature is among the critical environmental factors that limit soybean production. To elucidate the genetic basis for chilling tolerance and identify useful markers, we conducted quantitative trait loci (QTL) analysis of seed-yielding ability at low temperature in soybean (Glycine max), using artificial climatic environments at usual and low temperatures and recombinant inbred lines derived from a cross between two contrasting cultivars in terms of chilling tolerance. We identified a QTL of a large effect (LOD > 15, r (2) > 0.3) associated with seed-yielding ability only at low temperature. The QTL was mapped near marker Sat_162 on linkage group A2, where no QTL for chilling tolerance has previously been identified. The tolerant genotype did not increase the pod number but maintained the seed number per pod and single seed weight, namely, the efficiency of seed development at low temperature. The effect of the QTL was confirmed in a segregating population of heterogeneous inbred families, which provided near-isogenic lines. The genomic region containing the QTL also influenced the node and pod numbers regardless of temperature condition, although this effect was not primarily associated with chilling tolerance. These results suggest the presence of a new major genetic factor that controls seed development specifically at low temperature. The findings will be useful for marker-assisted selection as well as for understanding of the mechanism underlying chilling tolerance in reproductive organs.