Frequent generation of mutants with coincidental changes in multiple traits via ion-beam irradiation in soybean

Pigmentation of soybean seed coats via a mutation that abolishes production of multiple-phased siRNAs of chalcone synthase genes

Lack of pigmentation in seed coats of soybean is caused by natural RNA silencing of chalcone synthase (CHS) genes. This phenomenon is an evolutionary consequence of structural changes in DNA that resulted in the production of double-stranded RNAs (dsRNAs) that trigger RNA degradation. Here we determined that a mutant with pigmented seed coats derived from a cultivar that lacked the pigmentation had a deletion between DNA regions ICHS1 and a cytochrome P450 gene; the deletion included GmIRCHS, a candidate gene that triggers CHS RNA silencing via production of CHS dsRNAs. We also characterized CHS short interfering RNAs (siRNAs) produced in the wild-type seed coats that had CHS RNA silencing. Phased 21-nt CHS siRNAs were detected in all 21 phases and were widely distributed in exon 2 of CHS7, which indicates commonality in the pattern of RNA degradation in natural CHS RNA silencing between distantly related species. These results with the similarities in the rearrangements found in spontaneous mutants suggest that the structural organization that generates dsRNAs that trigger phased siRNA production is vulnerable to further structural changes, which eventually abolish the induction of RNA silencing.

Mutation of MYB36 affects isoflavonoid metabolism, growth, and stress responses in Lotus japonicus

Isoflavonoids are mostly produced by legumes although little is known about why and how legumes are able to regulate the biosynthesis of these particular compounds. Understanding the role of potential regulatory genes of the isoflavonoid biosynthetic pathway constitutes an important topic of research. The LORE1 mutation of the gene encoding the transcription factor MYB36 allowed the identification of this gene as a regulator of isoflavonoid biosynthesis in Lotus japonicus plants. The levels of several isoflavonoid compounds were considerably lower in two lines of Ljmyb36 mutant plants compared to the WT. In addition, we found that Ljmyb36 mutant plants were significantly smaller and showed a substantial decrease in the chlorophyll levels under normal growth conditions. The analysis of plants subjected to different types of abiotic stress conditions further revealed that mutant plants presented a higher sensitivity than WT plants, indicating that the MYB36 transcription factor is also involved in the stress response in L. japonicus plants.

Irradiation with carbon ion beams affects soybean nutritional quality in early generations

As people's demand for healthy diet increases, improving soybean seed nutritional quality is becoming as important as yield. Carbon ion beam radiation (CIBR) is an effective method to create soybean mutants, and thus breeding cultivars with better seed nutritional quality. In this study, the high-yield soybean line 'Dongsheng 28' was used, and three CIBR doses (100, 120, and 140 Gy) were used to explore the characteristics of quality separation and variation in the offspring of early mutant populations. Eleven quality traits, including protein, oil, sucrose, soluble sugar, iron (Fe), manganese (Mn), zinc (Zn), cupper (Cu), daidzin, glycitin, and genistin concentrations were analyzed in the M₂ and M₃ generations. The results revealed that the range of protein and oil concentration of all three CIBR doses changed by 38.5-42.9% and 18.8-23.8% in the M₂ and M₃ generations, respectively, while soluble sugar and sucrose concentrations changed by 48.1-123.4 and 22.7-74.7 mg/g, with significant effects by 140 Gy across the two generations. Therefore, around the optimum range, a higher CIBR dose is better for high protein, oil, and sugar varieties selection. In general, irradiation raised isoflavone concentrations, but 140 Gy had an inhibitory effect on isoflavone concentrations in the M₃ generation. Although a variety could not be released in the M₂ or M₃ generation, the results of this study have important guiding significance for the targeted cultivation of specific nutritional quality materials. For instance, a lower irradiation dose is preferable when breeding targets are higher isoflavones and Mn concentrations. It is essential to increase the irradiation dose if the breeding targets contain high levels of protein, oil, sucrose, soluble sugars, Fe, Zn, and Cu.

A Soybean Deletion Mutant That Moderates the Repression of Flowering by Cool Temperatures

Ambient growing temperature and photoperiod are major environmental stimuli that summer annual crops use to adjust their reproductive phenology so as to maximize yield. Variation in flowering time among soybean (Glycine max) cultivars results mainly from allelic diversity at loci that control photoperiod sensitivity and FLOWERING LOCUS T (FT) orthologs. However, variation in the thermal regulation of flowering and its underlying mechanisms are poorly understood. In this study, we identified a novel mutant (ef1) that confers altered thermal regulation of flowering in response to cool ambient temperatures. Mapping analysis with simple sequence repeat (SSR) markers located the mutation in the upper part of chromosome 19, where no QTL for flowering has been previously reported. Fine-mapping and re-sequencing revealed that the mutation was caused by deletion of a 214 kbp genomic region that contains 11 annotated genes, including CONSTANS-LIKE 2b (COL2b), a soybean ortholog of Arabidopsis CONSTANS. Comparison of flowering times under different photo-thermal conditions revealed that early flowering in the mutant lines was most distinct under cool ambient temperatures. The expression of two FT orthologs, FT2a and FT5a, was dramatically downregulated by cool temperature, but the magnitude of the downregulation was lower in the mutant lines. Cool temperatures upregulated COL2b expression or delayed peak expression, particularly at the fourth trifoliate-leaf stage. Intriguingly, they also upregulated E1, a soybean-specific repressor of FT orthologs. Our results suggest that the ef1 mutation is involved in thermal regulation of flowering in response to cool ambient temperature, and the lack of COL2b in the mutant likely alleviates the repression of flowering by cool temperature. The ef1 mutant can be used as a novel gene resource in breeding soybean cultivars adapted to cool climate and in research to improve our understanding of thermal regulation of flowering in soybean.